Manual Lead-free Soldering of LEDs from OSRAM Opto

Manual Lead-free Soldering of LEDs from OSRAM
Opto Semiconductors
Application Note
Abstract
In addition to a brief fundamental consideration of the manual lead-free and leadcontaining soldering process, this application
note describes the essential influencing
factors and their effect on the lead-free
soldering process.
Furthermore, the basic rules and specific
guidelines associated with the new manual
lead-free soldering process are illustrated.
Also, possible risks are discussed and the
general procedure of the lead-free soldering
process is described.
In conclusion an overview of the solderability
of the various LED types from OSRAM OS
are presented, along with their ability to be
reworked and repaired.
Introduction
With the introduction and ratification of
Directive 2002/95 (RoHS directive "on the
restriction of the use of certain hazardous
substances in electrical and electronic
devices") as of June 2006, many production
lines have already switched to lead-free,
RoHS-conformant technology or are currently still in a transition phase, in which both
lead-free and lead-containing processes are
executed in parallel.
In automated production sequences, the
implementation of lead-free soldering
processes has been carried out without
great difficulties, in spite of the smaller
processing window due to the higher melting
temperature of the new solder.
September, 2015
The quality, reproducibility and process
stability have achieved an equally high level,
although the solder heat resistance of
individual SMD components such as LEDs
makes it difficult to conform to the lead-free
soldering process.
In contrast, the introduction of a manual
lead-free soldering process is still awkward,
since it is more difficult to control.
Although today, manual soldering is almost
exclusively used for the manufacture of
prototypes and for repair or rework of
production components, quality assurance
represents the greatest challenge, here.
With manual lead-free soldering, the quality
is therefore essentially influenced and
determined by the solder materials and
equipment, the experience and ability of the
operator and a continuous process control.
It is therefore recommended to only carry
out manual lead-free soldering with appropriate equipment and trained personnel.
Page 1 of 12

In addition, it should be noted than not all
available LED types are suited for manual
soldering or repair.
Underlying considerations
In principle, manual soldering with lead-free
solder is not much more difficult than
soldering with lead-containing solder.
In order to achieve good results and solder
connections, the properties and differences
of the two soldering processes must be
thoroughly understood and considered from
a technical standpoint.
The essential differences between lead-free
solder and tin-lead compounds is first of all,
the higher melting temperatures (up to 40°C
higher than tin-lead compounds, depending
on the solder used), and secondly, the
poorer wetting characteristics of lead-free
solders.
For soldering, this means that the time
required for wetting the solder joints
increases and the lead-free solder takes
longer to spread.
In addition, differences can arise in the
appearance of the solder joints; lead-free
connections appear to be dull and matt
(without luster) in comparison to leadcontaining solder connections.
Figure 1: Different appearance of leadcontaining and lead-free solder
The quality and steadiness of soldering
created manually with a soldering iron is
generally influenced by several factors,
including:



the composition of the solder
the activity of the flux material
the thermal characteristics of the
soldering iron
September, 2015

