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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