AN 353: SMT Board Assembly Process Recommendations

AN 353: SMT Board Assembly Process Recommendations
SMT Board Assembly Process
Recommendations
AN-353-4.0
Application Note
This application note describes the board assembly process used in surface-mount
technology (SMT) and focuses on the SMT component-to-board reflow soldering
process and rework soldering if you are removing or replacing individual
components on already-assembled boards.
1
The information in this application note is for your reference only.
Conventional Tin-Lead and RoHS-Compliant Lead-Free Components
Altera® provides both the conventional tin-lead and restriction of hazardous
substances (RoHS)-compliant lead-free packages. Table 1 lists the second Level
Connections of each package type.
Table 1. Second Level Connections
Package Type
Lead frame
Ball-grid array (BGA)
1
Wire Bond
(Eutectic)
Wire Bond (RoHS
Compliant or
Pb-Free)
Flip Chip
Flip Chip (RoHS
Compliant or
Pb-Free)
Sn85Pb15
Matte Sn
—
—
Sn63Pb37 (Balls)
SAC305 (Balls)
Sn63Pb37 (Balls)
SAC305 (Balls)
While lead-frame and wire-bond packages can be provided as eutectic or lead-free,
currently Altera flip-chip packages are RoHS compliant. We use Exemption #15 for
the first Level Connections between the flip-chip die to the substrate (RoHS
Exemption #15: Lead in solders to complete a viable electrical connection between the
semiconductor die and the carrier within the integrated circuit flip-chip packages).
For the remainder of this application note, the term Pb-free is used to refer to the BGA
solutions provided with Pb-free package solder balls. This is considered the second
Level Connections between the package and the PCBs.
The recent directives and legislations by nations around the world have mandated
elimination of hazardous substances in the electronics industry. While elimination of
many of these substances do not have significant impact on reflow soldering of the
electronic component, there is one notable exception—lead. The elimination of lead
from solders requires special consideration in soldering of the Pb-free components on
to PCBs. This application note outlines the differences and recommends solutions to
develop the reflow process for Pb-free and RoHS-compliant devices.
101 Innovation Drive
San Jose, CA 95134
www.altera.com
October 2011
© 2011 Altera Corporation. All rights reserved. ALTERA, ARRIA, CYCLONE, HARDCOPY, MAX, MEGACORE, NIOS,
QUARTUS and STRATIX words and logos are trademarks of Altera Corporation and registered in the U.S. Patent and Trademark
Office and in other countries. All other words and logos identified as trademarks or service marks are the property of their
respective holders as described at www.altera.com/common/legal.html. Altera warrants performance of its semiconductor
products to current specifications in accordance with Altera's standard warranty, but reserves the right to make changes to any
products and services at any time without notice. Altera assumes no responsibility or liability arising out of the application or use
of any information, product, or service described herein except as expressly agreed to in writing by Altera. Altera customers are
advised to obtain the latest version of device specifications before relying on any published information and before placing orders
for products or services.
ISO
9001:2008
Registered
Altera Corporation
Subscribe
Page 2
Surface-Mount Technologies—Pb-Free Soldering Versus Tin-Lead Soldering
Altera has taken an industry leadership position and has adopted lead-free
technologies to provide solutions that align with industry requirements. In
semiconductors, lead is mainly used in packaging as a part of the eutectic solder used
as the surface finish for leaded packages and as solder balls for BGA packages. Altera
has proactively researched alternatives for lead compounds and has selected matte Sn
lead finish for leaded packages and Sn-3-4%Ag-0.5%Cu solder balls for BGA
packages. In addition, the thermal robustness of the packages has been improved by
selecting appropriate materials and processes to allow for the higher reflow
temperature compatibility required for assembling boards using lead-free solder
pastes.
f For more information about Altera’s Pb-free product offerings and solutions, refer to
the Packaging page of the Altera website.
Surface-Mount Technologies—Pb-Free Soldering Versus Tin-Lead
Soldering
This section outlines the differences between conventional tin-lead reflow soldering
and Pb-free soldering.
The reflow soldering process for Pb-free components is very similar to the
conventional eutectic (tin-lead) solder reflow process. Often, you can use the same
equipment set and process steps used for eutectic soldering for Pb-free soldering.
