Hello,
Could someone please explain what the term 'prepreg' is.
Thank you,
David Tremmel
-----Original Message-----
From: Robert Tarzwell [mailto:[log in to unmask]]
Sent: Tuesday, May 27, 2003 7:12 PM
To: [log in to unmask]
Subject: Re: [TN] Bad laminate and warped board
All very good therories but the reasons boards warp is many fold, usualy the
prepreg cant cause the problem unless many plies were layed ,or supplied
with the weave in the wrong direction, when you buy prepreg the weave
direction is printed on the vac bag. There are many places in a board shop
where warpage can be generated, The press cycle (too fast a cool down), the
press ( warped plates or plattens) The plating of unbalanced copper, the hot
air leveler, the ovens for solder mask, and white marking, the uv cure
machines , The biggest problem is not storing the boards in a flat position
when they are near or above the T/G the material is plastic like and easly
cures with a warp.
Can they be strightend, depends on the method used to warp them, If it is a
heat/bent in cooling problem yes a short cycle above the t/g and placed on a
flat surface with weights can return them to flat. but if it is a weave
problem no you can not permenitly strighten them. A quick test is to take a
board place it in a oven till the board is just above the T/G and take it
out and lay it on a flat surface with no weight, if the board goes flat it
was probably a handling issue, if its still bend then throw it out.
Long winded I know.
the major problem with a oven cycle to strighten the boards, is when you go
above the T/G you only get so many of them. each transition above T/G
removes one solder cycle the assembler can do before cracking in the vias.
As a pcb manufacture knows after 4-6 solder floates you get via cracking!!
I think you need to talk with the manufacture and let him know warped boards
above the IPC standard will not be accepted.
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