the angle of soldering, dependent on
the handling of the operator
the joint clearance of the two
surfaces
In addition to the abovementioned factors,
the solder connection is ultimately
dependent on the prevailing temperature
and effective time.
Important influencing factors and
their implications
Solder
The type of solder used represents the most
important parameter and has a decisive
influence on the entire soldering process
and on the subsequent connection.
Through the composition of the solder and
the associated properties such as solder
temperature,
wetting
and
oxidation
characteristics etc, a certain process window
for the soldering process is predefined.
Figure 2: Side by side comparison of the
process windows – lead-containing vs.
lead-free
As can be seen in Figure 2, the higher
melting point of the lead-free solder leads to
a reduction in the solder processing window
in comparison to that of lead-containing
solder.
The size of the window is determined by the
melting temperature of the solder and the
maximum allowable temperature, above
which damage to the component occurs.
In addition, the poorer wetting characteristics
Page 2 of 12
of lead-free solder causes a lengthening of
the processing time. Compared to leadcontaining solders, a factor of 2 to 3 can be
assumed.
Type of soldering iron
Many types of soldering irons are available.
The main differences are the heating power
and the precision of temperature regulation.
Figure 3: Side by side comparison of the
process window – lead-containing vs.
lead-free hand soldering
With
older
soldering
iron
models,
temperature measurement and regulation
usually occur at the internal heating element.
Due to the distance from the soldering tip,
large deviations from the actual temperature
of the soldering tip (up to 50°C) can occur.
Combined
with
the
higher
melting
temperature of lead-free solder, this
increases the risk of overheating or can lead
to temperature losses during the manual
soldering process.
Flux material
The use of a flux material basically serves to
activate the respective soldering surfaces of
the components.
That is, it dissolves the oxidation layer of the
surfaces by warming and at the same time,
prevents new oxidation of the solder, before
and during the soldering process.
The flux material simultaneously reduces the
surface tension of the flowing solder and in
this way, brings about better wetting
characteristics and a more favorable flowing
property.
With the use of flux material, one generally
has to consider whether the smoke gases
which arise, depending on the type of flux
material, represent a health hazard.
Regardless of this, it is generally recommended to provide sufficient ventilation, or
for longer periods of work, to utilize an
exhaust fan.
In addition, flux material also has an
influence on the durability of the soldering
iron tip. Since this flux material is more
aggressive than tin-lead compounds, a
reduction in operating life can occur. As a
result, the tip must be exchanged after a
shorter period of time.
September, 2015
Better results can be achieved for lead-free
solder with the help of modern soldering
stations which possess regulated heat
management as well as internal process
monitoring.
With modern soldering irons, temperature
regulation occurs at the soldering tip rather
than at the heating element. In addition, the
soldering irons are equipped with sufficient
heating power (> 80 W) and exhibit
extremely fast warming characteristics.
This ensures that all solder joints are
created with nearly the same temperature.
Solder temperature
In general, it is recommended to use the
lowest temperature possible, depending on
the solder used.
On the one hand, this prevents damage to
heat-sensitive components and on the other
the operating life of the soldering iron is
extended.
With an increase in temperature, the wetting
time for lead-free solder can indeed be
reduced, but this can damage certain types
of components and reduce the operating life
of the soldering iron.
Since the melting point of lead-free solder is
around 40°C higher than the melting point of
typical tin-lead compounds, the temperature
of the soldering tip must be set higher as a
consequence.
Page 3 of 12
In general, therefore, it is typical and also
acceptable that the temperature of the
soldering tip is set to be 50°C higher than
the melting point of the solder. However,
soldering temperatures are often selected
which are 100°C higher than the melting
temperature.
This excess temperature is ultimately
dependent on the heat capacity of the LED
to be soldered, the extent of the solder joint
and the size of the soldering tip.
Possible problems / risks
Prototyping
With manual soldering, the most common
problems are damaging the LEDs or the
circuit board (base material, solder resist
mask, pads etc.) and poor solder joints.
Soldering of larger LEDs with a higher heat
binding potential causes the greatest
difficulty in most cases.
An improvement can possibly be achieved
with the use of an additional heat source
(heating pad, IR radiator, etc.).
In case different alloys are used in parallel, it
