Guidelines for the Assembly of Micronas Packages

Guidelines for the Assembly of Micronas Packages
General
Literature
S p e c ial To p i c s
Guidelines for the Assembly
of Micronas Packages
Edition April 30, 2015
GL000017_004EN
Guidelines for the Assembly of Micronas Packages
Copyright, Warranty,
and Limitation of
Liability
The information and data contained in this document are believed to be accurate and
reliable. The software and proprietary information contained therein may be protected
by copyright, patent, trademark and/or other intellectual property rights of Micronas. All
rights not expressly granted remain reserved by Micronas.
Micronas assumes no liability for errors and gives no warranty representation or guarantee regarding the suitability of its products for any particular purpose due to these
specifications.
By this publication, Micronas does not assume responsibility for patent infringements or
other rights of third parties which may result from its use. Commercial conditions, product availability and delivery are exclusively subject to the respective order confirmation.
Any information and data which may be provided in the document can and do vary in
different applications, and actual performance may vary over time.
All operating parameters must be validated for each customer application by customers’ technical experts. Any new issue of this document invalidates previous issues.
Micronas reserves the right to review this document and to make changes to the document’s content at any time without obligation to notify any person or entity of such revision or changes. For further advice please contact us directly.
Do not use our products in life-supporting systems, military, aviation, or aerospace
applications! Unless explicitly agreed to otherwise in writing between the parties, Micronas’ products are not designed, intended or authorized for use as components in systems intended for surgical implants into the body, or other applications intended to support or sustain life, or for any other application in which the failure of the product could
create a situation where personal injury or death could occur.
No part of this publication may be reproduced, photocopied, stored on a retrieval system or transmitted without the express written consent of Micronas.
Micronas Trademarks
– HAL
Third-Party Trademarks
All other brand and product names or company names may be trademarks of their
respective companies.
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Contents
Page
Section
Title
4
5
6
8
8
1.
1.1.
1.2.
1.3.
1.4.
Introduction
Package Stress Resulting from Assembly
Lead Forming
Lead Plating
Failure Analysis
9
9
10
11
11
2.
2.1.
2.2.
2.2.1.
2.2.2.
Shelf Life, Storage, and Transportation
Moisture-sensitive Components (MSL Classification)
Storage and Transportation conditions
Maximum Storage Time
Internet Links to Standards Institutes
12
12
12
13
13
13
14
14
16
16
17
19
21
21
22
3.
3.1.
3.2.
3.2.1.
3.2.2.
3.2.3.
3.2.4.
3.2.4.1.
3.3.
3.3.1.
3.3.1.1.
3.3.1.2.
3.3.1.3.
3.3.1.4.
3.4.
Soldering and Welding
Basic Guidelines
Lead Soldering
Solder Paste
Solder Paste Fluxes
Contaminants to Avoid
Lead Soldering Processes
Reflow Profile
Lead Welding
Resistance Welding
Resistance Welding Process Development
Resistance Welding with Projections
Resistance Welding Trouble Shooting
Resistance Welding Process Definition and Monitoring
Laser Beam Welding (LBW)
24
4.
Attachment of the HAL Package to Subassemblies
26
26
5.
5.1.
Standards and Supporting Documentation
Protection against Electrostatic Damage
27
6.
Document History
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Guidelines for the Assembly of Micronas Packages
1. Introduction
Release Note:
Revision bars indicate significant changes to the previous edition.
The Hall-effect sensor is a monolithic integrated circuit that switches in response to
magnetic fields. If a magnetic field with flux lines perpendicular to the sensitive area is
applied to the sensor, the biased Hall plate forces a Hall voltage proportional to this
field. The Hall voltage is compared with the actual threshold level in the comparator.
The temperature-dependent bias increases the supply voltage of the Hall plates and
adjusts the switching points to the decreasing induction of magnets at higher temperatures. If the magnetic field exceeds the threshold levels, the open drain output switches
to the appropriate state. The built-in hysteresis eliminates oscillation and provides
switching behavior of output without bouncing.
Magnetic offset caused by mechanical stress is compensated for by using the “switching offset compensation technique”. Therefore, an internal oscillator provides a two
phase clock. The Hall voltage is sampled at the end of the first phase. At the end of the
second phase, both sampled and actual Hall voltages are averaged and compared with
the actual switching point. Subsequently, the open drain output switches to the appropriate state. The time from crossing the magnetic switching level to switching of output
can vary between zero and 1/fosc.
All Hall-effect sensors are equipped with an active offset compensation which minimizes the influence of mechanical stress on the magnetic characteristics. Although the
magnetic characteristics are robust against external stress effects, it is important to
take precautions to minimize stress from thermal or mechanical sources caused by
lead forming, lead soldering and welding or overmolding during the attachment of Halleffect sensor packages to subassemblies.
Avoiding stress during assembly will not only improve the electrical performance of the
device, but will also reduce the possibility of long-term reliability problems. These
guidelines provide information to minimize package stress which could compromise
both the performance and reliability of Hall-effect sensor (HAL) products.
