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Grainy SMT Solder Joint



Grainy SMT Solder Joint | 10 April, 2001

What's wrong if my solder joint is grainy ? what will be the effect on its strength ? what process problem is causing that ? regards

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Steve Brown


Grainy SMT Solder Joint | 10 April, 2001

One of the reasons for a grainy joint is the cooling of the joint after reflow. Check your reflow profile, not just the preheating and reflow section but also the cooling zones. It works the same way as molten rock. If it cools slowly you get pumice stone, if it cools quickly you get a volcanic glass.

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Grainy SMT Solder Joint | 10 April, 2001

You've told us nothing about your situation, process, boards, or anything!!!

If you leave some boards with very fine solder connections on the shelf for a cuppla years, the grain will become coarse.

What do you mean when you say "grainy"? What is the grain size, measured after sectioning?

"Not shiney" is a process indicator. I am an advocate of NOT using visual appearance to define a good and bad solder joint. [The number of hours spent by the specification community trying to define what a "disturbed or grainy" solder joint is would/should make all of us disturbed!] Two dull solder connection conditions are:

1) "Disturbed" solder joint: a solder joint that has an "angular, faceted" appearance that is caused by the solder joint being moved as it solidified.

2) "Grainy" solder joint: a solder joint that has a rough, gritty appearance that is caused by the solder microstructure giving the solder joint surface texture.

If you think you have a "disturbed" situation, look for equipment, handling, or fixture causes for the movement.

If you have a "grainy" situation, options are: * Try to find a way to cool the solder joint faster. * Don't get it so hot in the first place. This produces a finer solder joint microstructure without the fine solder joint surface texture. * Determine if materials added to solder are changing its appearance. For instance: Both gold and palladium in tin/lead solder can make connections look dull / grainy. * Consider that DI water can make solder connections look dull. * Remove crud [in barrels or on pads] that is floating to the top on the solder.

Use cross section analysis to prove that "graininess" is not a reliability issue. We did some extensive work in the 80's showing that a "grainy" solder joint is not a bad solder joint - only different, reflecting some process or design parameter influence.

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Michael Parker


Grainy SMT Solder Joint | 10 April, 2001

since you did not define what visual process you are inspecting with I will ask is this "grainy" appearance being detected with the naked eye only? Are you using magnification? if so, what power?

Is this a SMT solder or through hole solder problem?

A grainy look, with the naked eye only, is far too vague a description for us to understand your problem.

Using a microscope at 10X power, do you see "grainy" or has the surface imperfection changed?

For SMT solder, one common example of grainy to the eye that changes to "bumpy" under magnification would be if your solder paste has not reflowed completely, therefore the solder spheres have retained their shape. Possible causes of this kind of problem are: not enough heat during reflow (the solder did not melt completely), the flux in the paste dried out before reflow (you left the boards exposed to the air after paste print for many hours)

Please try to describe in detail what was done in the process before this defect was detected.

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