Gentlemen,

All you have said is fine concerning PTH type rework situations wherein relatively normal
temperatures are seen in the process. My concern is for SMT board types being subjected to long
periods, under the automatic rework machine knife, of extended temperature cycling.

With boards whith as many layers and as thick as those indicated, very high top heater settings are
required to provide adequate reflow for BGA devices, as an example. On a recent 16 layer board,
.100" thick, a top heater temperature exceeding 295 degrees C for 55 seconds (providing a
corresponding solder joint corner TC reading temperature meeting recommended solder paste reflow
profiles - within a 183 C. window not exceeding 100 seconds and a liquidous max point of 220 C.
while in liquidous not more than 45 seconds) was required to effect solder ball reflow, solder
wetting, and acceptable solder joints. This is hot stuff folks and takes "normal" boards, plated
through holes (with acceptable material Tg's and plating ductility) to new levels.

I do not know who's done the study in the outside world. But in our little corner, the numbers
dictate no more than three rework cycles. Normally, these requirements are based on lesser board
layers and thicknesses requiring much less severe thermal profiles. If any information exists
concerning the requirements spoken to herein, please let us all know.

Earl Moon

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