Mr. Engelmaier, (and TechNet Forum) Below, you touch briefly on the effect of non-functional pads (and their absence) on plated through holes. Has any one documented the effect of non-functional pads, or their absence, on plated through hole reliability? (Sometimes our FAB vendors have suggested eliminating non-functional pads to improve innerlayer processing / yields) I would value your input. Best Regards, Ken Devitt ITT Avionics Clifton, New Jersey 973-284-3880 Fax 973-284-3736 >---------- >From: [log in to unmask][SMTP:[log in to unmask]] >Reply To: TechNet Mail Forum. >Sent: Wednesday, August 27, 1997 8:30 PM >To: [log in to unmask] >Subject: Re: [TECHNET] [T] Annular Ring Limits > >Hi Les, >I don't know about the 'Ring and Ball' experiment, but I do know what happens >inside a PWB around the PTHs during heating to solder temperatures. >(1) At those 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 themal 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-funcional lands), and sometimes a space >between the barrel wall and the reseded epoxy. If there were no copper >plating, the hole diameter would decrease on heating. >(6) Thus, the PTH barrel in 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. > >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] >