Check out the IBM and Lucent NEPCON papers from the early 90's on Double
Reflow.
Regards,
George
George M. Wenger
Andrew Wireless Solutions
Senior Principal FMA / Reliability Engineer
40 Technology Drive, Warren, NJ 07059
(908) 546-4531 (office) (732) 309-8964 (cell)
[log in to unmask]
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Dale Ritzen
Sent: Tuesday, July 22, 2008 8:44 AM
To: [log in to unmask]
Subject: [TN] They solder well... But they don't work!
Importance: High
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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