Re: STENCILS

| 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 One other consideration is the type of solder paste that you are using. With the no-clean solder pastes one of the problems that has arisen is increased solder balls, or solder beads, which in case you are not familar with the term, are those little beads of solder generally seen between chip style components (Cer. caps, resistors, etc,). I have found that a simple reduction of aperture size is usually not enough. You will probably need to change the aperture design to a style that will reduce the amount of paste under the part, leaving only enough for the termination, and if you are using a chipshooter, enough to keep the part on the pcb/flex. Phil Zarrow wrote an excellent article on the different design considerations that was published in the April 1998 issue of Circuits Assembly. He also has a web page that the article is on, but I don't the location off the top of my head. I'm sure if you look for it, it will be very easy to find. Welcome to wonderful, and always challenging, world of stencil design. Good Luck Phil

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| | 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 | One other consideration is the type of solder paste that you are using. | With the no-clean solder pastes one of the problems that has arisen is increased solder balls, or solder beads, which in case you are not familar with the term, are those little beads of solder generally seen between chip style components (Cer. caps, resistors, etc,). | I have found that a simple reduction of aperture size is usually not enough. You will probably need to change the aperture design to a style that will reduce the amount of paste under the part, leaving only enough for the termination, and if you are using a chipshooter, enough to keep the part on the pcb/flex. | Phil Zarrow wrote an excellent article on the different design considerations that was published in the April 1998 issue of Circuits Assembly. | He also has a web page that the article is on, but I don't the location off the top of my head. I'm sure if you look for it, it will be very easy to find. | Welcome to wonderful, and always challenging, world of stencil design. | Good Luck | Phil On >31 mil pitch components go either 1:1 or reduce 1 mil per side (you can reduce further if the foil thickness is > 7 mil.), you may want to try an oval shape here also. <31 mil. components reduce width 10%, below 16 mil. reduce 20%. Remember your aspect ratio (foil thickness to aperture width) should never be below 1.5. Good Luck.

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| | | 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 | | One other consideration is the type of solder paste that you are using. | | With the no-clean solder pastes one of the problems that has arisen is increased solder balls, or solder beads, which in case you are not familar with the term, are those little beads of solder generally seen between chip style components (Cer. caps, resistors, etc,). | | I have found that a simple reduction of aperture size is usually not enough. You will probably need to change the aperture design to a style that will reduce the amount of paste under the part, leaving only enough for the termination, and if you are using a chipshooter, enough to keep the part on the pcb/flex. | | Phil Zarrow wrote an excellent article on the different design considerations that was published in the April 1998 issue of Circuits Assembly. | | He also has a web page that the article is on, but I don't the location off the top of my head. I'm sure if you look for it, it will be very easy to find. | | Welcome to wonderful, and always challenging, world of stencil design. | | Good Luck | | Phil | On >31 mil pitch components go either 1:1 or reduce 1 mil per side (you can reduce further if the foil thickness is > 7 mil.), you may want to try an oval shape here also. <31 mil. components reduce width 10%, below 16 mil. reduce 20%. Remember your aspect ratio (foil thickness to aperture width) should never be below 1.5. Good Luck.

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| | | | 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 | | : | | On >31 mil pitch components go either 1:1 or reduce 1 mil per side (you can reduce further if the foil thickness is > 7 mil.), you may want to try an oval shape here also. <31 mil. components reduce width 10%, below 16 mil. reduce 20%. Remember your aspect ratio (foil thickness to aperture width) should never be below 1.5. Good Luck.

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You may liketo contact Chemtech in the UK. http://chemteck.co.uk as they have a new hand book on stencils and a design guide due for release shortly there is also some stencil information as a survey on types and price on my web sites under surveys http://www.bobwillis.co.uk that may also be of interest to you | | 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 | One other consideration is the type of solder paste that you are using. | With the no-clean solder pastes one of the problems that has arisen is increased solder balls, or solder beads, which in case you are not familar with the term, are those little beads of solder generally seen between chip style components (Cer. caps, resistors, etc,). | I have found that a simple reduction of aperture size is usually not enough. You will probably need to change the aperture design to a style that will reduce the amount of paste under the part, leaving only enough for the termination, and if you are using a chipshooter, enough to keep the part on the pcb/flex. | Phil Zarrow wrote an excellent article on the different design considerations that was published in the April 1998 issue of Circuits Assembly. | He also has a web page that the article is on, but I don't the location off the top of my head. I'm sure if you look for it, it will be very easy to find. | Welcome to wonderful, and always challenging, world of stencil design. | Good Luck | Phil

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| | | | | 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 | | | : | | | On >31 mil pitch components go either 1:1 or reduce 1 mil per side (you can reduce further if the foil thickness is > 7 mil.), you may want to try an oval shape here also. <31 mil. components reduce width 10%, below 16 mil. reduce 20%. Remember your aspect ratio (foil thickness to aperture width) should never be below 1.5. Good Luck. First look at the technology on the board, anything below 0,63mm pitch I use a 140u electrformed stencil with a general reduction of 15% of all apertures. Pitches of 0,63mm and over I use an etched 150u stencil also with a 15% reduction. If your data uses flash pad, it is recommended that all the corners are rounded off. This combination gives us no bridging and the only solderballing we get is when the component overlaps the land more than it should. This error is remedied by further reducing the aperture, under the component. One last thing, we reflow in Nitrogen, and this helps considerably.

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