Techies....
Here's
some thoughts from a PCB fabricator....
First
of all, great input from everyone. As usual Dwight nailed
it.
Eventually all vias will fail under the right
conditions...
My
group acquired an IST tester earlier this from PWB Corp,
pin
point the trouble spots in our via process, i.e. laminate, oxide,
drill,
desmear, (etchback when required), electroless, copper
plate, etc.
The
three most significant impacts to via reliability were drill (hole wall
quality),
laminate (#1), and copper thickness (#2). We could
double the life cycles
of our
thermal cycles by changing the laminate or increasing the
copper
thickness.
There
is one subject, I did not see discussed... BGA rework....
Typically
your
BGA sites have the highest density of vias per square inch. Many
of
our
clients allow up to 5 rework cycles. The vast majority of via failures on
our
products are typically under BGAs... The Z axis
expansion of the laminate
after
several thermal excursions is remarkable.
Jim
Hole wall roughness is also a major determinant
of number of cycles to failure. The rougher the hole -- the more stress
risers for cracks to start propagating from. That's why I seen many
holes with thinner copper but clean drilling outlast thicker copper holes with
rougher drilling.
All in all, it's not so much a question of
whether a holewall will crack after thermal cycling. It's a
question of when it'll crack or how cycles it'll take to start cracking when
thermal cycled (wearout point on the classic reliability bathtub
curve).
Soon or later we all die and sooner or later hole copper will
start cracking if it sees enough temp cycling.
Dwight
At
08:15 PM 12/5/2002 -0800, Roger Mouton wrote:
Werner,
Your comments are well
taken. I think you know what I was getting at
Throughout the mid
80's I saw time and again where thin deposits cracked and
thicker ones
held together. And thanks for the clarification on
elongation
testing.
Best,
Roger
Mouton
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