Mumtaz,
The key indicator in your photo is the shape of the copper in the
via. That's a classic indicator of electrolytic Cu plating up to a skip in
electroless.
This defect probably was a latent defect that had some initial
continuity. That allowed it to escape into your assembly process.
This type latent defect escapes ET at the fab and opens up at the assembler
after seeing reflow temp.
You'd be well served to find out what this lot's ET fallout for opens was
at the flex shop. There very likely was some fallout for this defect at
your supplier's ET.
fwiw, we won't qual a supplier until they have a documented response plan
for opens detected at ET. We wrote that requirement in blood and etched it
in scar tissue on my back (and your friend/colleague Van Nguyen's). ;^)
Your supplier needs to verify whether any opens at their ET are voids or
not and then follow a documented MRB response plan for voids. In our qual
requirement, any lot w/ a void detected at the supplier ET must be
contained and go through a documented decision tree and reaction plan prior
to shipment (classifying type of void, lot risk of void type, MRB
disposition decision tree, yadda, yadda). When we see a void failure at
our site we then go back and audit the supplier process. We check on ET
results and look for evidence that their void response process was properly
exercised.
It's surprising how focused suppliers get at driving voids out of their
process when they aren't allowed to ship suspect product.
cheers,
Dwight Mattix
Sr Staff QE
At 12:18 PM 11/30/2005, paul reid wrote:
>We have done performing some reliability testing and failure analysis on
>flex circuits lately and this appears to be a typical failure mode for PTHs.
>It does not strike me as a classic void (unless this is a wedge void). The
>pictures seem to show copper that is cracked rather than copper that is
>missing. I would classify this failure as a barrel crack (as apposed to
>metal fatigue). This failure is likely to isolated areas of holes that have
>thin copper plating.
>
>I believe the hole qualify in these pictures would be classified as more or
>less average. The hole roughness may have smear removal as a contributing
>factor. We have seen electroless copper having a problem in covering
>adequately in deep fissures that can occurred in the hole wall of flex
>circuits. The electroless and electrolytic copper in these fissures can be
>thin and prone to break. The amount of stress that is exerted by the
>polyimide and adhesive system is minimal so marginal copper plating
>thicknesses often go undetected for awhile.
>
>Here is an indicator: If this is a circumferential void then it should be
>found at electrical test of the bare flex. If it is a crack developing due
>to the thermal excursions of assembly and rework, it should be found after
>assembly.
>
>Sincerely,
>
>Paul Reid
>
>
>Program Coordinator
>PWB Interconnect Solutions Inc.
>
>Tel: 613-596-4244 Ext. 229
>Fax: 613-596-2200
>
>
>-----Original Message-----
>From: Mumtaz Bora [mailto:[log in to unmask]]
>Sent: Monday, November 28, 2005 11:02 PM
>To: [log in to unmask]
>Subject: [TN] Via Cracking in flex cable
>
>
>Dear All,
>
>We have an issue with flex cables cracking in cellphone
>assembly. X-sections are showing voids, some residue and cracking in the
>via. It appears to be a combination of smear remove and copper plating
>problem. Any insight from the flex circuit and plating experts will be
>appreciated. I have asked Stephen Gregory to post the X-sections on his
>website.
>Thank-you
>
>Mumtaz
>
> D/ 252 - SMT Process and Component Quality
> Bldg. V238E
>Voice (858)-882-1967
>Fax (858)-882-3126
>Page (858)-635-1180
>Cell: (858) -449-7054
>email: [log in to unmask]
>Kyocera-Wireless Corp.
>10300, Campus Point Drive
>San Diego, Ca 92121,USA
>
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