Steve,
The solderablity of the pad has little to do with voiding. The primary
factor of voiding in any surface mount joint is based on the
Flux-Exclusion-Rate Model first proposed by Wanda O'Hare and Dr.
Ning-Cheng Lee. They wrote an article featured in SMT/ January 1996
titled "How Voids Develop in BGA Solder Joints".
The general idea is based on Newtonian mechanics of "F=ma" where the
time required to push the flux out of the molten solder is dependent on
the opposing forces trying to hold it in. Basically, the more fluid the
flux, the less force required.
This indicates that as more oxides exist in the paste, on the pad and on
the leads, the more residue that will be generated. Flux residue is
much more viscous than the flux itself, thus it has a harder time being
driven out of the joint. Some no clean flux has less voiding because it
is formulated to have less residue on the joint. In addition, the more
effectively you can control oxide generation on soldering surfaces, the
fewer voids that will be seen. This is why a nitrogen reflow atmosphere
helps reduce voiding.
If you pry off BGA's that have big voids in them, and you are lucky
enough to separate the balls from the pads, (normally you will tear the
pads out of the epoxy) you can look inside the void with a high powered
microscope and see the residue. In addition, with accelerated aging, or
old boards, you will see black surrounding the void in the bulk solder.
Presumably the black is corrosion induced by the entrapped flux. Notice
however that a Motorola study found small voids did not degrade the
reliability of the joint.
However, the biggest contributor of voiding is the flux viscosity
itself. Low solids fluxes (i.e. no clean flux) will exhibit less
voiding. If the flux is dried out too early in the oven, it will become
very viscous and remain in the joint, causing a void.
We use a water soluble paste that has virtually eliminated BGA voiding.
It is because the flux is so fluid and has such low surface tension that
it nearly completely runs out of the paste before liquidous temperature
is reached. Thus there is very little "mass" (F=ma) to drive out of the
joint.
Thanks
Ryan Grant
Process Development Engineer
MCMS
(208) 898-1145
FAX (208) 898-2789
[log in to unmask]
>-----Original Message-----
>From: Joy, Stephen C [SMTP:[log in to unmask]]
>Sent: Saturday, September 25, 1999 4:44 PM
>To: [log in to unmask]
>Subject: [TN] BGA voids
>
>Technetters,
>How do BGA solder ball voids form in the assembly process?
>I understand that they are generally due to a pad that is unsolderable, but
>what is the mechanism?
>Is this flux residue that volatizes, impurities or something else?
>Thanks,
>Steve
>
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