Brian
You do not have to go so far, If using pinless floating built-up
technique (riveting the inner layers without external prepregs) you will
easily create a situation where one prepreg will not align to other in 100%.
This also can cause to the glass fibers to not be 90º to each other in the
total book.
Shabtay
Eltek - Israel
> -----Original Message-----
> From: Brian Ellis [SMTP:[log in to unmask]]
> Sent: ? ?????? 13 2000 18:55
> To: [log in to unmask]
> Subject: Re: [TN] Warp/Bow after reflow
>
> MoonMan
>
> Please allow me to add
> 10) Quality of glass cloth: much of the cloth we are seeing today has
> been woven unevenly, often with the warp and weft locally not even at
> 90° to each other. This is rolled up under stress and goes straight into
> the treater. Result: when the prepreg is pressed and the resin melts,
> some of the resultant strain is relieved, but is partially dissipated in
> the adjacent layers, which also become stressed (if they aren't
> already). Just waiting for a passage above Tg for stress relief!
>
> Brian
>
> Earl Moon wrote:
> >
> > Wes,
> >
> > I just finished a consulting assignment in Connecticut concerning the
> very
> > same thing. I have done many of these contracts recently as everyone
> seems
> > to be having the same problems.
> >
> > As I told Tom Gervascio (another Siemens person), first read my article
> on
> > the SMTnet (their newsletter) in the September 1999 edition. It speaks
> > clearly, though technically, to this and other issues concerning MLB
> > technology. I am assuming boards, on the panel, are this type even
> though
> > double sided types also may perform this way.
> >
> > 1) The MLB construction, beyond balanced copper planes, as material
> > selections. Also, material availability becoming more of a problem due
> to
> > glass fiber shortages (much being diverted to fiber optics industry -
> what a
> > surprise!)
> >
> > 2) The lamination process not effectively managed as press cycle from
> ramp
> > through pressure, time, and temperatures used.
> >
> > 3) Lamination press issues as non parallel press plattens, thermal
> problems
> > as uniform heat transfer or heater elements not working, etc.
> >
> > 4) Number of books in each press opening along with thermal lag
> material,
> > etc.
> >
> > 5) Total cure not effected during lamination
> >
> > 6) Variations in your assembly process as reflow profiles, including
> cool
> > down.
> >
> > 7) The type scoring used and how well the process managed
> >
> > 8) A thin panel that is too large (how large are the panels?) especially
> > when not effectively supported during reflow.
> >
> > 9) Other variables not so common
> >
> > Earl Moon, Proof Of Design
> >
> >
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