would be advantageous to mark or label the
solder pads and possibly the components, in
order to provide information about the solder
used.
Since a higher temperature is required
during rework due to a change in the
composition of the material, it can happen
that both the components and circuit board
can be damaged in the process. Careful,
skilled work along with process and
temperature monitoring are thus strongly
recommended.
Furthermore, depending on the condition
and storage time of the components to be
processed, a more aggressive flux material
may possibly be required.
Basic rules for manual soldering
Poor solder joints most often occur if the
surfaces are not clean or are strongly
oxidized.
With lead-free compounds, this is seen more
often.
One possible remedy is the use of an
aggressive flux material. However, this can
lead to additional problems.
Since solder wire is produced with several
types of flux, some of which are more
corrosive than others, an initial test should
be performed with respect to its suitability.
A further possibility is the use of an
additional flux material for the components.
Rework
When reworking components already
soldered with lead-free solder, it should be
noted that not all alloys can be mixed
together. Some combinations can lead to
unreliable solder connections.
Normally the same alloy as for to the former
soldering is used.
September, 2015
Page 4 of 12
A good heat contact between the
soldering tip and the solder joint
(component and PCB) must be
created. This can only be achieved
with flowing solder.
The flux material should perform its
effect at the appropriate locations
and should therefore flow freely to
the locations to be soldered. This
also enhances the heat transfer.
The contact between the soldering
iron and the location to be soldered
should only be maintained until the
solder has freely flowed.
Only as much solder as needed
should be used. For stranded-wire
connections, the contour of the wires
should remain visible.
The LEDs must not be permitted to
move during the solidification
process.
Additional rules for lead-free soldering:
The temperature of the soldering tip
must be raised in comparison to that
required for lead-containing solder
(+25°C to +40°C)
The upper limit must not be
increased, as this would result in
delamination of the circuit board or
thermal damage to the components.
For the soldering procedure, this
means that the processing window
becomes narrower.
With lead-free solder, the flow
behavior is poorer; the solder time
increases by 50 – 100% in comparison to lead-containing solder.
Exertion of pressure during soldering
should be avoided so that the
soldering tip does become deformed
or the components will damaged.
Since lead-free solder is more
aggressive, there is more wear and
tear on the soldering tip. The soldering stations should be switched off
when not in use or when no standby
function is available.
The use of fast heating soldering tips
is preferable, since these are more
quickly placed into operation.
After the soldering the tip should be
cleaned and tin-plated.
or circuit traces which could interfere or
inhibit the wetting of the solder.
In order to minimize or prevent additional
effort, it is advantageous to populate new
circuit boards directly after manufacture, or
package them in a vacuum or inert gas for
later processing.
Contamination or progressive oxidation is
thereby prevented.
In case of rework of already populated
boards, this means that if necessary, the
boards should be preheated in an
appropriate oven, depending on storage
conditions and time.
Preheating serves to remove absorbed
moisture and prevent the so-called
"popcorn" effect with components.
The duration and temperature of the
preheating
procedure
is
individually
determined, dependent on the components
on the circuit board and the storage and
environmental conditions.
Tools and materials
As mentioned previously, lead-free solder
places special demands and requirements
on the soldering equipment.
Representative of equipment available on
the market, three appropriate and proven
soldering stations are listed here.
ERSA:
i-CON Soldering Station & i-Tool Soldering
Iron, 150 W
WELLER:
WD2M Soldering Station, 160 W
Lead Free Hand Soldering Process
In general, it is recommended to prepare
and provide all necessary tools, materials
and additional auxiliary tools before the
soldering process.
This also means that the circuit board should
be cleaned if necessary, in order to remove
oxidation or other impurities.
Care should be taken that cleansing itself
does not cause damage to the circuit board
September, 2015
METCAL:
Soldering Station PS 800
All three systems were specially developed
and optimized for lead-free soldering.
In comparison to older soldering stations for
example, the soldering system from ERSA
possesses precise temperature regulation at
the soldering tip and extremely fast warm-up
characteristics. In addition, it is equipped
with a process window alarm and an
Page 5 of 12
automatic standby sensor as well as other
user-friendly functions.
Other systems are similarly equipped.
