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Aperture Reduction for QFP (fine pitch)

Greg H.

#9455

Aperture Reduction for QFP (fine pitch) | 20 September, 1999

My stencil thickness is 6 mil and we got qfp's with fine pitch. how many percent should i reduce my stencil aperture using a 6 mil stencil for 0.5 mm pitch and 0.4 mm pitch ?

thanks

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#9456

Re: Aperture Reduction for QFP (fine pitch) | 21 September, 1999

| My stencil thickness is 6 mil and we got qfp's with fine pitch. | how many percent should i reduce my stencil aperture using a 6 mil stencil for 0.5 mm pitch and 0.4 mm pitch ? | | thanks | We use 6 mil lasercut stencils with 15% reduction and that�s working fine at least with our pad design doing manual and automatic printing. With really miserable paddesign for 0,5mm pitch we had more than trouble even with this, so paddesign is the first to look at before changing things that worked so well also in reducing solderballs.

There might be other suggestions Wolfgang

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CPI

#9457

Re: Aperture Reduction for QFP (fine pitch) | 21 September, 1999

| | My stencil thickness is 6 mil and we got qfp's with fine pitch. | | how many percent should i reduce my stencil aperture using a 6 mil stencil for 0.5 mm pitch and 0.4 mm pitch ? | | | | thanks | | | We use 6 mil lasercut stencils with 15% reduction and that�s working fine at least with our pad design doing manual and automatic printing. With really miserable paddesign for 0,5mm pitch we had more than trouble even with this, so paddesign is the first to look at before changing things that worked so well also in reducing solderballs. | | There might be other suggestions | Wolfgang | |

There are two things that you could do with this one. An aperture reduction or a step down. I will agree with wolfgang if you are going to reduce the aps go with 15%. A step down would simply reduce the stencil thickness around the QFP so if you ordered a 6 mil stencil I would have the QFP stepped down to 5 mil. Both methods will give you "approx" the same result 15% reduction. The choice is yours, just remember reducing your fine pitch apertures will lead to other problems such as paste release. You may find that your aps will clog more and you will get poor solder definition (have to clean you screen more often). What ever you do have the thing LASER CUT!!!!!

Chad |

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Scott Davies

#9458

Re: Aperture Reduction for QFP (fine pitch) | 21 September, 1999

| | | My stencil thickness is 6 mil and we got qfp's with fine pitch. | | | how many percent should i reduce my stencil aperture using a 6 mil stencil for 0.5 mm pitch and 0.4 mm pitch ? | | | | | | thanks | | | | | We use 6 mil lasercut stencils with 15% reduction and that�s working fine at least with our pad design doing manual and automatic printing. With really miserable paddesign for 0,5mm pitch we had more than trouble even with this, so paddesign is the first to look at before changing things that worked so well also in reducing solderballs. | | | | There might be other suggestions | | Wolfgang | | | | | | | There are two things that you could do with this one. | An aperture reduction or a step down. I will agree with wolfgang | if you are going to reduce the aps go with 15%. A step down would simply reduce the stencil thickness around the QFP so if you ordered a 6 mil stencil I would have the QFP stepped down to 5 mil. Both methods will give you "approx" the same result 15% reduction. The choice is yours, just remember reducing your fine pitch apertures will lead to other problems such as paste release. | You may find that your aps will clog more and you will get poor solder definition (have to clean you screen more often). What ever you do have the thing LASER CUT!!!!! | | Chad | | | | I've used a 10% aperture reduction for 0,025" pitch QFPs on a 0,006" laser cut stencil with no real problems, so I would agree with all you guys that a 15% reduction for 0,020" pitch or less would be appropriate. (And, as Chad so rightly said, there is no substitute for Laser Cut!)

Remember, in addition, that when reducing QFP apertures, you only need to specify a reduction on the shortest edge of the aperture. That little bit of extra paste you get on the board by leaving the longest edge equal to the nominal pad length can sometimes make a difference.

