I think if you saw my comments about this subject you will notice
I spoke about the connections opening after solder. Looking back at
what you said, the connections to the via were fine, but the Molex
connections (soldered) were open. I think you may find, they aren't
particularly open (they may be) but at a very high impedance.

The heat tends to break the connection, boil the resin in the hole
wall causing additional insulation between the hole wall and the
internal trace.

This too may have been avoided with the specification of a low
trace resistance during continuity test and a 100V during the
shorting test. The 100V test should go first.

Chris Robertson
[log in to unmask]
  ----- Original Message -----
  From: Lum Wee Mei
  To: [log in to unmask]
  Sent: Monday, December 15, 2003 6:04 PM
  Subject: [TN] Problem With PCB Opens - Another case study


  Hello there!

  SInce we are at this topic of open circuit on PCB, I like to bring up
  one more case study.

  I have some assembled boards, tested functioning and went through ESS.
  They are put aside and after some times, the engineer wanted to donwload
  a new firmware through the molex pins on the board. Oop, the software
  cannot get through. He take the meter and measure the connections
  between the molex pins to the IC chip and found them open. He touched up
  the solder joints at this molex connector and found it open. Then later,
  the connections are found to be opened again. All the connections are in
  the inner layers - 3 layers from top. Soldering of the molex connector
  is from the bottom - 8th layer. Other molex connectors along the same
  axis have not problem including the same connectors on other boards.

  What has been done : we take measurement on the connections. There are
  connections from the IC chip to the vias but found open circuits from
  vias to molex connector. It is surprise to find all the 5 connections
  opened while the ground and supply connections are intact. The signal
  traces are of 5 -6 mils.

  What could be the root cause of such defect? Anyone want to try? Can
  X-ray surface the open circuit?

  Note : No cross-section is allow on this assembled faulty board as it
  will be used.

  Regards,
  Wee Mei

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