Brad,
The vacuum press is not complicated. It is not really a press. We used a heavy duty silicone rubber bag that was designed for that purpose. I don't recommend that approach because the handling precautions associated with the silicone rubber are more than just annoying. Contact between the bag and the solderable surfaces rendered the boards unsolderable. Fingerprints can carry enough silicone to interfere with soldering. The website that Andrew included in his response has a section on vacuum bags that are made of more suitable materials. They also carry the bits and pieces that make it happen. These are not as durable and the silicone rubber bags, but then, you are not doing large scale production.
Harvey
\ufffdThanks for all the great responses.\ufffd\ufffd I guess I have a lot to think about now.
\ufffd
A question about temps \ufffd do I need to worry about fire risk, etc doing things this way?\ufffd I mean, I would assume not so much an issue on a low voltage board, but on my TVT where an actual direct to AC connection is made and there\ufffds a transformer..
\ufffd
The vacuum press idea sounds cool but might be beyond my skill level to assemble.\ufffd I have a vacuum machine used for filling inkjets.\ufffd It has a chamber about the side of a large coffee pot.\ufffd I imagine for smaller stuff that would work.\ufffd The larger boards though..
\ufffd
From: Homebrew_PCBs@yahoogroups.com [mailto:Homebrew_PCBs@yahoogroups.com]
Sent: Tuesday, July 18, 2017 10:58 PM
To: Homebrew_PCBs@yahoogroups.com
Subject: RE: [Homebrew_PCBs] Attaching two PCBs together\ufffd
\ufffd
Harvey, Brad \ufffd
A PCB in a vacuum bag is getting about 15 lb. per square inch clamping just from atmospheric pressure and a consistent pressure across the whole board That should help level the epoxy assuming it is not too viscous.\ufffd I absolutely agree with degassing the mixture and using a long pot life epoxy (30 minutes or more).\ufffd The vacuum does not have to be applied for the whole 24 hour cure time, just until the epoxy is well set up\ufffd and cannot reform bubbles.\ufffd The pump plans at http://www.veneersupplies.com/ do include a filter before the pump to protect it from outgassing and moisture.\ufffd They also have a pressure controlled switch so that the pump only runs when the vacuum falls to low.\ufffd I built one.\ufffd \ufffdIt was easy and it works great.
Regards,
Andrew\ufffd
From: Homebrew_PCBs@yahoogroups.com [mailto:Homebrew_PCBs@yahoogroups.com]
Sent: Tuesday, July 18, 2017 9:13 PM
To: Homebrew_PCBs@yahoogroups.com
Subject: Re: [Homebrew_PCBs] Attaching two PCBs together\ufffd
\ufffd
Brad,
As Andrew V. mentioned, vacuum bagging is an effective process for laminating. There are a couple of issues when using epoxy, or any resin that requires mixing. Mixing the resin and hardener introduces air bubbles into the epoxy. It is true that the vacuum will pull the air out, but a better way is to degas the resin before you apply it. Lots of descriptive material on the epoxy manufacturer's sites. The fly in the ointment is that the epoxy is curing while you are doing this. You need to use an epoxy with a relatively long pot life. Once the epoxy is degassed, you can apply it to your board and process in the vacuum bag. In both vacuum operations, it is important to have some kind of "filter" in the vacuum line to keep the volatiles that the vacuum pulls out of the epoxy out of the pump.Otherwise, the pump will eventually get gunked up with this material. A water bath might work, or a large chamber that slows down the flow to where the volatiles condense on the surfaces, rather than entering the pump.
Here is a reference to the degassing process: www.epotek.com/site/files/Techtips/pdfs/tip4.pdf
Another thing you should give a thought to, is how to produce an epoxy layer with a uniform thickness.
Harvey A.
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On 7/18/2017 6:57 PM, 'Andrew Volk' amvweb@... [Homebrew_PCBs] wrote:
\ufffd
Brad \ufffd
\ufffd
Use a vacuum press.\ufffd Vacuum is useful for many things, especially if you are also into woodworking (veneering), molding, etc.\ufffd The vacuum will draw the bubbles out.\ufffd Vacuum pumps are available from Harbor Freight.\ufffd Just need a few fittings and a reinforced hose and a chamber or bag in which to put the boards.\ufffd (See http://www.veneersuppliescom/pages/DIY__Vacuum__Press.html).
\ufffd
Andrew V.
\ufffd
From: Homebrew_PCBs@yahoogroups.com [mailto:Homebrew_PCBs@yahoogroups.com]
Sent: Tuesday, July 18, 2017 6:15 PM
To: Homebrew_PCBs@yahoogroups.com
Subject: RE: [Homebrew_PCBs] Attaching two PCBs together\ufffd
\ufffd
The material used to glue layers together in multilayer boards is called prepreg.\ufffd I don\ufffdt see prepreg sold online anywhere though.\ufffd Not even Ebay.\ufffd
\ufffd
From: Homebrew_PCBs@yahoogroups.com [mailto:Homebrew_PCBs@yahoogroups.com]
Sent: Tuesday, July 18, 2017 7:58 PM
To: Homebrew_PCBs@yahoogroups.com
Subject: Re: [Homebrew_PCBs] Attaching two PCBs together\ufffd
\ufffd
Brad,
I think you slipped a decimal point o the thickness of your boards- 0.031 and 0.0625".
There is another (better?) way, but it is problematical. You would get a much more uniform layer of adhesive by going with sheet adhesives. I my prior life, I was involved in some production that used sheet type adhesives to bond fairly big boards to heatsinks. The adhesive we used was an epoxy. I recall being able to buy sheet adhesives with or without a carrier layer, but since they were basically mixed, but uncured epoxy systems, they had to be kept in the freezer until use. The are other types of adhesives that also are available in sheet form. That was the good news. The bad news is that I couldn't find a place to buy these materials in reasonable quantities. I found an acrylic adhesive, recommended for this type of application, but it came on 3" rolls that were 250' long and 24" wide. Price was in the stratosphere. I looked at ebay, Amazon, McMaster-Carr and MSC to see if any of them carried these materials. Nada. A general Google search turned up some manufacturers, but no sellers at the retail level.
If you want to do your own search, look for sheet adhesives, adhesive preforms, bonding adhesive sheets.
If you should find a source, you will still be faced with the need to provide a fairly high, uniform pressure, and an elevated cure temperature. The material we used needed to withstand high temperatures, so we used something that cured over 150\ufffdC. Others are lower, but then it will depend on what you can find.
I did find some two sided adhesive mounting sheets at a craft store. No information available about what type of adhesive. They might work, but I am not so sure about how long they will stick, or if they might dry out.
Good luck in your search.
Harvey A.
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On 7/18/2017 11:14 AM, 'Brad' unclefalter@... [Homebrew_PCBs] wrote:
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So as I stated in a previous post, I have acquired some vintage 1973 PCB stock, which is perfect for a project I\ufffdm doing that replicates the 1973 TV Typewriter.\ufffd Except for one thing \ufffd the stock is 0.31\ufffd thick, rather than 0.625\ufffd.\ufffd So my thought was to epoxy two pieces together.\ufffd And that works, except despite my best efforts, I end up with these ugly voids you can see.\ufffd I\ufffdve tried putting the boards under a press with heavy pressure but there\ufffds always a few spots.
\ufffd
Is there a better way/adhesive to do this?\ufffd
\ufffd
I realize it\ufffdd be easier to use 0.625\ufffd stock from the outset, but stock of the correct vintage just isn\ufffdt available, and it means a lot of me to be using actual boards from the time.
\ufffd
Thanks!!
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