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July 2008

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From:
Liang Yin <[log in to unmask]>
Reply To:
TechNet E-Mail Forum <[log in to unmask]>, Liang Yin <[log in to unmask]>
Date:
Tue, 22 Jul 2008 09:06:52 -0400
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Could it be a typical 'black pad' issue with ENIG? Hand soldering might introduce local warpage of the board, which could open up weak joints. 

A 'dye and pry' test should tell you the location of the failed pads, and the correlation with the location of hand soldering.

Liang Yin
Process Research Engineer
Unovis Solutions



> Date: Tue, 22 Jul 2008 07:44:00 -0500
> From: [log in to unmask]
> Subject: [TN] They solder well... But they don't work!
> To: [log in to unmask]
>
> Here's a good one for the collective 'mind-meld'...
>
> We have a two-sided, non-RoHS compliant, all-SMT assembly (no PTH parts)
> that is reflowed under a leaded profile (cooler reflow temperatures than
> Pb-free). There is one 44-pin QFP on the topside of the board that
> consistently solders well (i.e. passes all IPC-A-610 Class 2 inspection
> criteria), but fails test until it is reflowed with a hand iron, a little
> flux and wire solder. We have about a 25% pass rate without reflowing the
> QFP. That jumps to around 95% once the QFP is reflowed. Several different
> date codes of the device have been tried with similar results.
>
> Thinking the ENIG pads under the device leads might be contaminated, we
> cleaned them on several test case 4-up panels, with no change in yield.
> Thinking it might be contaminated leads on the devices, our process guys
> gently scraped off the tinned coating on the leads of several devices and
> took it down to the Beryllium Copper finish. Same resultant yield.
>
> We've re-run the failing assemblies through a higher reflow temperature with
> extra 'tacky flux' on them, trying to get a better reflow and use the action
> of the flux to purge any contamination that might be interfering in the
> solder junction. When done, the solder junctions on the device look like a
> 'Target - Class 1,2,3' picture from IPC-A-610, but the yield is still around
> 25% unless we reflow the junctions with an iron, some flux and wire solder.
>
> There may be some things we're overlooking, but it's coming down to where we
> believe the wire bonds to the lead frame material could be the source of the
> problem. The intense, direct heat of the soldering iron might be enough to
> generate a reflow of the wire bond, resolving any bonding issues. But, on
> several different date codes? I will be getting the vendor involved shortly
> to test the devices and see if they come up with something. I'm fairly
> skeptical about doing this as typically the normal answer I get is 'all is
> working as designed'.
>
> We are all scratching our heads on this one... Any 'words from the wise'
> would be appreciated!
>
> Thanks!
> Dale Ritzen, CQA
> Quality Manager/ISO Management Representative
> Austin Manufacturing Services
> Email: [log in to unmask]
>
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