Afri,
From what you describe, with the top and bottom panel being affected only,
It would seem that the rate of heat rise is too fast. This leads to
entrapment of the volatiles and gas within the circuit.
How many plies of 7628 are you using? 1 ply would be insufficient to
encapsulate 2-3oZ of copper, leaving pockets against the copper in low
pressure areas. If only 1 ply, then try using 2 plies of 2113, or 2112
which will give the same presses thickness between layers as 1 ply 7628,
but will have sufficient resin for complete encapsulation.
You mention a specific heat rise, but don't say if this is the actual
package temperature, or the indicators on the press itself. The only way to
be certain of heat-rise is to measure within the package itself. I would
normally reccomend a nominal heat rise of between 5-8oC per minute between
70 and 140oC.
Placement of the thermocouple is also important.
Heat rise through the package as a whole, depends on the number of panels
you are attempting to bond in one package, how many layers of copper are in
the package, how much thermal buffer pad (Kraft etc.) are in use?
Using the vacuum assist on the press will also help, assuming a pull of
around -29 Bar. Pressure will play a significant part, especially with
thicker copper layers, but you don't mention what has been used.
I have probably raised more questions than have answered, but I hope the
above gives you some idea of where to look next.
Colin McVean
At 23:39 22/01/98 -0500, you wrote:
>Can anyone shed some light on a problem i am having in my lam dept.
>We are getting pockets of air (bubbles) trapped in low pressure areas
>towards the middle of the panel
>Seen only on the top or bottom panel of each book
>This only happens on cores with 2-3 oz cu.
>
>So far we have tried
>heat rise 12 -16 F /min
>b-stage with more resin- we are using 7628 only
>vacume assist press and hydrolic
>low pressure vs high
>no kiss cycle
>flatness of plates
>not able to modify the pettern
>
> thank you for any suggestions
>
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