For the soldering irons mentioned above,
several different soldering tips are also
available which can be specifically adapted
and optimized for the component.
General solder technique / procedure
The soldering technique and correct procedure is basically no different from the old
technique for lead-containing solder.
After setting the required working temperature (soldering tip ≤ 350°C for LEDs) the tip
should be cleaned before each use with a
moist sponge or by means of a dry pad
made of steel wool.
Figure 4: Tip Selection- correct geometry
for each application
As auxiliary tools, various sizes of tweezers
are recommended for better handling of
LEDs along with a desoldering braid or
pump for removal of solder during repair
work. In addition, the use of a so-called
"third hand", an adjustable fixture for holding
the circuit board, and a magnifying glass can
also be helpful.
In general,
ESD protection should
additionally be provided for the components
and/or the populated circuit board. This can
be achieved with a grounding armband,
grounded table or support, etc.
When soldering, the use of solder wire with
flux core is preferable. This is available in
several diameters and provides a sufficient
amount of flux in most cases.
As an example, solder wire from EDSYN
consisting of SnAgCu with NO-CLEAN flux
as per F SW34 can be used.
For soldering of LEDs, particularly for
miniature components, a diameter of 0.35
mm is sufficient.
Depending on the size of the LED or
component to be soldered, heavier solder
wire can be used.
With the use of solder wire with a flux core,
the solder and flux can spray out due to the
very quick warming of the solder.
The flux tends to carbonize in the process
and the desired effectiveness is reduced. An
improvement can be achieved with a Vformed notch in the solder wire, permitting
more effective use of the flux material.
September, 2015
Figure 3: Example of moist sponge for
cleaning soldering tips
Dry cleaning has the advantage that the
soldering tip is not abruptly cooled, and that
no contamination arises from dirty sponges.
In addition, the light scouring effect of steel
wool can also easily remove heavy
contamination and accumulated passive
layers.
Figure 4: Example of dry cleaners
After cleaning, the tip must be wetted again
with a sufficient amount of solder.
In the next step, the solder joint is heated.
Here, the solder pad and LED connection
contact are heated together by simultaneous
contact with the soldering tip.
Page 6 of 12
The LED is then attached with the addition
of a small amount of solder in the corner
between the soldering tip and the LED pin.
Afterwards, the solder wire should be pulled
away and one should wait for a short
moment.
Then, solder is again applied to the lead or
solder joint until the location has been
sufficiently filled with solder.
The solder wire is then pulled away and
finally, the soldering tip is removed from the
solder joint.
The other contact connections are soldered
in a similar manner.
Before replacing the soldering iron in the
holder, the tip should be checked once
again and re-tinned if necessary (procedure
according to the IPC recommendation).
Rework and repair procedure
The procedure for repair or rework of solder
connections differs somewhat from the
prementioned soldering technique, since the
solder connection is already present.
With repair, a defective component is
normally replaced with a functioning part.
The existing solder connection must be
melted and the solder removed by means of
a desoldering braid or a pump.
With rework, however, individual solder
joints are reworked because they are
possibly damaged or not sufficiently formed.
Here, it is also generally true that the
soldering iron tip should be cleaned and
wetted with solder before use.
1. Heat the solder connection until the
solder completely melts
2b. Apply appropriate solder if necessary
(rework)
The solder wire is applied to the surface of
the melted solder so that the solder is
melted there instead of at the soldering tip.
In order to prevent damage to the
component of the circuit board material, a
maximum contact time of 3 seconds should
not be exceeded.
3.
After the soldering tip is removed, the
connection solidifies again after a few
seconds and other leads of the component
can be soldered.
4.
Cleaning the solder joints
In case cleaning is required, it is
recommended to eliminate the flux residue
as soon as possible. As a rule, dried residue
adheres more tenaciously and can only be
removed with greater difficulty and by more
aggressive means.
Visual assessment of the solder joints
After soldering, a visual assessment should
be performed in any case, with respect to
the appearance and quality of the
connection.
The person carrying out this assessment
should be trained in this regard and have
sufficient experience. For a confident and
reliable assessment, criteria according to
IPC-Standard (IPC-A-610) are drawn upon
worldwide.
A few excerpts include:
In general, the soldering iron should be held
at the connection location with the largest
amount of solder. In order to achieve a good
