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TECHNET  July 2008

TECHNET July 2008

Subject:

Re: They solder well... But they don't work!

From:

"Kane, Amol (349)" <[log in to unmask]>

Reply-To:

TechNet E-Mail Forum <[log in to unmask]>, Kane, Amol (349)

Date:

Wed, 23 Jul 2008 08:08:18 -0400

Content-Type:

text/plain

Parts/Attachments:

Parts/Attachments

text/plain (134 lines)

What is the MSL on these parts? Have they been stored properly (MBB, desiccant and HIC?) We had a similar phenomenon happen here not too long ago and baking the parts prior to assembly improved the yield to over 95%

-Amol

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Guy Ramsey
Sent: Tuesday, July 22, 2008 10:06 AM
To: [log in to unmask]
Subject: Re: [TN] They solder well... But they don't work!

I had this experience once when I was QA Manager.

I walked into a techroom to observe a technician strum the leads of a QFP
with a dental pick. "Hear the difference?" This one will work fine now (He
was an accomplished musician and gave guitar lessons after hours). He
explained that he had just reflowed these with his solder pencil.

It turned out that there was nothing wrong with the soldering process. The
QFP was a logic array that needed to be programmed. The program did not have
a CRC. There was a timing problem. The programmer passed all parts, even if
there were errors in the download to the part.

Once we added a CRC we found that about half the parts failed to program
properly the first time. Reflowing the part and retrying while they were
warm from rework a much higher percentage accepted the program.

Fixing the clocking problem resulted in nearly 100% pass with both cool and
warm parts. It took many weeks of tossing the problem back and forth over
the wall from production to design to finally identify the root cause of the
problem . . . Bad code and a clocking problem.




-----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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