Unusual Solder Appearance

We are observing an odd surface texture on solder joints after reflow. The reflow oven that we are using isa BT300NCP (MannCorp) and the solder is indium 3.2 SAC305 Type4. We havetried multiple types of profiles to attempt to alleviate but with no success.Do you know what could cause this effect?

D.A.

Expert Panel Responses

Surface effects with lead-free SAC305 can be normal. SAC305 isprone 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 causedby several factors listed below.

Thermalprofile used especially TAL, peak temperature and cool down rate.
Metalfinishes of the parts that were soldered, some dissolution occurs which canimpart color and frosty surfaces to the joints.
Reflowatmosphere if air or nitrogen, will impact the cosmetics of the solder surface.
Physical/chemicalproperties of the flux itself. Some fluxes react more with the solder surfacethan others.
Flowproperties of the flux during the reflow process, some fluxes flow away morerapidly than others, giving a longer exposure to oxygen.
Excessiveoxidation of the parts to be soldered may impact the surface also.
PowderType, oxidation of the powder, shelf life of the paste, how the paste washandled prior to use, all may impact solder surface.

Although the solder surface looks frosty, dull or has shrinkageeffects it may not necessarily indicate a reliability issue. The solder joint may be good with excellent intermetallicbonding. If in doubt pull/shear testing can be done and cross-sectional studiesto examine the bond layers.

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 appearanceof the solder joint is dependent upon many variables, therefore many questionsneed to be asked relative to the appearance of that particular solder joint. My firstthought is the solderability of the solder paste. Has this been conducted? Secondly

Doall the joints have this appearance?
Dosome of the joints have this appearance?
Isthere additional conductors tied to this particular connection causing a heatsinking effect?
Isthere any large components in the vicinity of this component which could impactthe air flow to the solder joint?
Ifthe thermal profile consistent across the entire board?
Isthis condition on the first side of the board to be soldered?
Isthis condition on the second side of the board to be soldered?

As can bededuced I'm leaning towards a thermal issue. The reflow time is too short andpoor reflow occurred, the molten time is too long, so alloy separation occurredwith the molten metal. Thirdly, myinvestigation would revolve around the plating on the board and contaminationor compatibility would be looked at, and then I would also look at the componentplating and its compatibility with the solder paste alloy. Sometimes it is notas simple as it looks and more complex than anticipated.

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'shard 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 liquidus 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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