Lead-Free Defects in Reflow Soldering - How to Prevent Them
Feb 17, 2005
The IPC-610D Acceptability of Electronic Assemblies will shortly be available
and Section 5. Soldering will include information on acceptable and defective
lead-free solder joints. Lead-free SMT can be challenging, however this challenge
can be reduced if the basic changes associated with this transition are clearly
understood. Both paste selection and the reflow process will need to be
Tin-Silver-Copper alloys are the primary choice for lead-free SMT assembly.
Although there are other options available such as alloys containing bismuth
or indium and other elements, tin-silver-copper solders, also known as SAC alloys
are by far the most popular. They are used by approximately 65% of users, as
last surveyed by Soldertec, and their use is on the increase.
The lead-free SMT process differs from a 63/37 process in numerous ways.
A good understanding of these differences when using SAC alloys, will enable
process engineers to bring about the necessary changes to the SMT process and
reduce soldering defects, increase lead-free assembly reliability and maintain
Often when a manufacturer transitions to lead-free soldering an increase
in defects is noticed, in other cases a reduction in production yields. This
is too expensive in the market we are in these days. These negatives are often
the result of not properly implementing an appropriate process. Soldering defects are good
process indicators, and addressing them at the on-start will reduce the overall
costs of production.
A well-defined, optimized and controlled lead-free process will not
augment defect rates.
The main differences between a leaded and lead-free SMT process are summarized
- Solder physical properties, melting point, surface tension, oxidation
potential, metallurgy and metal leaching potential
- Higher peak temperatures
- Higher preheat temperatures
- Lead-free finishes for boards and components (preferred)
- Solder cosmetics and surface effects
- Solderability differences such as speed of wetting and spread
- Less self-centering or alignment of components
The liquidus temperature of SAC alloys is 217-220 Â°C; this is about 34Â°C
above the melting point of eutectic 63/37. This higher melting range requires
peak temperatures to achieve wetting and wicking to be in the range of 235-245Â°C.
Lower peak temperatures can be used with SAC solders such as 229Â°C. This lower
peak temperature often can only be used for boards with lower overall thermal
masses or assemblies, which do not have a large thermal mass differential across
the board. This lower peak temperature may also require extended times above
liquidus (TAL). If the solderability of the component or board is poor, this
lower temperature will also translate itself into poor wicking of solder and
Higher reflow profile temperatures will require the use of new solder paste
flux chemistries. Solder paste flux accounts for nearly 50% of the solder paste
Its ingredients characterize the paste's rheological properties, its ability
to print, avoidance of cold and hot slump, tack life, stencil life and abandon
time. The flux system is also the main contributor in void occurrence.
As the preheat is engaged during reflow, the flux system will prevent hot
slump, reduce the oxidation potential of the metals to be joined, deoxidize
the solder powder and remove oxides of the metals to be joined. The flux system
insures an oxide free and therefore solderable surface as to give the lowest
surface energies to enable spread and wicking of solder.
After reflow is complete the flux system must be easily removed in water
if it is a water washable paste or remain benign if it is a no-clean type paste.
With some no-clean solder pastes the residue must not undergo complete polymerization
as to remain pin-probeable.
The basic ingredients in a solder paste flux can be summarized as indicated
- Resins solid and liquid types
- Activators, organic acid and/or hydrohalides
- Solvents and co-solvents
- Gelling agents
- Chelating agents
Solder paste manufacturers have had to revisit most of these ingredients
to account for the higher temperatures experienced in the reflow operation.
Most of these ingredients are organic compounds and thermal stability up to
245Â°C is essential to avoid issues of decomposition, oxidation, charring and
polymerization of paste flux during reflow.
Lead-free solder pastes designed for lead-free alloys and also alloy specific
will function best and help prevent solder defects.
Typical defects associated with lead-free reflow soldering are:
Bridging, Solder balls and Mid-chip Balling
The first three defects
bridging, solder balls and mid-chip balling can arise from the solder paste
selection process. Since preheats are higher with lead-free, the hot slump character
of the paste is critical; solder pastes with good hot slump at higher temperatures
such as 185Â°C are needed. Traditional 63/37 pastes have already melted and flowed
at these temperatures; the gelling materials have broken down.
The example below, demonstrates this quite well, two no-clean lead-free
SAC, Type III powder solder pastes are shown.
The difference in chemistry is evident in their slump performance.
Both pastes were run through a reflow oven at 180Â°C. Paste B has better hot
slump properties than Paste A and would less likely cause bridges, solder balls
or mid-chip balling. For fine pitch components it is critical to select a lead-free
paste with a heat stable gelling agent, as indicated in Paste B.