However, some important differences must be taken into account for Pb-free
soldering, as the material set used for Pb-free soldering is different and higher reflow
temperatures are required. The important factors that must be considered for Pb-free
soldering are described in the following sections.
PCB Considerations
The important PCB consideration is the surface finish. Several PCB surface finishes,
such as Organic Solderability Preservatives (OSP) and metallic surface finishes (such
as electrolytic NiAu and immersion silver), are available in the industry. You need to
determine the PCB surface finish based on wetting, storage, planarity, and cost issues.
In addition, you must ensure that board materials can withstand reflow temperatures
without warpage or other damage. For most cases, FR-4 board material is acceptable,
but high-density and high-complexity applications may require different board
materials, such as high Tg FR-4 (above 170°C).
Solder Alloy and Flux Considerations
A wide range of solder paste alloys are available in the industry. The lead-free alloys
typically have higher soldering temperatures than eutectic solder. The SnAgCu family
of solder alloys is most commonly used for SMT manufacturing. The eutectic and
Pb-free solder alloy selected must be nonhazardous, mechanically reliable, thermal
fatigue resistant, have good wetting and relatively low melting temperature, and
must be compatible with a variety of lead-bearing and lead-free surface coatings (1).
SMT Board Assembly Process Recommendations
October 2011
Altera Corporation
Print Process Considerations
Page 3
When selecting flux chemistries suitable for Sn-Pb and Pb-free processing, you must
consider the flux activation temperature, activity level, compatibility with chosen
lead-free alloy and reliability properties, such as surface insulation resistance (SIR)
and electromigration.
Print Process Considerations
Sn-Pb and Pb-free pastes require special handling. You must take into consideration
the development of the printing process and the specific aspects of the lead-free paste,
as outlined in this section.
Solder Paste Handling
The shelf life and storage conditions of the lead-free pastes may be different from the
eutectic solder pastes. To avoid issues related to paste handling, you must strictly
follow the paste handling recommendations provided by the paste manufacturers.
Screen Printing
The printing process for lead-free pastes is identical to the process used for eutectic
solder pastes. You must follow the guidelines recommended by the paste
manufacturers to accommodate paste-specific requirements. In general, the lead-free
paste characteristics yield similar performance in terms of stencil life, aperture release,
print definition, and repeatability.
One important factor that you must consider in designing stencils is that lead-free
pastes have higher surface tension and do not wet or spread on the surface of pads as
easily as eutectic solder pastes. This higher surface tension can lead to exposed pad
finish material after reflow soldering. You can rectify this problem by modifying the
stencil aperture designs to increase the paste coverage on the pads.
Reflow Process Considerations
For Pb-free soldering, the most important factor to consider is the characterization
and optimization of the reflow process. The reflow process window for conventional
soldering is relatively wider than that of Pb-free soldering.
The melting point of eutectic solder is 183°C. The lower temperature limit for reflow is
usually 200°C. The upper limit is approximately 235°C, which is the maximum
temperature to which most components can be exposed (Table 3 on page 5). These
high- and low-temperature limits provide a process window of over 35°C.
The lead-free alloy used for BGA solder balls has a melting point of 217°C. This alloy
requires a minimum reflow temperature of 235°C to ensure good wetting. The
maximum reflow temperature is in the 245°C to 260°C range, depending on the
package size (Table 4 on page 5). This narrows the process window for lead-free
soldering from 10°C to 20°C.
October 2011
Altera Corporation
SMT Board Assembly Process Recommendations
Page 4
Reflow Process Considerations
The increase in peak reflow temperature, in combination with the narrow process
window, makes the development of an optimal reflow profile a critical factor for
ensuring a successful lead-free assembly process. The major factors contributing to
the development of an optimal thermal profile are the size and weight of the
assembly, the density of the components, the mix of large and small components, and
the paste chemistry you use.
To ensure that all components are heated to temperatures above the minimum reflow
temperatures and that smaller components do not exceed the maximum temperature
limits, you must perform reflow profiling by attaching calibrated thermocouples
embedded in the spheres of the larger BGA parts as well as other critical locations on
the boards (2). Because the components are subjected to higher reflow temperatures,
select the appropriate moisture sensitivity level (MSL) for the components and
component handling. You must strictly follow the storage recommendations.
