Hi Ken,
>(1)Can any one provide me the possible causes for the defects?
The loading conditions for the innerlayer separation come from the excursions
to soldering temperatures. As I previously wrote on TechNet:
The reliability concern with any of this is basically this:
(1) At soldering temperatures the epoxy is way above its Tg, is very soft,
and should be considered a high-viscosity liquid as far as behavior is
concerned.
(2) For analysis as to what is going on around a PTH, the PWB can be
considered an infinite plane in the x- and y-directions.
(3) While the epoxy tries to expand in all directions, it is constrained in
the x,y-directions by the glass reinforcements.
(4) Thus, the primary expansion is in the z-direction at roughly 3 times the
linear thermal expansion, because all the volumetric expansion is funneled
into the z-direction.
(5) The PTH hole, however, allows the epoxy to also expand into the space
occupied by the PTH if the PTH barrel wall is thin enough to by compressed.
That is why in cross-sections of PTHs with 1 mil of copper plating and less
one sees the PTH barrel bowed inward between innerlayer lands, particularly
in prepreg layers (higher resin content, less cure) and when there are larger
distances between lands (no non-functional lands), and sometimes a space
between the barrel wall and the receded epoxy. If there were no copper
plating, the hole diameter would decrease on heating.
(6) Thus, the PTH barrel in tension in the z-direction, with the largest
stresses near the PWB center, the PTH barrel experiences compressive hoop
stresses, innerlayer lands experience tensile stresses at their attachment to
the PTH barrel, and innerlayer lands near the PWB surface experience bending
moments due to land rotation resulting from the z-direction expansion of the
epoxy surrounding the PTH barrel.
(7) Thus, the higher the temperatures reached the worse the reliability
threat. I have heard of hand soldering operations performed with irons in
excess of 410C! Depending of the polymer glass transition temperature, that
might be a delta-T of 285C above Tg.
In your case, resin smear makes the interface to the inner layer copper foil
weak, causing separations at less severe loading conditions.
>(2)How much is a variation for this kind of defects within a same date code?
Resin smear, assuming your process is just out of spec, will occur to some
degree within all codes made prior to an adjustment to your processing.
>(3)What about the reliability of my assembled boards?
The depends on the severity of the application your PCBs will experience.
While the failure probability for your PCBs will be higher than it could be,
you may not experience any failures in benign applications within the life of
the product. More severe operating conditions however, are likely to
propagate partial inner layer separations to completion, and thus failure.
While I am not aware of a book that I could recommend to you in this context,
I give a workshop "Design, Manufacturing and Reliability Issues of
Small-Diameter/ High-Aspect-Ratio Plated-Through-Holes and Vias" that deals
with the underlying issues; also PC-D-279 "Design Guidelines for Reliable
Surface Mount Technology Printed Board Assemblies" would be helpful.
Werner Engelmaier
Engelmaier Associates, Inc.
Electronic Packaging, Interconnection and Reliability Consulting
23 Gunther Street
Mendham, NJ 07945 USA
Phone & Fax: 973-543-2747
E-mail: [log in to unmask]
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