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1.1.
Package Stress Resulting from Assembly
Plastic package cracking is the occurrence of fractures anywhere in or on a plastic
package. Over the years a vast range of mechanisms that lead to package cracking
have been characterized. Mechanisms vary from one package type to another, and
some may even be unique to certain package groups.
Most of the known mechanically-induced package cracks come from assembly, and
specifically, lead forming operations. Worn-out forming blades and punches can result
in large deflections of the leads and tie bar during processing, creating excessive
stresses at the lead-to-plastic interface. Cracks occur if these stresses exceed the
molding compound’s fracture strength. Single-stage lead forming also creates excessive stresses at the lead-to-plastic interface.
Despite the robust design and the use of advanced materials it is hence still important
to avoid any situations or conditions where the Hall sensor will be subject to forces from
compression, tension, torsion, and shearing onto the package, as well as on the leads.
Otherwise, this may be directly or latent hazardous to the functionality, the proper operation culminating in reliability issues.
Lead frames are prone to lead pulling, which is usually preceded by cracking at the
lead-to-plastic interface. Debris underneath the package during lead forming operations can produce large bending stresses on the package, which lead to cracks if they
exceed the plastic's fracture strength. Inadequate package nesting during lead forming
operations can also result in similar package cracks.
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Assembly of the HAL package into a subassembly will inevitably result in a shift in the
magnetic properties of the Hall-effect sensor. Micronas recommends thorough testing
of a large sample size of the finished product to verify that the products meets all performance and reliability goals.
F
(I
II)
F (II)
Fig. 1–1:
1.2.
F (I)
Stress sensitive locations.
(I) Force or bending applied to leads can damage wedge bonds and cause
package cracking.
(II) Force over wires can cause damage to wedge or ball bonds.
(III) Force over die face can cause die cracking and parameter shift
Lead Forming
Lead forming procedures are often carried out at customer facilities as preparation of
the package for subsequent assembly steps. The long package leads are imbedded in
the package body to a depth of approximately 1 mm. Therefore any stress applied to
the leads during lead forming operations induces a proportionally high stress in the
package body which can lead to internal damage of the electrical connections or HAL
device.
Micronas offers several preformed lead configurations. Consult with Micronas for the
availability of preformed leads which conform to your requirements as an alternative to
performing the lead forming operations.
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Fig. 1–2:
Preformed lead configurations
The following precautions will ensure that lead forming procedures do not induce damage to the HAL device, package body or leads:
– Leads must not be formed or cut closer than 1.2 mm to the package body.
– The bend radius resulting from the lead forming operation should not extend closer
than 1.2 mm to the package body.
– Leads must be adequately supported from above and below, so that the stress of the
forming operation does not induce any stress in the package body.
– Any axial pulling force on the package pins is unacceptable during the lead forming
operation.
– The package body must be mechanically isolated from the lead forming operation.
– Lead forming must be performed over a round anvil with a radius of at least 1 mm or
0.7 mm, according to the bending direction.
Rmin = 1.0 mm
Rmin = 0.7 mm
≥1.2 mm
Fig. 1–3:
Micronas
≥1.2 mm
Lead forming
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Guidelines for the Assembly of Micronas Packages
– During the bending operation, the leads should not be excessively clamped between
the anvil and pusher, preferably a roller forming tool should be used.
– The clamping force should not be high enough to change the shape of the leads.
– Incidental marks on the leads caused by lead forming operations are acceptable if
the penetration of the marks do not expose the leadframe base metal.
– Any marks in the leads made by clamping should have the same shape and depth
on all of the leads, which is an indication of uniform clamping.
1.3.
Lead Plating
All HAL packages have 100% matte tin plating (lead-free). This finish guarantees a
robust solder joint when typical solder reflow profiles are used. Besides having the lowest possible environmental impact, matte tin plating is backwards compatible with tinlead (SnPb) solders of all compositions. It can normally be processed at the same temperature used for typical SnPb solder alloys allowing the soldering of Micronas packages using an existing process, including processes which have a peak temperature
below the melting point of tin (232 °C). To get reliable solder joints, it is recommended
to exceed the pure tin melting of 232°C during the solder process. For lead-free solder
alloys, it is recommended to exceed a temperature of 240 °C during the solder process.
Micronas lead plating has a typical thickness of 11.5 µm which is compatible with all
soldering and welding processes and ensures excellent results. Typical industry standard plating thickness is higher which can lead to process problems such as splattering
or the formation of solder balls.
The materials and the process used for soldering have become increasingly important
due to the narrower process windows imposed by lead-free plating. 100% tin plating
will wet slower than SnPb alloys and require longer preheating thereby requiring the
use of fluxes which remain active for longer periods of time and maintain their properties at higher temperatures. Typically, fluxes with a higher activation level are necessary
in lead-free soldering processes.
1.4.