heat
transfer,
the
tip
should
be
simultaneously held against the solder pad
and the connection contact of the
component.
2a. Remove the liquid solder by means
of a desoldering braid or a pump (repair)
September, 2015
Solidification of the solder connection
Page 7 of 12
The solder joint should be uniform
and smooth in appearance (shiny is
not required)
The solder should taper off from the
inserted parts (small contact angle)
The surface of the solder joint should
be unbroken.
The contours of the soldered parts
should be recognizable in the solder
joint.
The solder joint
sufficient solder.
must
contain
Additional information and exact details can
be obtained from the IPC Standard.
Cleaning
It is simpler as well to resort to water-soluble
flux material. Meanwhile, there are systems
which also permit better wetting of lead-free
materials without nitrogen.
Important LED-specific points
In most cases, final cleaning is only
necessary to remove any flux residue which
may be present.
Essentially, other residue or contamination
should not be present.
Often, various cleansing solutions or
cleaning by means of an ultrasonic bath is
recommended by solder manufacturers.
With the presence of LEDs, however, this is
only conditionally or not at all possible.
In principle, isopropyl alcohol (IPA) can be
used, since this is also suitable and
approved for cleaning LEDs from OSRAM
OS.
If other cleansing solutions are applied, their
suitability should be tested beforehand,
particularly if there is associated damage to
the LED.
Because of worldwide regulations, cleansers
such as FREON or other compounds
containing chloroflurocarbons (CFCs) should
not be used.
Cleaning by means of an ultrasonic bath is
not recommended for LEDs.
The reason for this is that the influence on
the LEDs is dependent on the ultrasonic
power, the duration of treatment and the
cleansing solution used.
If ultrasonic cleaning cannot be avoided, it
must first be determined whether the LEDs
will be damaged in the process.
In the best and ideal case, cleaning is not
required if solder with so-called NO-CLEAN
flux is used.
September, 2015
With this type of flux material, it is not
necessary to remove the remaining residue
from the connections or circuit board in order
to guarantee reliability.
Since LED housings predominantly consist
of plastic (and ceramic for a few of the new
LEDs), the direct contact with a hot soldering
tip can often lead to damage of the device.
This applies exceptionally to the plastic
optics of the LEDs.
In addition, it should be noted that with
higher soldering tip temperatures, heat is
transferred more quickly to the housing via
the connection contacts.
The prescribed solder times should therefore
not be exceeded, since this can otherwise
damage the component.
It should also be noted that with the various
packaging types, the size and form of the
connection contacts vary as well.
For optimal soldering results, it is
recommended to use individually adapted
soldering tips.
If a soldering tip is too large or wide for
miniature components, for example, this can
lead to overheating and thus damages to the
component housing. If a small, narrow tip is
used for larger contacts, however,
insufficient heat is available for a good
solder connection.
Particular experience combined with special
care and higher demands are required for
the processing of LED with high power
housings. The reason for this is the heat
slug integrated in the package base. For
optimal heat transfer, this must be affixed or
soldered to the circuit board.
Soldering of the heat slug itself can only
occur with the help of solder paste and an
additional heating plate.
Page 8 of 12
Since the heat slug is embedded in the
package base, direct contact with the
soldering iron is not possible; for this reason,
rework at the heat slug cannot be carried
out.
Conclusion
Generally it is advisable to use an additional
heating plate for lead-free soldering of LEDs,
especially if an insulated metal substrate is
utilized.
The prerequisite for this, however, is that
one thoroughly understands the properties
and differences of the new solder and also
considers the corresponding process from a
technical standpoint.
In addition, with manual soldering, it is
generally recommended to take into account
the specific features of the component or
LED such as the package form, lead size,
etc. when defining the process window.
In the following, Table 1 provides an
overview of the manual solderability of
various LED types from OSRAM OS as well
as their repair and rework capability.
Equally good results and reliable solder
connections can also be achieved and
created with lead-free solder.
Basically, manual soldering with lead-free
solder is not much more difficult than
soldering with lead-containing solder, so
long as work is performed with appropriate
equipment, qualified employees and the
fundamental ground rules are strictly
maintained.
LED TYPE
EXEMPLAR
IMAGES
MANUALLY
SOLDERABLE
REWORK
POSSIBLE
CAPABLE OF
REPAIR
Radial or
Through-hole
LED
3 mm ARGUS LED
4 mm OvalLED
3 & 5 mm LED
5 mm MultiLED
Ultra Flux