Although nitrogen is not required, Altera recommends including nitrogen in the
reflow process as its presence helps achieve better wettability and widen the process
window. Nitrogen is especially beneficial when temperature differential across the
board is large. Additionally, nitrogen improves the appearance of solder joints by
inhibiting the effects of oxidation.
Manufacturing processes for each PCB are unique and may require unique solutions
to ensure acceptable levels of quality, reliability, and manufacturing yield. Because of
differences in equipment, materials, and PCB density and sizes, you will need your
own unique process parameter development and validation.
To ensure that all packages are successfully and reliably assembled, the reflow profiles
studied and recommended by Altera are based on the JEDEC/IPC standard
J-STD-020 revision D.1 (3).
Figure 1 shows the range of temperature profiles compliant with the JEDEC/IPC
standard J-STD-020 revision D.1 (3).
Figure 1. IR/Convection Reflow Profile (IPC/JEDEC J-STD-020D.1)
>T
Supplier T p c
User T p <
- Tc
Tc
T c -5°C
Supplier t p
User t p
Tp
Temperature
Max. Ramp Up Rate = 3°C/s
Max. Ramp Down Rate = 6°C/s
TL
T smax
tp
Tc -5°C
tL
Preheat Area
T smin
ts
25
Time 25°C to Peak
Time
SMT Board Assembly Process Recommendations
October 2011
Altera Corporation
Reflow Process Considerations
Page 5
Table 2, Table 3, and Table 4 list the reflow parameters and peak temperatures as
recommended by JEDEC (3). Industry studies have shown that the Ramp-to-Spike
(RTS) process yields better results for the Pb-free assembly. This RTS profile type
offers better wetting and less thermal exposure than the Ramp-Soak-Spike (RSS)
profile typically used in eutectic soldering.
Table 2 lists the reflow profile recommendations.
Table 2. Reflow Profile Recommendations (JEDEC/IPC J-STD-020D.1) (1)
Profile Feature
Sn-Pb Eutectic Assembly
Pb-Free Assembly
3°C/ second maximum
3°C/ second maximum
Minimum preheat temperature (TsMIN)
100°C
150°C
Maximum preheat temperature (TsMAX)
150°C
200°C
60–120 seconds
60–120 seconds
183°C
217°C
Time above liquidous temperature TL (tL)
60–150 seconds
60–150 seconds
Peak temperature (TP)
Refer to Table 3
Refer to Table 4
Average ramp-up rate
(TsMAX to TP)
Preheat time (TsMIN to TsMAX) (tS)
Temperature (TL)
20
Time within 5°C of peak TP
Average ramp-down rate (TP to TsMAX)
Time 25°C to TP
seconds (2)
30 seconds (2)
6°C/ second maximum
6°C/ second maximum
6-minutes maximum
8-minutes maximum
Notes to Table 2:
(1) All temperatures refer to the topside of the package, measured on the package body surface.
(2) Tolerance for TP is defined as a supplier’s minimum and a user’s maximum.
Table 3 lists the Sn-Pb eutectic process peak reflow temperature (TP).
Table 3. Sn-Pb Eutectic Process—Peak Reflow Temperature (TP) (1),
(2)
Package Thickness
Volume mm³ < 350
Volume mm³  350
< 2.5 mm
235°C
220°C
 2.5 mm
220°C
220°C
Notes to Table 3:
(1) The package volume excludes the external terminals such as balls, bumps, lands, and leads.
(2) The tolerance of all classification temperatures is ±2°C.
Table 4 lists the Pb-free process peak reflow temperature (TP).
Table 4. Pb-Free Process—Peak Reflow Temperature (TP) (1)
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 to Table 4:
(1) The package volume excludes the external terminals such as balls, bumps, lands, and leads.
October 2011
Altera Corporation
SMT Board Assembly Process Recommendations
Page 6
Reflow Process Considerations
Typical Reflow Oven Setting for Altera’s Board-Level Reliability Studies
Altera uses a PCB board 40 cm long by 10 cm wide and 2.36 mm thick for board-level
reliability studies. The following oven settings are based on an eight-zone production
reflow oven.