Failure Analysis
For failure analysis, the devices have to be returned as single device, stripped down
from the application module. The sensor has to be free of resin, glue or other moulding
material. The terminal length must be more than 2.5 mm for TO92 packages, for SMD
devices it has to be the original length.
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2. Shelf Life, Storage, and Transportation
2.1.
Moisture-sensitive Components (MSL Classification)
For moisture-sensitive packages, it is necessary to control the moisture content of the
components. Penetration of moisture into the package molding compound is generally
caused by exposure to ambient air. In many cases, moisture absorption leads to moisture concentrations in the component that are high enough to damage the package during the reflow process. Thus it is necessary to dry moisture-sensitive components, seal
them in a moisture-resistant bag, and only remove them immediately prior to board
assembly to the PCB. The permissible time (from opening the moisture barrier bag until
the final soldering process), which a component can remain outside the moisture barrier
bag, is a measure of the sensitivity of the component to ambient humidity (Moisture
Sensitivity Level, MSL). The most commonly applied standard IPC/JEDEC J-STD-033
defines eight different MSLs (see Table 2–1).
Please refer to the “Moisture Sensitivity Caution Label” on the packing material, which
contains information about the moisture sensitivity level of our products. IPC/JEDEC-JSTD-20 specifies the maximum reflow temperature that shall not be exceeded during
board assembly at the customer’s facility.
Table 2–1:
Level
Micronas
Moisture Sensitivity Levels (according to IPC/JEDEC J-STD-033)
Floor Life (out of bag)
Time
Conditions
1
Unlimited
30°C / 85% RH
2
1 year
30°C / 60% RH
2a
4 weeks
30°C / 60% RH
3
168 hours
30°C / 60% RH
4
72 hours
30°C / 60% RH
5
48 hours
30°C / 60% RH
5a
24 hours
30°C / 60% RH
6
Mandatory bake before use. After bake, must be reflowed
within the time limit specified on the label.
30°C / 60% RH
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2.2.
Storage and Transportation conditions
Unsuitable conditions during transportation and storage of components can cause
problems like poor solderability, delamination, and package cracking effects.
Due to this proper conditions for transportation and storage of components have to be
ensured.
The following standards in Table 2–2 should be taken into account:
Table 2–2:
Standard
Description
IEC 60721-3-0
Classification of environmental conditions: Part 3: Classification of
groups of environmental parameters and their severities; introduction
IEC 60721-3-1
Classification of environmental conditions: Part 3: Classification of
groups of environmental parameters and their severities; Section 1:
Storage
IEC 60721-3-2
Classification of environmental conditions: Part 3:Classification of
groups of environmental parameters and their severities; Section 2:
Transportation
IEC 61760-2
Surface mounting technology – Part 2: Transportation and storage
conditions of surface mounting devices (SMD) - Application guide
ISO 14644-1
Clean rooms and associated controlled environments – Part 1:
Classification of airborne particulates
Table 2–3:
10
IEC standards
General storage conditions – overview
Product
Condition for Storing
Wafer/Die
N2 or MBB (MBB= Moisture Barrier Bag)
Component – moisture sensitive
MBB (JEDEC J-STD-033)
Component – not moisture sensitive
1K2 (IEC 60721-3-1)
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2.2.1.
Maximum Storage Time
Micronas guarantees 2 years of solderability for the following packages:
SOT89B-1, SOT89B-2, SOT89B-3, TO92UA-1, TO92UA-2, TO92UA-3, TO92UA-4,
TO92UA-5, TO92UA-6,TO92UT-1, TO92UT-2, TO92UT-3, TO92UP-1, SOIC8-1 and
SOIC8-2
The conditions to be complied with in order to ensure problem-free processing of active
and passive components are described in standard IEC 61760-2.
Furthermore the ZVEI-Guideline for Long-Term Storage of Components. Subassemblies and Devices is to be taken into account.
2.2.2.
Internet Links to Standards Institutes
American National Standards Institute (ANSI)
Association Connecting Electronics Industries (IPC)
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Guidelines for the Assembly of Micronas Packages
3. Soldering and Welding
3.1.
Basic Guidelines
When package leads are soldered or welded at customer facilities, a few basic guidelines must be observed:
– Keep the package leads as long as possible. Long leads isolate the package body
from the high temperature necessary to form good solder or weld joints. Long leads
are easier to form during final alignment and help isolate the package body from any
stress incurred. Long leads allow for any thermal mismatch between package leads
and the part to which the package is being welded.
– Stay away from the dambar cut area with soldering or welding. This dimension is
about 1.2 mm on the lead starting from the package body and often referred to as
“F1” in package drawings. Solderability is not guaranteed in the dambar cut area and
closer to the package body.
– Maintain the lowest possible temperature and shortest process time necessary for
good solder or weld joints. The temperature at the package body must not exceed
260 °C. The use of a process with a too high a temperature can damage the HAL
device. On the other hand, prolonged heating due to a low temperature process can
also result in heat damage.
