I have tried that before and it is usually worse. The way the process works
is you would have two lamination cycles. The first lamination cycle would be
a problem because you are trying to laminate a 2 mil product over 4 oz cu.
Normally a 2 mil dry film would be able to encapsulate up to a 1.5 mil high
circuit. Your circuit height for this application would be more that 3
times that height. The air entrapment would be extreme.
After the first coat is applied then the 1 mil cover sheet is removed. The
mask is still soft because it has not been polymerized (with UV exposure).
When you do the second lamination cycle the soft mask compresses with the
original coating and you end up with less than 4 mils. My experience is this
would yield around 3 mils final thickness. During the second lamination
cycle, some of the air entrapment from the first cycle will be eliminated,
but the board should still have enough air entrapment to still be
unacceptable.
As Jeffery stated below another option is Conformask. This product allows
you to remove the cover sheet while the board is still hot and there are
still effects of vacuum present on the board surface. Once the cover sheet
is removed the mask draws down to conform (hence the name Conformask) to the
board topography. This allows the mask to flow between conductors and would
normally eliminate air entrapment. In this case I do believe you are beyond
the capabilities of Conformask. Conformask is available two or three
thickness. The thickest is 2.3 mils. I believe they have a 1.5 and 1.0 mil
versions also. The 2.3 mil Conformask was designed to encapsulate conductors
up to 4 mils. It would be close. The fear is with 4 oz cu you might thin out
the mask over the conductors to the point that it would not survive your
assembly process.
I still think the best thing (assuming your customer would not accept LPI
alone) is to use LPI first followed by dry film.
My 2 cents.
Bob Metcalf
Circuit Solutions
-----Original Message-----
From: TechNet [mailto:[log in to unmask]]On Behalf Of Mike Sewell
Sent: Friday, June 03, 2005 6:28 AM
To: [log in to unmask]
Subject: Re: [TN] Dry Film "Soda Strawing"...an update.
What about two applications of 2 mil Vacrel....? Is a thinner DF mask more
compliant or do other issues crop up?
Mike Sewell
Production Manager
LaBarge inc.
|---------+----------------------------->
| | Jeffrey Bush |
| | <Jeffrey.Bush@VTCI|
| | RCUITS.COM> |
| | Sent by: TechNet |
| | <[log in to unmask]> |
| | |
| | |
| | 06/03/2005 06:37 |
| | AM |
| | Please respond to |
| | TechNet E-Mail |
| | Forum; Please |
| | respond to Jeffrey|
| | Bush |
| | |
|---------+----------------------------->
>---------------------------------------------------------------------------
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| To: [log in to unmask]
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| cc:
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| Subject: Re: [TN] Dry Film "Soda Strawing"...an update.
|
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Steve -
You will not be successful with 8140 material with a 4 oz copper finish.
There are conformable materials like conformask that will model the
conductor finish. If the OEM will not budge on the DFSM finish, you can
apply an LPI initially, then overcoat with DFSM. The initial LPI
coating levels the base to the conductor somewhat and should resolve
this issue.
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: Stephen Gregory [mailto:[log in to unmask]]
Sent: Thursday, June 02, 2005 6:36 PM
To: [log in to unmask]
Subject: [TN] Dry Film "Soda Strawing"...an update.
Hi all!
We just had a conference call with the fab vendor about our soda
strawing
issue.
They acknowledged that the fabs are defective, but asked for a deviation
to notes on the fab drawing. We can't approve any deviation without
asking
our customer first.
The drawing calls out for 4-oz copper, and 4-mil dry-film solder mask.
The
mask being used is Dupont Vacrel 8140. The assembly is a power supply.
They want to apply a LPI, then apply the dry film. I understand that
applying
a 4-mil dry film over 4-oz. copper traces is difficult and challenging,
but
can
it be done without these kind of soda strawing defects?
Kind regards,
-Steve Gregory-
Senior Process Engineer
LaBarge Incorporated
Tulsa, Oklahoma
(918) 459-2285
(918) 459-2350 FAX
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