Table 5 lists the Pb-free reflow oven settings.
Table 5. Pb-Free Reflow Oven Settings
Zones
Temperature setting
1
2
3
4
5
6
7
8
180°C
190°C
195°C
195°C
210°C
240°C
270°C
275°C
Belt Speed
32 inch/ minute
Targeted peak temperature
205°C to 245°C
Time above liquidus (217°C)
60 to 90 seconds (Approximately 70 seconds)
Package Type
1517-Pin FineLine BGA (Flip Chip)
Table 6 lists the eutectic reflow oven settings.
Table 6. Eutectic Reflow Oven Settings
Zones
Temperature setting
1
2
3
4
5
6
7
8
150°C
160°C
160°C
160°C
170°C
195°C
250°C
255°C
Belt Speed
32 inch/ minute
Targeted peak temperature
205°C to 215°C
Time above liquidus (217°C)
Package Type
60 to 90 seconds (Approximately 60 seconds)
1517-Pin FineLine BGA (Flip Chip)
Compatibility with Pb-Free Assembly Rework
Per the JEDEC standard J-STD-033, the Pb-free area array components (Table 4) must
be capable of assembly rework at 260°C within eight hours of removal from dry
storage or bake. This requirement is not applicable for the eutectic Sn-Pb area array
components.
To reduce thermal stress on boards and components, you must control the peak
temperatures below the recommended maximums (Table 4) and minimize the
temperature gradients across the board. High temperatures can put significant stress
on plated through-holes and barrels, which can lead to cracking. High first-pass
temperatures on double-sided assemblies increase the amount of second-side
oxidation, which can cause solderability problems on the second pass.
Altera has worked extensively with leading EMS companies and has successfully
demonstrated that the Pb-free parts can be soldered in air atmosphere (5).
However, for high-density, two-sided assemblies, you can alleviate the problems
related to a narrow process window by selecting modern reflow ovens with forced
convection and more heating zones with tighter process controls on reflow
parameters. Reflow ovens equipped with nitrogen reflow atmosphere have shown to
improve wettability at lower peak temperatures and reduce temperature gradients
across the board and have proven beneficial for double-sided assemblies (6).
SMT Board Assembly Process Recommendations
October 2011
Altera Corporation
Post-Reflow Inspection
Page 7
Post-Reflow Inspection
Industry studies have shown that using automated x-ray inspection systems are
effective for both the eutectic and lead-free solder joints. You may have to optimize
the x-ray inspection systems to take into account the contrast differences of the
lead-free solder and the differences in solder fillet shape and length.
You can also use Automated Optical Inspection (AOI) and visual inspection methods
for inspecting solder joints other than BGA joints. Lead-free solder joints are not as
shiny as eutectic solder joints. Therefore, inspectors must be trained to distinguish the
lead-free solder joints from the eutectic solder joints. Optimize the AOI system
parameters to account for changes in the solder fillet shape and the reflection
characteristics of the solder joint surface (4).
f For more information, refer to the latest IPC-A-610D Standard.
Manual Soldering and Rework
Because higher soldering temperatures are required for lead-free solders, set the
solder tip temperature higher. The higher soldering temperature requires that you
keep the soldering iron clean and coated with the solder alloy. Lead-free solders are
more sensitive to the effects of a dirty soldering iron. If not cleaned and coated, the
higher soldering temperatures can result in the soldering iron tip becoming oxidized.
You can improve the soldering performance with more active solder flux and
soldering in nitrogen atmosphere. In addition, the technicians performing the
operation must be trained thoroughly in lead-free soldering operation.
BGA Rework
The rework process for Pb-free BGAs is similar to that used for eutectic BGAs. The
BGA rework process typically consists of the following steps:
1. Thermal profiling
2. Removal of defective component
3. Site redressing
4. Solder replenishment or flux application
5. New component placement
6. Reflow soldering
7. Post reflow inspection
The rework machine must be capable of handling both eutectic and lead-free
processing temperature. The equipment must be a preheat system equipped with a
vision system that can accurately place fine-pitch components, hot gas airflow control,
and have software capable of thermal profiling and editing rework sequences.
