Hi, Werner,
Thankyou very much. I guess my intuition on these things isn't borne out in
practice. I would be very interested in the IPC workshop, but the
difficulty is persuading the folks here that the cost of sending me from
Singapore to the US for such a thing is not a waste of time and money. The
more "local" ones in China might be more possible.
Peter
Werner Engelmaier <[log in to unmask]> 03/10/2002 11:20 AM
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Please respond to "TechNet E-Mail Forum."; Please respond to Engelmaier
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cc: (bcc: DUNCAN Peter/Asst Prin Engr/ST Aero/ST Group)
Subject: Re: [TN] PCB design with mask over vias.
Hi Peter,
You write:
>Copper would be a better filler, [the barrel] would still be relatively
inflexible, though,
>in place of one or two mils of barrel plating.
>Expansion and contraction in [the z-]direction causes the relative (to a
given datum)
>positions of the internal layers to move up and down as the'yre carried
>with the board material. At the same time, the copper plating in the
>interconnecting holes has to "give" to accommodate this and minimise the
>displacement between the layer in the board and the point to which it
>connects to the barrel.
>If the "give" (ductility) is constrained by the hole being full of solder,
>the displacement between a layer and its connection point to the barrel
>increases, increasing the risk of shearing the joint between barrel and
>layer .... unless a solder-filled hole has a LCTE in the Z direction more
>closely matched to that of the board material than does a plated barrel
>alone (?).
You have an inaccurate mental picture of what is physically happening.
(1) the ductility of the copper barrel is independent of plating thickness
or
the nature of any possible filler;
(2) the reason thicker copper plating produces more reliable PTV barrels
lies in the fact that thicker copper barrel walls are more able to resist
the
deformation, "give," the expanding resin tries to impose;
(3) thus any filler that helps the barrel resist deformation will increase
reliability--the problem is that the modulus of elasticity of solder fill
is
too low to be much help;
(4) there has never been an observed " shearing [of] the joint between
barrel
and layer"--the cause of inner-layer separation (ILS) is not shearing, but
tension due to the compression of the PTV barrel[-tank];
(5) that is why ILS is observed on larger-diameter PTVs, and barrel
cracking
on smaller diameter PTVs.
Perhaps you would be interested in my workshop on PTV reliability at
IPC-EXPO
in Long Beach. I am also trying to arrange such a workshop in Beijing or
Shanghai in April and/or Shenzhen in May of 2003.
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], Website: www.engelmaier.com
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