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July 2008

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From:
Dale Ritzen <[log in to unmask]>
Reply To:
TechNet E-Mail Forum <[log in to unmask]>, Dale Ritzen <[log in to unmask]>
Date:
Tue, 22 Jul 2008 07:44:00 -0500
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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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