Poor wetting of terminations and pads
Non-wetting or insufficient
wetting is also encountered. It must be understood that different metallization
will exhibit differing spread and wicking characteristics and also flux activity
will play an important role. Lead-free SAC alloys during solderability testing
using wetting balance instruments demonstrated the best wetting when water
washable flux systems were used. No-clean flux systems containing less activator
and/or free of halides demonstrated lower wetting speeds and lower maximum force
Bare copper OSP boards, which have seen more than one thermal cycle, are
prone to incomplete pad wetting. While pure tin, silver immersion finishes exhibit
better solder spread. Ni/Au if the nickel is not affected with impurities or
oxides will normally solder well. Below are two examples, one with SAC alloy
on copper and the other on silver immersion; both QFP's were reflowed in air,
using a SAC no-clean paste ROLO type flux. This type of paste is free of halides
as described by the IPC-STD.
Poor solderability, insufficient wetting, poor wicking of solder, and large
contact angles can also result from an inadequate thermal profile. It is very
important to achieve good thermal equilibrium across the whole board, this becomes
more important with lead-free since the peak temperature window is narrower.
SAC alloys melts at 217Â°C while the peak temperature needs to be in the range
of 235-245Â°C. Here the difference is under 20Â°C. In 63/37 systems the difference
is closer to 30Â°C.
If BGA's are present on the lead-free assembly, these components act as heat
sinks, the solder paste may not completely reflow under the BGA, while other
smaller components may show good soldering. It
becomes very important to establish good thermal profiling points across the
board, including under BGA's. To properly insure wetting has occurred completely,
optical inspection or X-ray inspection may be necessary.
A test board is essential for the first lead-free assembly to insure thermal
requirements are met across the board. The diagram to the left, shows the proper
way to measure the heat applied to the balls in the grid array.
The photo on the left shows balls, which have not undergone reflow due to
insufficient heat. By measuring the temperature accurately at the ball site,
this can be avoided. The temperature at the ball site had not seen 217Â°C the
melting point of SAC balls.
The photo in the center shows what happens when excessive temperature is
seen by the BGA, in this case the temperature was measured at about 265Â°C at
the ball site.
The photo on the right shows the proper collapse of lead-free balls with
the thermal profile properly set. The standoff distance may be higher with lead-free
SAC due to its higher surface tension.
There are other reasons why lead-free reflow demonstrates poor wetting and
the main causes are summarized below:
- Solder paste activity level is
- Excessive preheat temperatures
- Too long a preheat
- Difficult to solder finishes
- Insufficient time above liquidus
- Excessive oxidation of parts to
Lead-free solder pastes require activation to be sustained beyond traditional
tin-lead systems up to 217Â°C and beyond for SAC alloys. Like traditional 63/37
no-clean pastes, such as ROLO types, the prevention of oxidation to parts and
boards is critical. Flux classifications such as ROM1 may contain halides and
are therefore better able to cope with oxides or difficult to solder parts.
Tin-Silver-Copper solders wet most metal surfaces more slowly and adequate
times above the melting point of the solder is needed to achieve good wicking
and solder spread. Normally the range is 60-90 seconds with peak temperatures
If soldering is jeopardized by oxidation of parts to be soldered, this can
be verified using solderability test methods such as the wetting balance test.
Voids in lead-free joints and BGA's
Much has been written about
void prevention when soldering with lead-free solder pastes containing tin-silver-copper.
Excessive solder voids can create a reliability issue especially in applications
where the lead-free assembly will be exposed to thermal cycling conditions or
in applications where the assembly will be exposed to vibration, or flexing
during box builds. Also voids can reduce thermal performance and reduce electrical
It must also be stated that smaller voids can in cases increase reliability
by changing the crack pattern. Studies have shown that there is no reduction
in reliability when voids are present to up to 25% by volume in the joint. Voids
can act as stress relievers, due in part to the compressive nature of air pockets.
This is documented in the technical paper, Voiding: Occurrence and Reliability
Issues with Lead-free, by Martin Wickham of the National Physical Laboratory.
Some causes of voids in joints are summarized below:
- Solder paste chemistry
- Solder surface tension effects
- Thermal profile
- Oxidation of the outer surface
of solder joints
- Termination geometries, joint
- Metallization of finishes for
boards and components
- Component board out-gassing during
Lead-free alloys such as SAC alloys have slight higher surface tensions when
compared to 63/37. It is important to select a solder paste which has a flux
chemistry designed for the higher preheats and peak temperatures. Choosing a
solder paste, which does not contain resins and activators which decompose at
these higher temperatures, is the primary factor in void reduction. Good
solder paste manufacturers are designing flux systems for lead-free alloys.