– Minimize latent spring stress in the package leads – Due to lead forming tolerances,
a certain degree of additional forming is often necessary during joining operations. If
the package leads need to be deformed and then soldered or welded, this spring
energy will be stored in the lead and subsequent exposure to elevated temperatures
could result in movement of the leads in the package body, which could lead to electrical failures.
3.2.
Lead Soldering
HAL packages (SOT89-x, SOIC8-x, TO92Ux-x, SOT23, TSSOP14) are suitable for typical soldering methods. The following guidelines on solder pastes, fluxes, contaminants
to avoid and general processing equipment, conditions and parameters will help avoid
damaging the package during the soldering process.
For above mentioned packages except the ones listed in Section 2.2.1 solderability is
guaranteed for one year from the date code on the package, provided that the product
is stored at a maximum temperature of 30 °C and a maximum relative humidity of 85%.
For usability of products older than one year in date code please contact Micronas.
Under these conditions, no Dry Pack is required.
Micronas packages are non-hermetic and the epoxy molding compound used to form
the package body can absorb moisture and other contaminants from the atmosphere.
The absorbed moisture can lead to internal delamination or external package cracking
when the package is exposed to elevated temperatures during soldering or welding
operations, due to the pressure exerted on the package from the absorbed water when
it vaporizes. Therefore, it is important that the storage requirements are fulfilled.
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3.2.1.
Solder Paste
Solder paste provides the metals and cleaning agents used to produce the mechanical
and electrical interconnection between package leads and contact surfaces within the
subassembly. The purpose of the solder paste is to provide clean and wettable surfaces to which the metal solder will adhere, thus producing a reliable electrical interconnection. The flux operates as the cleaning agent to remove potential oxides and other
contaminants from both the package leads and the contact surfaces. Additionally, the
flux also protects the newly cleaned metal surfaces, thereby preventing re-oxidation
during the soldering process while the contact surfaces are at an elevated temperature.
3.2.2.
Solder Paste Fluxes
The fluxing system is composed of a flux and binder system and can be divided into
three typical categories: rosin-based (or solvent-clean); water soluble (or aqueousclean); and no-clean. All three flux systems and flux cleaners are compatible with
Micronas HAL packages with the exception of flux cleaners containing 1,1,1-tricloroethane and trichloroethylene. The solvents can lead to corrosion within the package.
Rosin-based fluxes are available in non-activated (R), mildly activated (RMA) and activated (RA) formulations. These flux types contain an activating agent which increases
the wettability of the metal surfaces through the removal of surface oxides. The residue
resulting from the use of rosin-based fluxes is corrosive and must be thoroughly
removed from the subassembly after soldering.
The performance of the flux needs to be evaluated. A mild no-clean flux, for example,
may not be acceptable under production conditions. Micronas recommends the use of
halide-free water soluble fluxes because they are effective cleaners and have a very
low environmental impact. No-clean fluxes have an even lower impact on the environment, but may not be effective enough under typical production conditions. No-clean
fluxes should only be used when clearances within the subassembly are too small to
allow the use of cleaning agents or when local environmental restrictions do not allow
the use of flux removal cleansers.
3.2.3.
Contaminants to Avoid
During assembly the high temperatures can have long-term effects on the reliability of
the package. Leaching of contaminants through the use of water for the removal of flux
can lead to the formation of corrosive compounds after the package has been assembled and put into service by the end user. Halide compounds are the largest contributor
for corrosion. Any materials used in assembly containing halides must be avoided,
including solder paste, fluxes and overmolding materials.
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3.2.4.
Lead Soldering Processes
Typical mass-production soldering is performed through either wave soldering or reflow
soldering, though hand soldering may still be used for the attachment of certain components. The appropriate combination of solder, flux and process parameters must be
established and qualified through experiments prior to production start.
The solder-reflow process can be used for SMD and through-hole packages. The process profiles used must be optimized to ensure that the package body is not exposed
to high temperatures. This can be difficult since the maximum temperature required for
a lead-free solder process is higher that the typical peak SnPb process temperature.
Parts by Micronas have pure Sn plating; however, the parts can be processed by a
SnPb soldering process if the condition is observed that the peak temperature exceeds
232 °C. For a pure Sn soldering process, the temperature needs to exceed 240 °C.
The wave soldering process is not recommended for SMD packages. Through-hole
packages can be wave soldered. It is important that adequate spacing between the
package body and the PCB board be maintained to minimize overheating of the package.
Hand soldering can be used for through-hole packages, but is not recommended for
SMD packages. For hand soldering a self-regulating soldering iron should be utilized
with a maximum temperature of 380 °C (for 4 s – 5 s; tested according to EIAJ
ED-4701/300). If necessary, shielding should be used to avoid overheating the package
body. Hot air gun type soldering equipment should never be used for manual soldering.
This equipment generates excess volumes of hot air which can flow uncontrolled over
the PCB resulting in damaged solder joints on the adjacent components near the package being soldered.
3.2.4.1.
