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February 2000

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From:
Brian Ellis <[log in to unmask]>
Reply To:
TechNet E-Mail Forum.
Date:
Fri, 25 Feb 2000 17:41:19 +0200
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Lou

1. Solder balls are formed by a number of mechanisms. The two main mechanisms are 1) solder paste reflowing into discrete particles, rather than agglomerating into a mass, so that little "islands" are left behind and 2) sputtering caused by flux volatiles "exploding" when passing through the solder wave.. I agree with Bob Willis that they are very common. In both cases, they may be reduced to reasonable quantities by fine-tuning the solder process. The mask quality is a key issue, but not necessarily an absolute one.

A mechanism which your production manager may be thinking of with wave soldering is the "explosion" of blow holes projecting solder. This is known but relatively rare, especially if the boards are baked immediately before assembly - an evil process but, unfortunately, an often necessary one.

2. The only reasonable production process control is one of the various ionic contamination testers (there are more than the two you mention, both made by the same company: I suggest you research this before making up your mind because some of the others have valuable features not present in those cited). If you want more details, please e-mail me privately. As for the frequency, this will depend, to some extent, on the stability of your processes. I would recommend hourly tests to start with, until you are satisfied that the results are consistent, when you could start reducing the frequency to a minimum of twice per day.

I am surprised that you use water-soluble for all your processes bar wave soldering. It would make good, economic sense to use water-soluble for this, as well. You would have the advantage of a wider operating window, fewer retouches, greater tolerances towards poorly solderable components and you could also wash the lot in just DI water (possibly with some cleaning aids, but much cheaper than anything used to clean off rosin flux). Furthermore, if you don't use a rosin flux, you would not need to test for it, would you?

Brian

Lou Hart wrote:

> Hi Technetters,
>
> (1)  Solder balls:  I feel a little sheepish to bring up a subject that has been discussed many times before, I'm sure, but let me get an opinion.  Our production manager thinks solder balls on assemblies come from holes in the boards (always FR4) that got wet during cleaning after surface mount reflow and were not dried before wave solder.  He says the vaporizing water blows solder around the board, leading to balls.  Searching through the TechNet archives, I found a posting from Bob Willis in Jan 96 where he says "(solder balls) are always present in some from.  The mask is the key issue, if they can not stick to the mask the problem is solved".  Can I get your comment(s).
>
> (2)  Assembly cleanliness:  How do people assess assembly cleanliness after manufacture?  We use several kinds of flux at different stages of operation here.  Surface mount uses water-based, rework and touchup use organic acid, wave solder uses RMA.  Our cleaner has a formulation supposed to get rid of RMA, but everything goes through it.  What cleanliness measures are or should be used after these cleanings?  I'm most concerned about the final assembly, but am curious about the intermediate stages as well.  The IPC test methods manual has some tests for ionic contamination (Omegameter and Ionograph seems most feasable for production) and some for rosin residue (that seem to require some expensive equipment and take a while).  Questions I'm asking myself are:  Should we be checking both ionic contamination and rosin residue?   And, of course, with what frequency(ies)?
>
> Thanks for any comments.
>
> Lou Hart
> Quality Assurance Manager
> Compunetix
> 412-858-6184
>
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