The surface layers see the bulk of the stress from CTE-Z movements. The
delamination is caused from shearing the bond of epoxy to copper. The
interface at the copper is the weakest point and as the CTE mismatch has
the copper moving at differing rates than the resin/glass, the adhesion
fails and delamination occurs.
Materials with higher Td levels has a lower CTE and therefore do not
expand as readily as standard FR4 at certain temperatures. A lower
expansion coefficient allows for a longer maintenance of the bond at
higher thermal excursions. The decomposition process that begins to
occur at temp X through Td also has measurable reductions the packages
tensile and bond strengths.
I believe at the temperatures you are reflowing you may see some varying
results due to PCB build, copper bonding chemistries, laminate material
types and lamination cycle. Given this you may experience intermittent
failures without a more stable material.
Jeffrey Bush
Director, Quality Assurance and Technical Support
VERMONT CIRCUITS INCORPORATED
76 Technology Drive - POB 1890
Brattleboro, Vermont 05302
Voice: 802.257.4571.21 Fax: 802.257.0011
http://www.vtcircuits.com
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Tempea, Ioan
Sent: Thursday, August 25, 2005 11:09 AM
To: [log in to unmask]
Subject: Re: [TN] Blistering of PCB at lead-free temperatures
Gentlemen,
thank you for the fast response.
Clarifications:
* I am talking about delamination blisters. The board looks like
the surface of the outer layers is about 30% detached from the others
and only the vias are holding them together. Of course, there are
several independent blisters, not only one that big.
* the profile is having a short (20 sec) soak at about 170C. I
actually need to soak it more, to lower the temp delta between the
transformer and the rest of the comps
* I did not check what the Td is, but how will it impact
delamination? Prepreg burning off and detaching?
Regards,
Ioan
> -----Original Message-----
> From: TechNet [mailto:[log in to unmask]]On Behalf Of Tempea, Ioan
> Sent: Thursday, August 25, 2005 10:19 AM
> To: [log in to unmask]
> Subject: [TN] Blistering of PCB at lead-free temperatures
>
>
> Hi Technos,
>
> wanted to start a pilot run, but the first PCB that we soldered ended
up full of blisters when it came out of the oven. It is a 6 layers, full
ground and power planes, about 6"x7", not a high density one, pretty
light stuff on it, the toughest part is a transformer which needs a
little more heating. The profile peaks at 250C on the PCB itself and the
PCB is made out of FR4, Tg 170.
>
> I said to myself that it must be humidity, so I baked 1 board, 8H @
125C, and passed it through the reflow again. Same blistering, almost to
the same extent. Then I took a totally different PCB that was hanging
around on my desk for about 3 months, reflowed it at 250C and got just a
tiny blister, probably due to humidity this time.
>
> The questions I have are:
>
> is 250C too much for a properly manufactured PCB?
>
> If it's not the humidity what else could it be?
>
> Any other test that I could do to get a better idea of the root cause?
>
> Thanks,
> Ioan
>
>
>
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