Reflow Profile
Table 3–4:
SnPb eutectic process – classification temperatures (Tc)
Package Thickness
Volume mm3
<350
Volume mm3
≥350
<2.5 mm
235 °C
220 °C
≥2.5 mm
220 °C
220 °C
Table 4-2 Pb-Free Process - Classification Temperatures (Tc)
Package
Thickness
Volume mm3
<350
Volume mm3
350 - 2000
Volume mm3
>2000
<1.6 mm
260 °C
260 °C
260 °C
1.6 mm - 2.5 mm
260 °C
250 °C
245 °C
>2.5 mm
250 °C
245 °C
245 °C
Note 1: At the discretion of the device manufacturer, but not the board assembler/user, the maximum peak package body temperature (Tp) can exceed the
values specified in Tables 4-1 or 4-2. The use of a higher Tp does not change the classification temperature (Tc).
Note 2: Package volume excludes external terminals (e.g., balls, bumps, lands, leads) and/or nonintegral heat sinks.
Note 3: The maximum component temperature reached during reflow depends on package thickness and volume. The use of convection reflow processes
reduces the thermal gradients between packages. However, thermal gradients due to differences in thermal mass of SMD packages may still exist.
Note 4: Moisture sensitivity levels of components intended for use in a Pb-free assembly process shall be evaluated using the Pb-free classification
temperatures and profiles defined in Tables 4.2 and 5-2, whether or not Pb-free.
Note 5: SMD packages classified to a given moisture sensitivity level by using Procedures or Criteria defined within any previous version of J-STD-020,
JESD22-A112 (rescinded), IPC-SM-786 (rescinded) do not need to be reclassified to the current revision unless a change in classification level or a
higher peak classification temperature is desired.
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tptp
TTPP
Ramp-up
Ramp-up
Temperature
Temperature
TTLL
Critical
CriticalZone
Zone
TTLLtotoTTPP
ttLL
smax
TTsmax
tsts
Preheat
Preheat
smin
TTsmin
Ramp-down
Ramp-down
t t25
25°C
°CtotoPeak
Peak
Time
Time
Fig. 3–4:
Reflow profile for TO and SOT packages
Profile Feature
Sn-Pb Eutectic
Assembly
Pb-Free
Assembly
Temperature Min (Tsmin)
Temperature Max (Tsmax)
Time (ts) from (Tsmin to Tsmax)
100 °C
150 °C
60-120 seconds
150 °C
200 °C
60-120 seconds
Ramp-up rate (TL to TP)
3 °C/second max.
3 °C/second max.
Liquidous temperature (TL)
Time (tL) maintained above TL
183 °C
60-150 seconds
217 °C
60-150 seconds
Peak package body temperature (TP)
235 °C
260 °C
Time (tP) within 5 °C of the specified
classification temperature (TC)
20 seconds
30 seconds
Ramp-down rate (TP to TL)
6 °C/second max.
6 °C/second max.
Time 25 °C to peak temperature
6 minutes max.
8 minutes max.
Preheat / Soak
Note: This reflow profile is based on J-STD-020D and is valid for TO and SOT package only. The customer is responsible to check all of the possible component,
board assembly and product design combinations for the different applications.
This standard cannot address all of the possible component, board assembly and product design combinations. However, the standard does provide a test method and criteria for commonly used technologies. Where uncommon or specialized components or
technologies are necessary, the development should include customer/manufacturer
involvement and the criteria should include an agreed definition of product acceptance.
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3.3.
Lead Welding
The HAL package leads are compatible with resistance and laser beam welding. The
success of any welding process is dependent upon process parameters, process conditions and the material to which the package is to be joined. Micronas does not give
any implied or express warranty as to the ability to weld Micronas packages and can
not provide specific welding processes for its products.
Micronas recommends the following welding processes if the package is to be welded
to a subassembly: resistance welding or laser welding.
Welding of copper terminals should be performed with the longest possible lead length
which helps maintain a lower overall package body temperature. The lowest temperature consistent with a stable welding process should be used. The welding process
temperature needs to be established through experiments to ensure that the leads can
be welded to the subassembly without undo high temperature, resulting in spattering
and solder ball formation. Shielding of surrounding area may be necessary to avoid
potential problems such as the bridging of nearby contacts and traces.
3.3.1.
Resistance Welding
There are three basic types of resistance welding process: solid state or cold welding;
fusion welding; and reflow braze welding. Micronas packages are most suitable for the
fusion welding process.
In a fusion welding process, either similar or dissimilar materials are heated above their
melting points so that the two materials flow together forming an alloy nugget with a
larger grain structure than the original materials. Typically a high welding energy for
either short or long weld times is used, depending upon material properties. To avoid
damaging the package, a short weld time is preferred. The joined materials exhibit
good tensile, peel and shear strengths.
Since resistance welding requires high temperatures, a certain volume of material at or
near the weld joint will be altered. This area is known as heat affected zone (HAZ). The
properties of the material in the HAZ undergo a change which may not be beneficial to
the welded joint. The welding process should be developed to minimize the size of the
HAZ.
