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PCB Sandwich possibility?

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Vonneh

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Hello,
I was wondering if it is possible to etch a board capable of soldering directly onto another board. What I want to do is remove a surface mount IC from a board, solder an adapter board directly ontop of the board then place another surface mount IC on the adapter board. I have to do this a number of times and the method I have been using is soldering wirewrap wires directly from the board to the chip and this takes up a lot of time. Any comments or suggestions are appreciated. Please let me know if anything needs to be clarified or pictured provided. Thanks.
 
Your best bet would be to use a small board with spring clips mounted on the back side that would contact the SMD pads on the original board. "Spring clip" isn't really what I mean, but it's too late to think clearly enough to enunciate the actual item I mean; it's a very small curved springy piece of metal that will make contact with the SMD pads on the original board, and it would be soldered on the back of the second board so that it could act as if it were the original SMD part. You could either reflow solder it on, or (if it's not to be permanent) you could just mount it with screws or clips or temporary glue or whatever, and it would contact the SMD pads.

If you'll give me a better idea of what exactly you're trying to accomplish, and why you would want to remove an SMD chip from a board and wire it up with wire-wrap wire in the first place, I might be able to come up with a more creative dodge.
 
Maybe the small telescopic spring - what used to hold the wristwatch band - useable. (depending from IC raster....)
 
Thanks for the suggestions, I never thought of using small wires/springs. As for using screws to screw the top board onto the bottom board I don't think I will be able to do that because there are so many traces on the board there really isnt any room to drill through.
Basically what I want to do is remove a SOIC chip that is 32 pins and replace it with a 32 pin PLCC. These chips are very hard to find and the only ones I can find come in 32PLCC form. So either I would need to solder the chip with some wire wrapping wires or use an adapterboard as I explained above.
Could you explain what you mean by reflow soldering the adapter board to the main board?
Lastly, any suggestions on how to solder these spring clips to the bottom of the board? Is is possible just to have a 28SOIC pin layout on a board and basically just sandwich two SOIC layouts together and heat the top of the board so it solders together?
If you need pictures of what I have done so far just let me know. I think the above describes what I need though.
Thanks again.
 
Hi,
how about this:
build a top board with a 32-soic pattern on both sides, each pad through-connected with a via at its center.
You place the top exactly over the bottom, and hand-solder (with soldering iron) supplying solder through each via downwards. Capillarity will take care of the rest (hopefully).
Regards,
DS
 
Hey,
That sounds like it would be very easy to implement. I'll send out to get a board etched and let you know how it goes. Before I do this however, I will do a design in eagle and take a screenshot to see if you guys think it will work. Please check back later in the week. Thanks.
P.S. I found this and this is what I want to do basically but with a different layout.
**broken link removed**
 
One problem with that layout, the green resist coating will prevent capillary action from drawing the solder through the vias (feedthroughs, whatever you want to call them). I'd suggest a via right in the middle of each of the bottom-side pads. That way, when you apply the solder to the via on the topside, it will flow easily to the pad on the bottom. That of course means that the original SMD would have to be either bigger or smaller than, or offset from, the new one... you wouldn't want the pads to overlap.

The picture you showed is very much what I was thinking of in my original suggestion, but with blobs of solder on the bottom side instead of springy little contacts like I had in mind. I'm guessing that for a permanent installation, what you showed is more appropriate, and certainly more durable. In that case, you wouldn't apply any solder to the vias- just a soldering iron. Be sure that the manufacturer knows not to put that green solder resist material over those vias. I'm not sure Eagle will let you make a resist mask; I haven't done any PCB design with it yet.

Reflow is the process by which SMDs that have been secured in place with glue but not yet soldered are soldered in place in a manufacturing environment. The pads are pre-loaded with solder, and the SMD part is glued in place with a dab underneath, with its contacts on the solder on the pads. Then the board assembly is run through an infrared furnace that melts the solder and solders the SMD in place. Provided you know exactly what you are doing, you can accomplish the same effect with a soldering iron, once the SMD is secured (as you no doubt have reason to know).

Run the layout past us. I'll try to remember to comment.
 
Good point Schneibster, I actually never specify solder mask on the vias so it didn't occur to me.
Alternatively instead of a pad-with-via you could craft a small through-hole pad and use that 32 times. That way you are sure there won't be any solder mask.
DS
 
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