I've often been tempted to try something a little different when specifying QFP apertures. Like, for example, using long, tapered triangular apertures arranged in alternating directions. The objective being, get the maximum amount of paste down, but also maximise the distance between the paste deposits. If anyone has tried anything along these lines, it would be good to hear about it.

Scott

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#9459

Re: Aperture Reduction for QFP (fine pitch) | 21 September, 1999

| My stencil thickness is 6 mil and we got qfp's with fine pitch. | how many percent should i reduce my stencil aperture using a 6 mil stencil for 0.5 mm pitch and 0.4 mm pitch ? | | thanks | Greg: I was going to link you to the IPC land pattern calculator, but I got this:

"IPC-SM-782 Land Pattern Calculator Look up surface mount land patterns, or calculate your own! (This site is temporarily down until further notice)"

Say, is "temporarily down until further notice" webtalk like "Honey, I�m a little pregnant"?

First the pandering:

Wolfi: Yes on laser cut. Chad: Yes on 5 mil and laser cut. Scott: Yes on laser cut and reduction philosophy � keep the length and reduce the width.

You didn�t ask for this, but:

� Use a Type 4 paste. � Use solder mask webbing between pads.

Try this, inluded is some justification of the selections

Pkg / Pitch / Aprtr / Thick / Aspect ratio/ Area ratio/ Ease QFP / 20 / 10x50 / 5 / 2.0 / 0.8 / Fairly easy QFP / 16 / 7x50 / 5 / 1.4 / 0.6 / Tough

Where: Dimensions are mils Aspect ratio = W/T SB GT 1.2 for laser cut Area ratio = (LxW)/(2x(L+W)xT) SB GT 0.66

good luck

Dave F

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#9460

Re: Aperture Reduction for QFP (fine pitch) | 21 September, 1999

snip

| I've often been tempted to try something a little different when specifying QFP apertures. Like, for example, using long, tapered triangular apertures arranged in alternating directions. The objective being, get the maximum amount of paste down, but also maximise the distance between the paste deposits. If anyone has tried anything along these lines, it would be good to hear about it. | | Scott | Scott: No reason not to try it. Also consider a "zipper pattern stencil." Aptly named, because the paste deposit looks like a zipper.

To do this, make the apertures normal pad width, but one half (minus a little bit) of the normal pad length. Position "every other" aperture so that it prints paste on the outside of portion of the land pattern and have the "not every other" apertures deposit paste on the center of the land pattern. Use solder mask webbing between pads.

Ta

Dave F

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#9461

Re: Aperture Reduction for QFP (fine pitch) | 21 September, 1999

| | | | My stencil thickness is 6 mil and we got qfp's with fine pitch. | | | | how many percent should i reduce my stencil aperture using a 6 mil stencil for 0.5 mm pitch and 0.4 mm pitch ? | | | | | | | | thanks | | | | | | | We use 6 mil lasercut stencils with 15% reduction and that�s working fine at least with our pad design doing manual and automatic printing. With really miserable paddesign for 0,5mm pitch we had more than trouble even with this, so paddesign is the first to look at before changing things that worked so well also in reducing solderballs. | | | | | | There might be other suggestions | | | Wolfgang | | | | | | | | | | | | There are two things that you could do with this one. | | An aperture reduction or a step down. I will agree with wolfgang | | if you are going to reduce the aps go with 15%. A step down would simply reduce the stencil thickness around the QFP so if you ordered a 6 mil stencil I would have the QFP stepped down to 5 mil. Both methods will give you "approx" the same result 15% reduction. The choice is yours, just remember reducing your fine pitch apertures will lead to other problems such as paste release. | | You may find that your aps will clog more and you will get poor solder definition (have to clean you screen more often). What ever you do have the thing LASER CUT!!!!! | | | | Chad | | | | | | | | I've used a 10% aperture reduction for 0,025" pitch QFPs on a 0,006" laser cut stencil with no real problems, so I would agree with all you guys that a 15% reduction for 0,020" pitch or less would be appropriate. (And, as Chad so rightly said, there is no substitute for Laser Cut!) | | Remember, in addition, that when reducing QFP apertures, you only need to specify a reduction on the shortest edge of the aperture. That little bit of extra paste you get on the board by leaving the longest edge equal to the nominal pad length can sometimes make a difference. | | I've often been tempted to try something a little different when specifying QFP apertures. Like, for example, using long, tapered triangular apertures arranged in alternating directions. The objective being, get the maximum amount of paste down, but also maximise the distance between the paste deposits. If anyone has tried anything along these lines, it would be good to hear about it. | | Scott |A trapezoidal shape to the apertures will also aid in paste release. Some lazers (maybe older or poorly maintained?) will leave a fairly rough wall and can benefit from electropolishing after cutting. John Thorup

