Wow Jan,
You have: "PCBs approx 450mm long, 50mm wide, 1.7mm thick, 12 layers, 17u
copper inner
layers, Isola IS420 material, boards are produced with longest dimension in
warp direction in PCB production panel" —and you expect a 75um tolerance after
subjecting the PCB to board drying and leadfree reflow.
Just consider that in PCBs you are marrying 4 different materials [laminate,
prepreg, woven glass-fiber reinforcement, copper] together all of whom have
significantly different physical properties, particularly at different T's and
in different directions.
The laminate (which is fully cured) and the prepreg (not fully cured) have
different resin contents, go through glass transition (which changes molecular
structure and is not fully reversible, only nearly so), start to decompose at
SAC solder T's (actual weight loss, which is irreversible).
The glass-cloths is in your PCBs laid up with the warp direction in the
longest dimension in the PCB production panel making your PCBs quite non-uniform in
the x and y directions [the normal practice is to alternate warp and fill
directions on alternate layers to equalize the different properties in the warp
and fill directions—even with that, it is not uncommon to measure CTE-values
different in x and y by as much as 2 ppm/C].
The copper layers are mostly etched away in irregular [in the sense of
physical behavior] patterns.
—and you expect a 75um tolerance after subjecting the PCB to board drying and
leadfree reflow?!
Werner
Future workshops:
Pb-Free Soldering Processes—Survival, Quality, Reliability, July 17/18, Thal,
Switzerland
Pb-Free Soldering Processes—Survival, Quality, Reliability, July 21/22, Bad
Segeberg, Germany
Pb-Free Soldering Processes—Survival, Quality, Reliability, August 18, O
rlando
Reliability Issues with Lead-Free Soldering Processes, September 22,
Schaumburg
Failure Mode and Root Cause Analyses Reliability (Fatigue, Brittle Fracture,
ENIG), September 22, Schaumburg
Solder Joint Reliability: Parts 1 & 4, Oct. 9, Moscow, Russia
Solder Joint Reliability: Parts 1 & 4, Oct. 17, Timisoara, Rumania
Solder Joint Reliability: Parts 1 & 4, Oct. 22, Tallinn, Estonia
**************
Gas prices getting you down? Search AOL Autos for
fuel-efficient used cars.
(http://autos.aol.com/used?ncid=aolaut00050000000007)
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