And you mention only TC: the coefficient of hygroscopic expansion at
constant temperature between, say, 25% and 75% RH (allowing time for
stabilisation) is likely to be as much again per %RH.
Just the copper has a TCE of 16.6 µm/m/°C and that's inorganic. Organic
materials have other molecular characteristics which render them
mechanically unstable and not always predictable.
Brian
Werner Engelmaier /* wrote:
> 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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