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se

#9462

Re: Aperture Reduction for QFP (fine pitch) | 22 September, 1999

| | | | My stencil thickness is 6 mil and we got qfp's with fine pitch. | | | | how many percent should i reduce my stencil aperture using a 6 mil stencil for 0.5 mm pitch and 0.4 mm pitch ? | | | | | | | | thanks | | | | | | | We use 6 mil lasercut stencils with 15% reduction and that�s working fine at least with our pad design doing manual and automatic printing. With really miserable paddesign for 0,5mm pitch we had more than trouble even with this, so paddesign is the first to look at before changing things that worked so well also in reducing solderballs. | | | | | | There might be other suggestions | | | Wolfgang | | | | | | | | | | | | There are two things that you could do with this one. | | An aperture reduction or a step down. I will agree with wolfgang | | if you are going to reduce the aps go with 15%. A step down would simply reduce the stencil thickness around the QFP so if you ordered a 6 mil stencil I would have the QFP stepped down to 5 mil. Both methods will give you "approx" the same result 15% reduction. The choice is yours, just remember reducing your fine pitch apertures will lead to other problems such as paste release. | | You may find that your aps will clog more and you will get poor solder definition (have to clean you screen more often). What ever you do have the thing LASER CUT!!!!! | | | | Chad | | | | | | | | I've used a 10% aperture reduction for 0,025" pitch QFPs on a 0,006" laser cut stencil with no real problems, so I would agree with all you guys that a 15% reduction for 0,020" pitch or less would be appropriate. (And, as Chad so rightly said, there is no substitute for Laser Cut!) | | Remember, in addition, that when reducing QFP apertures, you only need to specify a reduction on the shortest edge of the aperture. That little bit of extra paste you get on the board by leaving the longest edge equal to the nominal pad length can sometimes make a difference. | | I've often been tempted to try something a little different when specifying QFP apertures. Like, for example, using long, tapered triangular apertures arranged in alternating directions. The objective being, get the maximum amount of paste down, but also maximise the distance between the paste deposits. If anyone has tried anything along these lines, it would be good to hear about it. | | Scott |

A good chem etch stencil with trapizoidal apetures, electropolished is ok too, down to .020" pitch. Step stencils can be used, but you have to have the real estate around the qfp to properly size the step or it won't print well in the direction of squeegee travel.

Zipper patterns and other offset deposit methods rely on good wetting properties and flux activity, not always a given. Watch for reflow profile changes in offset deposits.

Look at solder paste with smaller solder balls. Look at soldermask hieght and pad(land) coplanarity on the board.

The 15% reduction is a good start, you can get your stencil manufacturer to reduce the apt.s for you if your designs vary alot or you don't have control of the design data in-house.

GoodLuck..Dan.