The voiding potential information is often available for use during the paste
Optimizing the reflow profile as to remove any volatiles by extending the
preheat times and increasing the time above liquidus will also help in reducing
void entrapment. Insuring components and boards are free of moisture and plating
contaminants will also help to reduce voids. It has been shown that copper OSP
tends to produce slightly higher volume of voids when compared to Ni/Au and
silver immersion, which produce much less.
In some cases joint geometries are contributors. Components such as leadless
chip carries or large flat surfaces, perpendicular to the board will prevent
out-gassing during the soldering process; this results in void increases. Solder
flux by-products both liquid and gases, will have to slowly make their way upwards. Component
geometries, which prevent the proper upward flow, will usually result in an
increase in voids.
Tombstoning defects with lead-free
Lead-free may increase the
uplifting of smaller components. This is due in part to the reduced wetting
behavior of lead-free alloys. Component placement is more important with lead-free
alloys since less centering will occur during reflow. This can increase the
incidence of tombstones.
SAC305 tends to reduce tombstones, this alloy has a concentration of 96.5
Tin, 3.0 Silver and 0.5 Copper and has melting range of 217-220Â°C. Because
of the small pasty range the component prone to tombstone is tacked by the initial
melting phase of the alloy.
A solder paste, which exhibits excessive out-gassing during the initial stages
of the melting of the solder powder, will also increase tombstone defects. The
paste manufacturer must carefully choose resins and solvents, which do not decompose
or vaporize at the melting point of the alloy.
De-wetting with lead-free
De-wetting is often due to a lack of
flux activity. This behavior rarely occurs with water-washable type pastes since
these pastes are highly activated. Lower activity solder pastes in the category
of ROLO, halide free no-cleans pastes tend to create this on more difficult
finishes such bare copper OSP or on Ni-Au where the nickel base metal, may have
experienced oxidation or plating contamination.
Below are test coupons on which SAC no-clean paste was applied to two surfaces.
The test coupons were then reflowed in air using the manufacturer's recommended
thermal profile. The one on the right shows de-wetting while the one on the
right exhibits good wetting. The pooling of the solder was due to the base metal
being difficult to solder to. The molten solder initially spread across the surface
but not a good enough intermetallic bond was formed, resulting in surface tension
pulling the solder away.
Ways to reduce or prevent de-wetting with lead-free SMT are:
- Select a paste with excellent
activity up to the melting point of the alloy, up to 217Â°C for SAC alloys
- Use a more active flux system
- Insure metals to be joined are
oxide-free as possible
- Insure base metals are solderable
with the selected flux type
- Reduce the preheat time or temperatures
as to preserve flux activity
- Increase time above liquidus (217
Â°C), if flux activity is good
Excessive dullness and surface effects with lead-free
SAC alloys offer solder joints which are less reflective than 63/37; the
contact angles tend to also be higher and spread is less. These are not
considered defects but only cosmetic.
If air reflow is used, SAC joints will be less bright and show surface effects
such as crazing which are due to the intermetallics within the solder and oxidation
If nitrogen reflow is used the joints will be more reflective and spread
will be enhanced.
Below are two photos. The one on the left is 63/37, while the other shows
joints done with SAC305 alloy.
Lower peak temperatures and lower times above liquidus will reduce both intermetallic
growth but also increase the overall brightness of the solder joints.
Proper training will be required when transitioning to lead-free assembly.
Operators will need to be given quality acceptance criteria for solder joints
that will look quite different from traditional leaded systems.
About the author:
Peter Biocca is Senior Market Development Engineer with Kester in Des Plaines,
Illinois. He is a chemist with 22 years experience in soldering technologies.
He has presented around the world in matters relating to process optimization
He has been working with lead-free for over 7 years. He has been involved
in numerous consortia within this time and has assisted many companies implement
lead-free successfully. He is an active member of IPC, SMTA, and ASM.
He is the author of many technical papers delivered globally. He is a Certified
SMT Process Engineer.
further information please contact Peter Biocca at Kester, telephone
972.390.1197 or via email at email@example.com
OK International, Photos of BGA Optical Inspection and Cosmetic Joint Comparisons.
Bob Willis U.K., Photos BGA with SAC alloy.
Gintic Manufacturing Consortium, Singapore. Lead-free Report.
Kester Des Plaines, Illinois, Applications Laboratory, Photos Paste Slump,
Voiding: Occurrence and Reliability Issues with Lead-free, Martin Wickham,
National Physical Laboratory, U.K.
Lead-free Electronics, 2004 Edition, Sanka Ganesan; Michael Pecht, Calce