Hugh Scott Miller asked:

>I have a question relative to solder balls and hope that someone can
>shed some light on this issue.
>
>1.  What causes solder balls?

Many possible causes, but one which is pretty common:

Larger rectangular parts will displace paste when placed, and the paste
under the "belly" of the chip tends to form a ball at reflow.

There is a very interesting article by Matthew Pak in the March issue of
"Circuits Assembly" magazine, p. 86.

>2.  Could solder balls be the result of incompatiability between flux
>and soldermask?

Yes, there have been known cases where improperly cured mask relates to
solder ball formation, especially the wave solder variety. You might get
suspicious if you just changed PWB suppliers, or they just changed mask
types.

>3.  How do you eliminate solder balls?

Identify all the potential causes, rank them by likelihood and impact, then
deal with them one-by-one.

Check your reflow profiles. I have also heard that reducing the pot temp of
a wave solder pot to like 475 F is useful. But don't change 3-4 things at
once and miss the real cause.

>4.  What is the IPC acceptability standards for solder balls?

IPC-A-610, Rev. B Acceptability of Electronic Assemblies, paragraph 4.1
specifies that solder balls greater than 0.13 mm (0.005") is a
non-conforming defect for class 2. More than 5 solder balls < 0.13 mm per
600 mm2 surface area is also a defect. Most of your Bellcore type criteria
say that the max diamter ball can't be larger than 50% of the minimum
lead/pad spacing.


good luck,


Jerry Cupples
Interphase Corporation
Dallas, TX USA
http://www.iphase.com/

data is not information is not knowledge is not wisdom
so just give me the straight wisdom and save all that other spam.

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