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3.3.1.1.
Resistance Welding Process Development
The metallurgy of the materials to be welded determines the type of electrode and the
welding process parameters to be applied. When developing a new welding process,
there are two general categories of metals to consider: conductive (e.g. Al, Cu, Ag, Au);
and resistive (e.g. steel, Ni, Ti, W, Mo). Electrically conductive metals have a higher
thermal conductivity and are softer. Electrically resistive metals produce higher heat
during welding and produce better welds.
These two categories also apply to the material used for the electrodes. Therefore,
conductive electrodes should be used to weld resistive parts and resistive electrodes
should be used to weld conductive parts. From this it follows that when welding dissimilar materials, the anode and cathode electrodes must also be of dissimilar materials.
The parameters which have the strongest influence on the resistance welding process
are as follows: welding current strength; welding on-time; secondary voltage; welding
force; and electrode material and shape.
The correct settings are highly dependent upon the materials and shapes to which the
package leads are to be joined. Additional variations in the materials which can have
an affect on the welding parameters are: degree of surface oxidation; material age; or
fluctuations in the composition of the alloys.
The best approach for the establishment of process parameters is to start with the
shortest welding on-time and lowest secondary voltage and then gradually increasing
the current strength while maintaining a high force. Should the required results with
maximum current strength not be achieved, the secondary voltage should be increased
and the welding trials repeated starting with the lowest current strength.
If the required results are still not reached, the welding on-time should be increased.
Again, welding trials are made while gradually increasing the current strength and secondary voltage as described above. A gradual reduction in the welding force may be
needed to reach the required results, because a lower welding force results in an
increase in the intensity of the welding. When changing the welding force, always verify
that no splashing occurs.
The crystal structure of the weld is highly dependent upon the rate of cooling directly
after completion of the welding process. Generally, the faster the cooling, the finer the
crystal structure, and therefore the stronger the weld. Since the small package pins can
not quickly remove the heat, the heat should be conducted away from the welded joint
through the electrodes. Therefore it is advisable to hold the electrodes in the closed
position for a short period after the welding on-time has elapsed. This parameter is
usually known as welding hold time.
As assistance to the process development, the following comments regarding process
parameters should be considered:
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Guidelines for the Assembly of Micronas Packages
Welding on-time
The welding on-time is set short for joining materials with good thermal conducting
properties (e.g. Cu, Ag, Al); materials with poor thermal conducting properties (e.g. St,
W, Mo) require longer on-time.
Secondary voltage
A higher secondary voltage results in a higher welding strength. Increasing the secondary voltage also increases the current density, and if too high, the surfaces of the joining parts or the electrodes can become burned which leads to splashing, scorch marks
or pores. These effects can be minimized by reducing the secondary voltage or
increasing the welding force.
Welding force
The welding force to be applied depends on the materials to be joined. A higher welding force is used for materials with high electrical resistance in order to reduce the contact resistance at the joining interface. A lower force is used for materials with low electrical resistance and in this case, the increased contact resistance promotes rapid
heating. A welding force which is set too low results in metal splashing due to arcing
between the materials to be joined.
Electrode material
The electrode material has a major influence on both the quality and the uniformity of
the welds under production conditions. A mechanically stable electrode with low deformation under load is important in reaching a stable production welding process.
Dissimilar workpiece and electrode material combinations provide the best welding
since these combinations result in a higher resistance. Electrodes made from conductive material are used to weld electrically resistive materials (e.g. stainless steel, nickel)
and electrodes made from non-conductive are used to weld electrically conductive
materials (e.g. Cu, Au).
Electrode shape
The shape of the electrode has a large influence on thermal dissipation. A large contact
area between the electrodes and the materials to be joined improves heat conduction
and thereby increasing the weld strength and reducing surface deformation and discoloration. On the other hand, a small contact area can be beneficial when joining materials with low electrical resistance, enabling fusion temperature with low current.
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3.3.1.2.
Resistance Welding with Projections
Differences in part thickness, geometry and materials result in unbalanced thermal
loading which can lead to splattering, weak welds and inconsistent weld quality. Projections, small raised areas which have been stamped into one of the parts to be joined,
create a uniform weld heat balance between the two joining parts. Projections can take
on many forms, but there are two shapes which are often used when welding package
pins to a leadframe or substrate:
Elongated spherical projection – This type of projection is used for welding package
leads to leadframes or substrates which are made of thermally conductive material
such as copper and brass. This shape increases the localized welding heat and allows
for a wide tolerance in the lead placement.
Elongated flat projection – This type of projection is a variation of the elongated spherical projection, whereby the top area has a wide and flat surface. The flat surface creates a weld heat balance when the package pins are being welded to thermally resistive materials. This shape also allows for a wide tolerance in the lead placement.