Miniature
Components
SmartLED
ChipLED



MiniTOPLED
PointLED



FIREFLY
FIREFLY RGB
Micro SIDELED
SIDELED
Multi Color Micro SIDELED
Multi SIDELED
MIDLED



Mini Top
Looker
Side Looker
 = possible
 =conditionally possible
 = not possible
Table 1a: Overview of manual solderability and rework/repair capability
September, 2015
Page 9 of 12
LED TYPE
EXEMPLAR
IMAGES
MANUALLY
REWORK
CAPABLE OF
SOLDERABLE
POSSIBLE
REPAIR
Top Looker
TOPLED
TOPLED with lens
Multi TOPLED
RG-TOPLED
RG-Multi TOPLED
6-lead MultiLED
6-lead ChipLED
STABLIX
DISPLIX
DURIS E3
DURIS S10
TOPLED Compact
MIDLED



Power LED
Power TOPLED
Power TOPLED with lens
Adv. Power TOPLED
Adv. Power TOPLED Plus




Ceramic LED
CERAMOS
CERAMOS Flash
Multi CERAMOS
CURAMOS
Ceramic LED
OSLON SX/MX/LX
OSLON SSL
OSLON Signal
OSLON Square
OSLON Compact
Ceramic LED
OSRAM OSTAR Compact
OSRAM OSTAR Lighting Plus
OSRAM OSTAR SMT
OSRAM OSTAR Stage
OSRAM OSTAR Medical
Epoxy SMD
(Bottom onlyterminated)
only with special
equipment and solder
paste

OSLUX Platform
High Power
LED
Dragon Platform


only with special
equipment and solder
paste


only with special
equipment and solder
paste

 =conditionally possible
Page 10 of 12
only with special
equipment and solder
paste

only with special
equipment and solder
paste

only with special
equipment and solder
paste

only with special
equipment and solder
paste









Heat slug not
possible
only with additional
heating plate
 = not possible
Table 1b: Overview of manual solderability and rework/repair capability
September, 2015


Heat slug only with
solder paste
 = possible

only with special
equipment and solder
paste
OSLON Black Series
OSLON Black Flat
TOPLED Compact 4014
DURIS P5
DURIS E5
DURIS S2
DURIS S5
DURIS S8
DISPLIX oval
OSRAM OSTAR Projection Cube
SYNIOS P2720
SYNIOS E4014
Flash LED

Appendix
Don't forget: LED Light for you is your place to be whenever you are looking
for information or worldwide partners for your LED Lighting project.
www.ledlightforyou.com
Revision History
Date
Revision History
Aug. 2007
March 2013
Sept. 2015
Publishing of application note
Update Table 1
Update Table 1, Change of Company Info & Disclaimer
Authors: Andreas Stich, Kurt-Jürgen Lang
ABOUT OSRAM OPTO SEMICONDUCTORS
OSRAM, Munich, Germany is one of the two leading light manufacturers in the world. Its
subsidiary, OSRAM Opto Semiconductors GmbH in Regensburg (Germany), offers its
customers solutions based on semiconductor technology for lighting, sensor and
visualization applications. OSRAM Opto Semiconductors has production sites in
Regensburg (Germany), Penang (Malaysia) and Wuxi (China). Its headquarters for North
America is in Sunnyvale (USA), and for Asia in Hong Kong. OSRAM Opto Semiconductors
also has sales offices throughout the world.
For more information go to www.osram-os.com.
September, 2015
Page 11 of 12
DISCLAIMER
PLEASE CAREFULLY READ THE BELOW TERMS AND CONDITIONS BEFORE USING THE
INFORMATION SHOWN HEREIN. IF YOU DO NOT AGREE WITH ANY OF THESE TERMS
AND CONDITIONS, DO NOT USE THE INFORMATION.
The information provided in this general information document was formulated using the utmost
care; however, it is provided by OSRAM Opto Semiconductors GmbH on an “as is” basis. Thus,
OSRAM Opto Semiconductors GmbH does not expressly or implicitly assume any warranty or
liability whatsoever in relation to this information, including – but not limited to – warranties for
correctness, completeness, marketability, fitness for any specific purpose, title, or noninfringement of rights. In no event shall OSRAM Opto Semiconductors GmbH be liable –
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September, 2015
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