Making SMD boards - wither paste stencils?

[Frank Gentges]

Mike,

We have used the laser toner transfer with some success.  We found that a specially set up laminating machine was needed to get a reliable transfer of the toner to the PC board.  We used an HP LJ3 but other printers at the time we tried did not deposit enough toner to work.  We could turn the toner density up full bore on the LJ3 and that made that part of it all work.

The resolution was not great and doing little 50 mil pins just would not work.  The toner would squish out in the laminator from the pressure and the lines would start to merge.

We used the special paper we got from Mouser and the backing would loosen up from the toner and just float away.  That worked fine.  We did not need any touch ups.

Our concern after all this is we had made one board and unless we were very careful, later boards were a crap shoot spoiled by some failure along the line.

We now have been using 

pcb-pool.com 

They are in Ireland and provide reliable quality work.  We do the layout in PCB Free 

www.freepcb.com/

and earlier in Eagle with silk screen and solder masks.  Importantly, they provide a free stencil that looks to be stainless steel.  The stencil has become an essential element of making things with PC boards and surface mount now.  

Given all the stuff we were buying and time spent with toner transfer, this proved a better approach.  Once you have proved the design from a prototype, they have good pricing on small quantities.  If you generate Gerbers for your prototype, you can get other makers to make boards if you need a quantity of boards.  We did make up a fixture to place the stencil over the board to assure alignment while putting on the solder paste.

We modified a toaster over with a solid state temperature controller so we can follow the solder paste manufacturer's time and temperature curve.  We are getting some real nice looking reflow soldering from this.  The controller uses a thermocouple and an attached notebook computer.  We did pretty good earlier with a "barefoot" toaster oven so the controller just gave us a consistent product.

Our work has led us to using others for the PC board and we no longer try to make our own as it takes too much time and money with marginal results.  If you are doing through hole parts and 0.1 lead spacing you may find the toner transfer still an acceptable approach.

This reply has gotten a bit long but I hope it helps everyone understand what we have evolved through and may save some time and money on their next project.

Frank K0BRA


On Nov 14, 2010, at 8:46 PM, Mike O'Dell wrote:

> i've been thinking a lot about how one makes
> PCBs for home-brew designs using SMDs.
> 
> after talking with a number of folks,
> the consensus seems to be that making a board
> using toner-transfer (plus pen touch-ups)
> and etching with ferric chloride will work
> acceptably well.
> 
> when i asked about making stencil for solder
> paste, the 2.5 suggestions were: laser cutter,
> vinyl cutter (a plotter with a sapphire scalpel
> instead of a pen), or (.5) use a syringe and
> dot the board manually.
> 
> it seems to me that there should be a way to make
> a paste stencil that isn't any more complex than
> making the board, and the article i referenced
> before on the "Ediburgh Etch" for intaglio print-making
> may provide the answer. (I almost wrote "solution" but
> stopped in time.) the trick is adding some citric acid
> to the ferric chloride. it dramatically improves the
> etching performance.
> 
> using toner-transfer, one ought to be able to image
> the stencil on very thin brass shim stock or even
> heavy copper foil. paint the back and edges with
> a resist and then let the etchant chew all the
> way through the metal to make the stencil.
> 
> i bring this up to ask if anyone knows of a better
> approach before i try this out?
> 
> while for small designs one can make a stencil
> manually, there is something to be said for
> a stencil that is made using the same tape-out
> as the board.
> 
> however, if people have manually made stencils
> (like for the charleston SDR board??), what
> techniques seem the best and where is the cross-over
> between manual dotting with a paste syringe and
> making a stencil?
> 
> 	-mo
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