Printed Circuit Board Assembly & PCB Design SMT Electronics Assembly Manufacturing Forum

Printed Circuit Board Assembly & PCB Design Forum

SMT electronics assembly manufacturing forum.


STENCILS

Maurice McClain

#15376

STENCILS | 18 June, 1998

WHAT SHOULD THE RATIO BE OF STENCIL APERATURE TO PAD TO GET GOOD QUALITY RESULTS. BY THAT I MEAN, A MINIMM OF SOLDER BALLS, AND LITTLE TO NO BRIDGING, SHORTS, ETC.. RIGHT NOW WE ARE USING A 1:1 RATIO, BUT ARE INVESTIGATING REDUCING THE APERATURE SIZE. IS THERE A STANDARD PRACTICE THAT HAS CONTINUOUSLY SHOWN GOOD RESULTS? ANY ADVICE WOULD BE GREATLY APPRECIATED. MAURICE MCCLAIN

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Igmar Grewar

#15380

Re: STENCILS | 18 June, 1998

| WHAT SHOULD THE RATIO BE OF STENCIL APERATURE TO PAD TO GET GOOD QUALITY RESULTS. BY THAT I MEAN, A MINIMM OF SOLDER BALLS, AND LITTLE TO NO BRIDGING, SHORTS, ETC.. | RIGHT NOW WE ARE USING A 1:1 RATIO, BUT ARE INVESTIGATING REDUCING THE APERATURE SIZE. IS THERE A STANDARD PRACTICE THAT HAS CONTINUOUSLY SHOWN GOOD RESULTS? ANY ADVICE WOULD BE GREATLY APPRECIATED. | MAURICE MCCLAIN We reduce apertures only on fine pitch components. For us, a 15 to 20% reduction works fine. We also specify the stencil thickness so that the apeture width of the smallest aperture is greater than 1.5 times of the stencil thickness. When the aperture gets smaller than 1,5 times of stencil thickness, you have to reduce the stencil thickness otherwise the paste gets stuck in the aperture.

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Russ Miculich

#15379

Re: STENCILS | 29 June, 1998

| WHAT SHOULD THE RATIO BE OF STENCIL APERATURE TO PAD TO GET GOOD QUALITY RESULTS. BY THAT I MEAN, A MINIMM OF SOLDER BALLS, AND LITTLE TO NO BRIDGING, SHORTS, ETC.. | RIGHT NOW WE ARE USING A 1:1 RATIO, BUT ARE INVESTIGATING REDUCING THE APERATURE SIZE. IS THERE A STANDARD PRACTICE THAT HAS CONTINUOUSLY SHOWN GOOD RESULTS? ANY ADVICE WOULD BE GREATLY APPRECIATED. | MAURICE MCCLAIN Maurice, It has been my experience that since most solder pastes have a slump ratio of 1.2 to 1, an undercut for 16mil to 20mil pitch should be around 20-25%. The 25% undercut will allow for solder flow not running off the pad should you be slightly off center on your initial print alignment. If you are doing 12 mil center to center - get ready for undercutting of 40% due to surface tension in the aperature vs. pad area stichtion. Also, what Igmar said is correct - you need to reduce the thickness of the stencil. Typically 4 to 5 mil thick works pretty well for most fine pitch. Cleaning the stencil preferably with vacuum understencil cleaner is necessary. The actual stencil material and the way the stencil apertures are cut (laser or chem etch) will affect the release of your paste also. My email is r.miculich@smtech.com - feel free to communicate.

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Russ Miculich

#15378

Re: STENCILS | 29 June, 1998

| WHAT SHOULD THE RATIO BE OF STENCIL APERATURE TO PAD TO GET GOOD QUALITY RESULTS. BY THAT I MEAN, A MINIMM OF SOLDER BALLS, AND LITTLE TO NO BRIDGING, SHORTS, ETC.. | RIGHT NOW WE ARE USING A 1:1 RATIO, BUT ARE INVESTIGATING REDUCING THE APERATURE SIZE. IS THERE A STANDARD PRACTICE THAT HAS CONTINUOUSLY SHOWN GOOD RESULTS? ANY ADVICE WOULD BE GREATLY APPRECIATED. | MAURICE MCCLAIN Maurice, It has been my experience that since most solder pastes have a slump ratio of 1.2 to 1, an undercut for 16mil to 20mil pitch should be around 20-25%. The 25% undercut will allow for solder flow not running off the pad should you be slightly off center on your initial print alignment. If you are doing 12 mil center to center - get ready for undercutting of 40% due to surface tension in the aperature vs. pad area stichtion. Also, what Igmar said is correct - you need to reduce the thickness of the stencil. Typically 4 to 5 mil thick works pretty well for most fine pitch. Cleaning the stencil preferably with vacuum understencil cleaner is necessary. The actual stencil material and the way the stencil apertures are cut (laser or chem etch) will affect the release of your paste also. My email is r.miculich@smtech.com - feel free to communicate.

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Jon Gruett

#15377

Re: STENCILS | 1 July, 1998

| WHAT SHOULD THE RATIO BE OF STENCIL APERATURE TO PAD TO GET GOOD QUALITY RESULTS. BY THAT I MEAN, A MINIMM OF SOLDER BALLS, AND LITTLE TO NO BRIDGING, SHORTS, ETC.. | RIGHT NOW WE ARE USING A 1:1 RATIO, BUT ARE INVESTIGATING REDUCING THE APERATURE SIZE. IS THERE A STANDARD PRACTICE THAT HAS CONTINUOUSLY SHOWN GOOD RESULTS? ANY ADVICE WOULD BE GREATLY APPRECIATED. | MAURICE MCCLAIN Maurice, You bring up some typical issues that arise when an assembler starts to push the envelope on their stencil designs. The bottom line on paste printing is putting the correct amount and volume onto the pads, right? Well in my experience it all comes down to 1)aperture design and 2)type of stencil being used. There really are two basic types of stencils. Those made with subtractive technologies and those made with additive technologies. We are all familiar with the subtractive, chem etch and laser. But the latest addition to the stencil market is the Electroformed stencil. It is made with an electroplating type of process and is literally built at the atomic level one atom at a time. Because it is an additive process you aren't restricted to fixed thicknesses of stencils as you are with the laser and chem etch. The people at AMTX can help you design the correct aperture size for your pads and they gaurantee that you will get greater than 95% paste release with the E-Fab stencil. This will aid you tremendously in the printing process. Give Dick Clouthier a call at AMTX and ask him how he would design your apertures for your application. He has emerged as a dominant expert in the field of paste printing and aperture design. His number is 716-396-6974. Good luck, JON

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