Hi Sarah,
>1) Q: what is the difference between IL separation and post separation?
A: Post separation is the old, and now obsolete, term for innerlayer (IL)
separation.
>2) Q: what causes the occurrence of such kind of defect?
A: The compressive forces on the PTV barrel from the hydrostatic pressure
generated by the thermally expanding resin forces the PTV barrel to
plastically deform (buckle) inwards. On cooling, the now shrinking resin
retracts, causing tesile forces on the PTV barrel and the innerlayer
interconnects. This causes separation of the barrel from the resin and not
infrequently, IL separation. Il separation is aided by the land roation of the
innerlayer lands due to the movement of the expandingresin in the z-direction.
For this reason, IL separations are primarily found in layers 2, 3, N-1, and
N-2. The cause of IL separation is the same as for PTV barrel cracking; what
fails first depends on which is the weaker link in terms of both quality
(drilling, smear removal, etchback, plating thickness/foil thickness balance,
copper properties) and load distribution (hole size, PCB thickness, Tg, Tmax,
T-gradients)
>3) Q: how can we prevent this type of problem from occurring?
A: Limit the maximum temperatures experienced by the PCB (worst offender
processes are manual soldering/rework operations), limit the number of all
soldering thermal excursions (HASL, reflow, wave, manual soldering,
rework/repair), use high Tg resins, use E3 copper foil, avoid thin innerlayer
Cu foil with thick barrel plating (balance).
>4) Q: at what layer does this problem usually occur?
A: IL separations are primarily found in layers 2, 3, N-1, and N-2; PTV barrel
fractures near the center of the PCB.
>5) Q: how can you distiguish separation from resin smear?
A: With great difficulty, if at all. But resin smear only causes IL separation
at the foil/electroless Cu interface; IL separations can also occur at the
plated Cu/electroless Cu interface and in the IL foils near the IL
interconnects.
Werner Engelmaier
Engelmaier Associates, L.C.
Electronic Packaging, Interconnection and Reliability Consulting
7 Jasmine Run
Ormond Beach, FL 32174 USA
Phone: 904-437-8747, Fax: 904-437-8737
E-mail: [log in to unmask]
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