Chris and Robert,

Even if I am the only one, I have to disagree with what you wrote:

1. Tg is a physical property of every material, incidentally it is
measured in degrees Celsius, not Fahrenheit.  It is not true that it
"really applies for board assembly", it applies to the whole life of the
material.

2. The cumulative temperature of 2 solder reflows is WAY above Tg of any
FR4 material (and most other PCB materials for that matter).  Actually,
just one reflow is above Tg of most FR4 materials.

3. If your theory was true a PCB would never even make it out of a board
shop.  A quick cumulative sum of the temperatures that a board sees
(after cure) is easily over 1000F (or 500C if you prefer).  Never mind
the 2 (or 3) reflows that most boards go through in assembly.

4. Via cracking does not depend only from the thickness of plating but
also form the quality of the copper plating, namely its elongation
properties, and (quite important) from the Z CTE of your material of
choice.

5. Plating 4 oz of copper may be good for a high power board,
unfortunately most of today's designs do not fit the bill.  Try plating
4 oz in a 10:1 (starting) aspect ratio hole which is quite common for
any BGA application... It is not exactly straight forward.

This does not mean that temperature and time do not affect the
performance and physical properties of a circuit board.  They obviously
do, but not nearly to the extent that you describe.  I am not an expert
but I have been making printed circuit boards for many years now.  I did
not run any experiments or write any books but I am quite sure that most
PCB in service today on this planet (and orbiting around said planet)
are low, medium or high Tg FR4, 1 ounce boards.  Last I checked most of
them are working quite well.

Adeodato Vigano
Compunetics, Inc.
PCB Division

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