I have tried laser cut plastic stencils but they are not a joy to use. The raised cut edges are about 20% higher than the plastic material itself and the squeegee and paste get caught up on them. It leaves a thin layer of paste on top of the plastic between the cutouts. Using a $80 stainless stencil is like liquid gold compared to plastic. Part of the issue is the ability to remain flat against the PC board as the squeegee is pulled across. Solder paste will build up in any voids under the stencil if the bottom of the stencil is not absolutely flat against the PC board. When you are dealing with stencils in the 0.005" - 0.010" thick range, having a 0.005" gap between the stencil and board results in islands of paste. If I get a bad stenciling I scrape the board with a razor blade and start again.
IMO using a syringe by hand is better than a plastic stencil for prototypes. This is why I'm looking at automating this process. I really don't have an issue with solder paste drying out. Batches of stenciled boards have been exposed to air for as long as 5 hours before they hit the oven. That's pushing the limit a little as a small number of resistors will tombstone because the solder paste is not sticky enough to hold down both sides of the resistor when reflowing. Three hours is still very workable. Some parts like JST SMT vertical connectors cannot be picked by a vacuum so they have to be placed by hand after the P&P. I'm also one 8mm feeder short of what I need for this board so some other parts need to be placed by hand. The P&P is not perfect either. It has about a 99% placement rate which I think is pretty good. This means a resistor may have rotated a bit when slammed into the solder paste and no longer contacts both pads on the board. There is a visual verification and manual "correction" process done for each board. The majority of boards I run are double sided so all SMT processes are performed twice. This is one of the reasons I would like a paste printer because the manual stencil printer requires a lot of adjusting and fixturing changes when I go from side 1 (flat, no parts) to side 2 with lumpy parts on the back side. It would be better if I actually made fixtures for each type of board but where does the time go. A paste printer with adjustable width rails on a conveyor would lock a board in place even if the board had parts mounted on the bottom side. I envision a stepper driven back gauge to set the panel width to 125mm, etc. depending on the board. The back gauge would have a little bit of spring tension for board size variations (fixed position front guide). I made fixtures to run very small boards through the P&P (postage stamp sized if individually routed). It is a spring loaded fixture that keeps the PC board height the same as if there were no fixture. I also have adjustable aluminum extrusion fixtures to gently suspend the boards as they go through the oven for the 2nd side of the board. The oven conveyor would damage the bottom side parts if the board were sitting directly on the oven conveyor itself. The boards are still at melt temperature when pulled off the conveyor for about 3 seconds and subject to bumping, etc. An outbound conveyor board stacker/cooler is also on the drawing board. Dennis > -------Original Message------- > From: cogoman <cogo...@optimum.net> > To: Enhanced Machine Controller (EMC) <emc-users@lists.sourceforge.net> > Subject: Re: [Emc-users] solder paste > Sent: Aug 07 '12 00:11 > > On 08/06/2012 09:49 PM, Jon Elson wrote: > > I make my own stencils for these. It is kind of a pain, but I'm CHEAP! > How about the cheapest CNC to build, an XY plotter like this: > > https://www.youtube.com/watch?v=WyLPpGdfR7s > > but with the laser from an old DVD burner for the pen. You could > produce plotter output files and convert them to G-code, and then cut > them out of thin plastic (just shield your eyes while it runs, or box it in. > > Getting the tool set in line would take some time, but once you get > the hang of it, it would save time in the long run. ------------------------------------------------------------------------------ Live Security Virtual Conference Exclusive live event will cover all the ways today's security and threat landscape has changed and how IT managers can respond. Discussions will include endpoint security, mobile security and the latest in malware threats. http://www.accelacomm.com/jaw/sfrnl04242012/114/50122263/ _______________________________________________ Emc-users mailing list Emc-users@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/emc-users