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SY

#9463

Re: Aperture Reduction for QFP (fine pitch) | 22 September, 1999

| | | | | My stencil thickness is 6 mil and we got qfp's with fine pitch. | | | | | how many percent should i reduce my stencil aperture using a 6 mil stencil for 0.5 mm pitch and 0.4 mm pitch ? | | | | | | | | | | thanks | | | | | | | | | We use 6 mil lasercut stencils with 15% reduction and that�s working fine at least with our pad design doing manual and automatic printing. With really miserable paddesign for 0,5mm pitch we had more than trouble even with this, so paddesign is the first to look at before changing things that worked so well also in reducing solderballs. | | | | | | | | There might be other suggestions | | | | Wolfgang | | | | | | | | | | | | | | | | | There are two things that you could do with this one. | | | An aperture reduction or a step down. I will agree with wolfgang | | | if you are going to reduce the aps go with 15%. A step down would simply reduce the stencil thickness around the QFP so if you ordered a 6 mil stencil I would have the QFP stepped down to 5 mil. Both methods will give you "approx" the same result 15% reduction. The choice is yours, just remember reducing your fine pitch apertures will lead to other problems such as paste release. | | | You may find that your aps will clog more and you will get poor solder definition (have to clean you screen more often). What ever you do have the thing LASER CUT!!!!! | | | | | | Chad | | | | | | | | | | | | I've used a 10% aperture reduction for 0,025" pitch QFPs on a 0,006" laser cut stencil with no real problems, so I would agree with all you guys that a 15% reduction for 0,020" pitch or less would be appropriate. (And, as Chad so rightly said, there is no substitute for Laser Cut!) | | | | Remember, in addition, that when reducing QFP apertures, you only need to specify a reduction on the shortest edge of the aperture. That little bit of extra paste you get on the board by leaving the longest edge equal to the nominal pad length can sometimes make a difference. | | | | I've often been tempted to try something a little different when specifying QFP apertures. Like, for example, using long, tapered triangular apertures arranged in alternating directions. The objective being, get the maximum amount of paste down, but also maximise the distance between the paste deposits. If anyone has tried anything along these lines, it would be good to hear about it. | | | | Scott | | | | A good chem etch stencil with trapizoidal apetures, electropolished is ok too, down to .020" pitch. Step stencils can be used, but you have to have the real estate around the qfp to properly size the step or it won't print well in the direction of squeegee travel. | | Zipper patterns and other offset deposit methods rely on good wetting properties and flux activity, not always a given. Watch for reflow profile changes in offset deposits. | | Look at solder paste with smaller solder balls. Look at soldermask hieght and pad(land) coplanarity on the board. | | The 15% reduction is a good start, you can get your stencil manufacturer to reduce the apt.s for you if your designs vary alot or you don't have control of the design data in-house. | | GoodLuck..Dan. | | I agree with laser cut stencils. If you are going to the stencil step-down, metal squeezees cannot be used. You may have the following options: 1) Use 6 mil stencil. Have a pad width of 11.5mil for 25 mil pitch and 8.5-9.0 mil for 20 mil pitch with prinitng at 45deg 2) Use 5 mil stencil. have a pad width of 12 mil for 25mil pitch and 8 mil for 20 mil pitch Hope it helps. Regards........SY .

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park kyung sam

#9464

Re: Aperture Reduction for QFP (fine pitch) | 30 September, 1999

| My stencil thickness is 6 mil and we got qfp's with fine pitch. | how many percent should i reduce my stencil aperture using a 6 mil stencil for 0.5 mm pitch and 0.4 mm pitch ? | | thanks | we have placed 16 mil components for 3 years. why don't you think of different aperture shape. just like peanut shape................... don't think the aspect ratio. just think defect free of 16mil component. as you know the solder paste is tacky good luck.....

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Steve Thomas

#9465

Re: Aperture Reduction for QFP (fine pitch) | 7 November, 1999

And you still have sufficient volume left when you do that? I'm new to this, so I'm not questioning the validity of your statement, just soaking up what I can.....thanks.

Steve

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#9466

Re: Aperture Reduction for QFP (fine pitch) | 9 November, 1999

Steve: Sure, but let's figure it out for you!!! ;-) Here�s some sample calculations. Your results may vary, void where prohibited, etc

20 PITCH:

Pinched aperture volume: 0.010"X0.050"X0.005" = 2.5 uin^3 Zippered aperture volume: (0.015"X~0.5)X0.063"X0.006" = ~2.8 uin^3

16 PITCH:

Pinched aperture volume: 0.007"X0.050"X0.005" = 1.8 uin^3 Zippered aperture volume: (0.012"X~0.5)X0.063"X0.006" = ~2.3 uin^3

Since I figured this for you, you need to report back to us on which of these pinched or zippered apertures are easier to print.

Ta

Dave F

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