Projections improve the welding process primarily in two ways. Firstly, the projection
results in the highest resistance occurring at the joining point between the two parts,
point R7 in the following diagram:
F
I
1
R1
R3
3
5
R5
R7
7
R6
6
R4
4
R2
2
F
Fig. 3–5:
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I
Resistance welding
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Guidelines for the Assembly of Micronas Packages
Secondly, the projection confines the weld current to a small area thereby producing a
localized, consistent weld heat allowing the use of lower weld current settings. The use
of projections enables the application of simple flat electrodes which are less expensive
than complex shapes and easy to maintain. Furthermore, the life of the electrodes is
extended because the lower current settings result in electrodes which run cooler in
production.
Regarding the design of projections, during formation of the projections the base material dimensions should not be significantly changed or distorted. Furthermore, the projections must be adequately rigid in order to avoid collapse during the welding process.
Most production problems stem from improper projection geometry or placement.
Avoid designing projections which are too small or too large and avoid placing projections too closely together.
Typical results achieved using a projection welding process with tests conducted by
Micronas:
Fig. 3–6:
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Projection-welded HAL device
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Guidelines for the Assembly of Micronas Packages
3.3.1.3.
Resistance Welding Trouble Shooting
To assist in solving typical problems associated with resistance welding, the following
table should be consulted:
3.3.1.4.
Problem
Solution
Electrode sticking
Use electrode with higher conductivity
Clean joining surfaces and electrode surfaces
Increase welding force
Incomplete welding
Increase welding current or on-time
Decrease welding force
Improve thermal balance
Improve alignment between mating parts
Clean joining surfaces
Surface melting
Decrease welding current or on-time
Increase welding force
Use cooled electrodes
Weld cracking
Increase welding hold time or force
Decrease welding on-time
Weld porosity
Clean joining surfaces
Increase welding force
Decrease welding current
Surface marring
Decrease welding force, time or current
Increase electrode tip size
Use softer electrode
Metal splattering
Clean joining surfaces
Increase welding force
Decrease welding on-time or current
Resistance Welding Process Definition and Monitoring
The welding process must be clearly defined, documented and verified. The following
steps outline the usual steps required to reach these goals.
– Weld process parameter optimization – Typically through the use of DOE method
– Parameters typically used to define weld quality – Peel strength, tensile or shear
strength; allowable part deformation; nugget diameter; nugget penetration; optical
requirements.
– Correlation of weld quality and weld monitor – Typically through the measurement of
one or more of the parameters listed above and the definition of acceptable target
values to be reached.
– Establishment of process limits
– Documentation of welding program and monitor
– Regular auditing of welding program and monitor
– Establishment of regular equipment inspection and maintenance
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Guidelines for the Assembly of Micronas Packages
3.4.
Laser Beam Welding (LBW)
LBM is a welding technique using a laser beam as a concentrated heat source, producing small, deep welds at a high throughput. The spot size of the laser should be small.
The depth of penetration is proportional to the amount of power used, but is also
dependent on the location of the focal point. Penetration is maximized when the focal
point is slightly below the surface of the workpiece. Advantages of LBW are narrow
welds, little or no distortion of the workpiece, minimal HAZ and exceptional metallurgical quality in the weld nugget.
Typically a pulsed solid state laser beam is used to weld thin materials. Nd: YAG1)
lasers are often used in this application.
As stated, heat is generated by the conversion of light energy. All metals reflect light to
some degree, with gold and silver high on the list and carbon steel low on the list. Gold,
silver, copper, and aluminium are therefore difficult to weld requiring intense energy
usually available from high energy peaking pulses or resorting to light absorbing coatings such as graphite on the weld joint surfaces to reduce their reflectivity. The 1.06
micron wavelength of the Nd:YAG laser is more readily absorbed than the longer 10.6
micron wavelength of the CO2 lasers, therefore, in this respect more suited for welding
highly reflective materials. However, though metallic reflectivity is a factor, once melted,
the reflectivity essentially disappears at the Curie temperature. Therefore most metals
are readily welded. The intense energy of the beam quickly melts the surface, from
which thermal conductance progresses to achieve penetration.
Tests have been conducted by Micronas to show the weldability of HAL packages to
both brass and copper leadframes using LBW. Good results were achieved using the
following equipment:
– Laser source: Single-mode fiber laser
– Beam spot diameter: 20 µm
– Laser power: 200 W
– Feed speed: 800 mm/s
– Beam focal length: 80 mm
– Shielding gas: Argon
The technique utilized was circular welding with a path diameter of 30 µm and 9 revolutions for each welded pin. For good reproducibility, it was important to ensure that there
was no gap between package leads and leadframe.
Using this equipment and technique, a welded depth of 100~200 µm was reached, with
a joint diameter of about 200 µm. No defects such as pores or holes were observed.
The following pictures show typical results.
