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February 20, 2013

Unusual Solder Appearance

Unusual Solder Appearance
We are observing an odd surface texture on solder joints after reflow. The reflow oven that we are using is a BT300NCP (MannCorp) and the solder is indium 3.2 SAC305 Type4.

We have tried multiple types of profiles to attempt to alleviate but with no success. Do you know what could cause this effect?


D.A.

Experts Comments

Surface effects with lead-free SAC305 can be normal. SAC305 is prone to micro-fissuring during cool down. Joints could look at times more reflective to dull, some frosty, some not. Often surface conditions are purely cosmetic and can be caused by several factors listed below.
  1. Thermal profile used especially TAL, peak temperature and cool down rate.
  2. Metal finishes of the parts that were soldered, some dissolution occurs which can impart color and frosty surfaces to the joints.
  3. Reflow atmosphere if air or nitrogen, will impact the cosmetics of the solder surface.
  4. Physical/chemical properties of the flux itself. Some fluxes react more with the solder surface than others.
  5. Flow properties of the flux during the reflow process, some fluxes flow away more rapidly than others, giving a longer exposure to oxygen.
  6. Excessive oxidation of the parts to be soldered may impact the surface also.
  7. Powder Type, oxidation of the powder, shelf life of the paste, how the paste was handled prior to use, all may impact solder surface.
Although the solder surface looks frosty, dull or has shrinkage effects it may not necessarily indicate a reliability issue. The solder joint may be good with excellent intermetallic bonding. If in doubt pull/shear testing can be done and cross-sectional studies to examine the bond layers.
image
Peter Biocca
Senior Market Development Engineer
Kester
Mr. Biocca was a chemist with many years experience in soldering technologies. He presented around the world in matters relating to process optimization and assembly. He was the author of many technical papers delivered globally. Mr. Biocca was a respected mentor in the electronics industry. He passed away in November, 2014.
The appearance of the solder joint is dependent upon many variables, therefore many questions need to be asked relative to the appearance of that particular solder joint.

My first thought is the solderability of the solder paste. Has this been conducted?  Secondly
  • Do all the joints have this appearance?
  • Do some of the joints have this appearance?
  • Is there additional conductors tied to this particular connection causing a heat sinking effect?
  • Is there any large components in the vicinity of this component which could impact the air flow to the solder joint?
  • If the thermal profile consistent across the entire board?
  • Is this condition on the first side of the board to be soldered?
  • Is this condition on the second side of the board to be soldered?
As can be deduced I'm leaning towards a thermal issue. The reflow time is too short and poor reflow occurred, the molten time is too long, so alloy separation occurred with the molten metal.

Thirdly, my investigation would revolve around the plating on the board and contamination or compatibility would be looked at, and then I would also look at the component plating and its compatibility with the solder paste alloy. Sometimes it is not as simple as it looks and more complex than anticipated.
image
Leo Lambert
Vice President, Technical Director
EPTAC Corporation
At EPTAC Corporation, Mr. Lambert oversees content of course offerings, IPC Certification programs and provides customers with expert consultation in electronics manufacturing, including RoHS/WEEE and lead free issues. Leo is also the IPC General Chairman for the Assembly/Joining Process Committee.
It's hard to say based on the photo. I see what appears to be two possible things occurring, Stress lines in the joint and component finish dissolution. I'll address the stress lines first.

Stress Lines
There are several things to check/compare. You can rule out the variables to hone in on the problem. Some process questions I'd pursue are: Does this only occur with one assembly, one joint / component finish or  one paste type? Is this common to only one board finish? Are there any surface contaminates on the board prior to pasting / reflow? Is this a No Clean flux left on the joint?  If so try removing the flux and inspect the joint finish. You may find that the joint is actually acceptable.

Some other possibilities are that you may be above lliquidous too long, cooling too fast or there may be board movement during the reflow process, while the joint is cooling. All of these can contribute to joint appearance and integrity.

Component Finish Dissolution
You may have a poor finish on your component termination. In some cases component terminations may either have minimal metalization needed to form a joint. Often the metal that is present will leach into the rest of the solder joint. This looks like what may be the case based on the color difference on the top of your component where there is less solder.

To ;rule out component finish problems you can start with solderability tests. In some cases the termination may disappear leaving only the ceramic, if this is a chip. If the part passes solderability tests then the part is probably not the problem.

If suspect then you can have the termination / plating evaluated at a lab which will tell you if there are problems with the base metal, contamination, composition etc. Once all variables have been ruled out you should be able to pinpoint the problem.
Robert Culpepper
Sr. Manufacturing Engineer
TransCore Amtech Technology Center
Mr. Culpepper has over 20 years in electronic manufacturing, primarily focusing on PWA assembly.
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