Site redressing is a crucial process in lead-free rework. There are two methods for site
redressing—the soldering iron or wick method and the copper coupon redress
method. The success of the redressing techniques depends on the technician’s skill
and training.
October 2011
Altera Corporation
SMT Board Assembly Process Recommendations
Page 8
Backward Compatibility
The components being reworked have been exposed to moisture, they must be baked
before reflow. The baking parameters depend on the MSL of the package.
Thermal profiling is very important and you must measure the temperature at the
solder joints. Thermal profiles must be developed for component removal and the
component replacement process. In addition, board preheating has proven to be
beneficial during reflow to reduce thermal gradient related stresses on adjacent
components. Also, using nitrogen gas improves wettability and reduces
manufacturing defects during the rework process (6).
Backward Compatibility
In this application note, backward compatibility refers to the soldering process. Both
eutectic and Pb-free devices from Altera have the same form, fit, and function. If you
switched from Altera’s eutectic Sn-Pb products to Pb-free products, no board design
changes would be required. However, the finish materials for PCBs, solder paste, and
SMT reflow profiles will need adjustment. Altera does not recommend using the
eutectic BGA packages in the Pb-free process.
Altera lead-frame packages (for example, plastic J-lead chip carriers [PLCC], plastic
quad flat pack [PQFP], or power quad flat pack [RQFP]) are backward compatible.
The components are soldered with Sn-Pb solder using the Sn-Pb soldering process.
Altera lead-frame packages use matte Sn plating on the leads, which are compatible
with both Pb-free and Sn-Pb soldering alloys.
However, Altera does not recommend soldering Pb-free BGA packages (for example,
PBGA, super BGA, FBGA, Lidless FBGA, Flip Chip FBGA, or FCmBGA) with Sn-Pb
solder using the Sn-Pb soldering process. The eutectic Sn-Pb soldering process usually
has a peak reflow temperature of 205°C to 220°C. At this temperature range, the
SnAgCu BGA solder balls do not properly melt and wet to the soldering surfaces. As
a result, reliability and assembly yields are compromised.
References
1. Lau, John and Liu, Katrina, “Global Trends in Lead-free Soldering—Part I and II,”
Advanced Packaging, January and February 2004.
2. Baldwin, Daniel and Kazmierowicz, Philip, “Taking the pain out of lead-free
reflow,” www.assemblymag.com, December 2003.
3. JEDEC/Electronic Industries Alliance, Inc, “Moisture/Reflow Sensitivity
Classification for Non-hermetic Solid State Surface Mount Devices,” March 2008.
4. Bath, Jasbir, “A Manufacturable Lead-Free Surface Mount Process,” January 2003.
5. Bath, Jasbir; Chou, Chris; Lam, Samson; Wu, Roy; and Yoon, Sam, “Lead-Free BGA
Assembly,” July 1, 2004.
6. Åström, Anders, “The Effect of Nitrogen Reflow Soldering in a Lead-Free
Process,” September 2003.
SMT Board Assembly Process Recommendations
October 2011
Altera Corporation
Document Revision History
Page 9
Document Revision History
Table 7 lists the revision history for this application note.
Table 7. Document Revision History
Date
Version
October 2011
September 2010
4.0
3.0
Changes
■
Combined AN-353 with AN-81.
■
Minor text edits.
■
Changed title from “Reflow Soldering Guidelines for Pb-free Packages” to
“Reflow Soldering Guidelines for Lead-Free and RoHS-Compliant Packages”
■
Updated Table 1.
■
Updated Figure 1.
■
Added note to page 1.
■
Added the “Compatibility with Pb-Free Assembly Rework” section.
■
Applied new template.
■
Minor text edits.
February 2009
2.0
Technical changes to Table 1.
July 2004
1.0
Initial release.
October 2011
Altera Corporation
SMT Board Assembly Process Recommendations
Page 10
SMT Board Assembly Process Recommendations
Document Revision History
October 2011
Altera Corporation
Was this manual useful for you? yes no
Thank you for your participation!

* Your assessment is very important for improving the work of artificial intelligence, which forms the content of this project

Download PDF

advertisement