1)
22
neodymium-doped yttrium aluminum garnet; Nd:Y3Al5O12
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Guidelines for the Assembly of Micronas Packages
Micronas
Fig. 3–7:
Side view, brass leadframe
Fig. 3–8:
Cross section, brass leadframe
Fig. 3–9:
Side view, copper leadframe
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Guidelines for the Assembly of Micronas Packages
4. Attachment of the HAL Package
to Subassemblies
Most processes used for the attachment of the package body to subassemblies, including gluing, conformal coatings, potting or overmolding, induce additional stress to the
package which can result in changes to the electrical characteristics of the HAL sensor.
The involved grouting materials usually cannot guarantee a complete hermetical sealing due to their chemical and intrinsic properties on the one hand and due to processing-related bubbles, pores, and inclusions on the other hand. This necessitates the use
of ion-free grouting materials, as well as a strictly zero-porosity-processing of the materials.
Micronas does not give any implied or express warranty as to the ability to the use of
adhesive on the package body to attach the sensor to the subassembly, the use of conformal coatings, the ultrasonic welding of the package body or the use of an overmolding process.
The best method for attachment of the package is through the use of an extra housing
into which the HAL sensor is fitted. The housing can then be attached to the subassemblies using any of the above mentioned attachment processes.
When designing the housing, the following points should be considered:
– The slip-fit housing should be specially designed for the package which can be
placed into the housing without excessive force; avoid press-fit tolerances in the
housing design which can cause stress in the package and lead to parameter shift of
the HAL device. For the time during insertion it is also necessary to ensure an adequate fixing of the HAL sensor in such a way that only a minimum of forces are transferred to the device itself.
– In case of unfavorable conditions, Hall sensors, which are not form-fit to a slip-fit
housing or to other encasements, may be damaged by mechanical influence or
vibration.
– Special attention should be paid to Mold-Flash when designing slip-fit housing.
Mold-Flash is very thin remains of the molding material at the gate or the venting of
the mold tool. When touching, it breaks from the package, but in certain cases it may
affect the assembly, depending on the design of the slip-fit housing. The following
drawing gives an example for the TO92 package.
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Fig. 4–10: Mold-Flash for TO92 packages
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5. Standards and Supporting Documentation
For information concerning Moisture Sensitivity Level (MSL) classification and the
associated package handling techniques, the documents offered from the International
Electrotechnical Commission (IEC), Japan Electronics and Information Technology
Industries Association (JEITA), Association Connecting Electronics Industries (IPC) or
Joint Electron Device Engineering Council (JEDEC) can be very useful. Additionally,
information concerning optimum soldering processes can also be obtained from these
organizations.
The evaluation of solder joints formed with 100% tin solder requires additional knowhow when compared to the evaluation of SnPb solder joints. The following standards
should be consulted.
IPC-A-610, Acceptability of Electronic Assemblies is the most widely used standard
published by the IPC. With multiple language versions, it has an international reputation
as the source for end product acceptance criteria for consumer and high reliability
printed wiring assemblies.
IPC/EIA J-STD-001 Requirements for Soldered Electrical and Electronic Assemblies
has emerged as the preeminent authority for electronics assembly manufacturing. The
standard describes materials, methods and verification criteria for producing high quality soldered interconnections. The standard emphasizes process control and sets
industry-consensus requirements for a broad range of electronic products.
JEDEC J-STD-002, Solderability Tests for Component Leads, Terminations, Lugs, Terminals and Wires describes a test method which provides optional conditions for preconditioning and soldering for the purpose of assessing the solderability of device
package terminations.
5.1.
Protection against Electrostatic Damage
IEC 61340-5-1, Protection of electronic devices from electrostatic phenomena - General Requirements describes the a system for protection of electronic components
against electrostatic phenomena
IEC/TR 61340-5-2, Protection of electronic devices from electrostatic phenomena User guide describes the application and quality control.
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6. Document History
1. Guidelines for the Assembly of HAL Packages, Nov. 26, 2009; GL000017_001EN.
First release of the document.
2. Guidelines for the Assembly of Micronas Packages, March 1, 2010;
GL000017_002EN. Second release of the document.
Major changes:
– Section 1.2 “Lead forming”: details changed
– Section 1.4 “Failure Analysis” added
– Section 2.1 “Basic Guidelines” changed
– Fig. 2–9: “Side view, copper leadframe” changed
3. Guidelines for the Assembly of HAL Packages, July 24, 2012; GL000017_003EN.
Third release of the document.
Major changes:
– Section 1.1 “Package Stress Resulting from Assembly”: text added
– Section 3 “Attachment of the HAL Package to Subassemblies”: text added
4. Guidelines for the Assembly of Micronas Packages, April 30, 2015;
GL000017_004EN. Fourth release of the document.
Major changes:
– Section 2: “Shelf Life, Storage, and Transportation” added
Micronas GmbH
Hans-Bunte-Strasse 19 ⋅ D-79108 Freiburg ⋅ P.O. Box 840 ⋅ D-79008 Freiburg, Germany
Tel. +49-761-517-0 ⋅ Fax +49-761-517-2174 ⋅ E-mail: docservice@micronas.com ⋅ Internet: www.micronas.com
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