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

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solder mask between qfp pads

Gerry S.

#10140

solder mask between qfp pads | 14 August, 1999

One of the DFM suggestions I've seen is to remove the solder mask web, i.e. the mask between pads on 20-mil QFP devices. The rational is that if the mask is thicker than the pad the mask can prevent the stencil from making intimate contact with the pad and allowing solder paste to squeeze under the stencil and encouraging solder bridging. Another school of thought seems to be that the solder mask prevents bridging between pads. Anyone have experience/preference in solder mask or lack of solder mask between pads?

Gerry

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Dean

#10141

Re: solder mask between qfp pads | 15 August, 1999

| One of the DFM suggestions I've seen is to remove the solder mask web, i.e. the mask between pads on 20-mil QFP devices. The rational is that if the mask is thicker than the pad the mask can prevent the stencil from making intimate contact with the pad and allowing solder paste to squeeze under the stencil and encouraging solder bridging. Another school of thought seems to be that the solder mask prevents bridging between pads. Anyone have experience/preference in solder mask or lack of solder mask between pads? | | Gerry | I have enciountered both techniques in CM and have heard very little of performance enhancements other than better gasketing. As far as preventing bridging, I think its a draw. Both solder mask coatings and glass / epoxy act as solder resist. win - win situation. If you're solder is bridge, maby there are issues in your print process (alignment, repeatability, etc) or P&P process.

Dean

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Earl Moon

#10142

Re: solder mask between qfp pads | 16 August, 1999

| | One of the DFM suggestions I've seen is to remove the solder mask web, i.e. the mask between pads on 20-mil QFP devices. The rational is that if the mask is thicker than the pad the mask can prevent the stencil from making intimate contact with the pad and allowing solder paste to squeeze under the stencil and encouraging solder bridging. Another school of thought seems to be that the solder mask prevents bridging between pads. Anyone have experience/preference in solder mask or lack of solder mask between pads? | | | | Gerry | | | I have enciountered both techniques in CM and have heard very little of performance enhancements other than better gasketing. As far as preventing bridging, I think its a draw. Both solder mask coatings and glass / epoxy act as solder resist. win - win situation. If you're solder is bridge, maby there are issues in your print process (alignment, repeatability, etc) or P&P process. | | Dean | I agree with Dean. Solder mask between fine pitch pads, or any others for that matter do little to prevent bridging. Paste application issue resolution should be first targeted for prevention.

Earl Moon

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Charles Stringer

#10143

Re: solder mask between qfp pads | 16 August, 1999

| | | One of the DFM suggestions I've seen is to remove the solder mask web, i.e. the mask between pads on 20-mil QFP devices. The rational is that if the mask is thicker than the pad the mask can prevent the stencil from making intimate contact with the pad and allowing solder paste to squeeze under the stencil and encouraging solder bridging. Another school of thought seems to be that the solder mask prevents bridging between pads. Anyone have experience/preference in solder mask or lack of solder mask between pads? | | | | | | Gerry | | | | | I have enciountered both techniques in CM and have heard very little of performance enhancements other than better gasketing. As far as preventing bridging, I think its a draw. Both solder mask coatings and glass / epoxy act as solder resist. win - win situation. If you're solder is bridge, maby there are issues in your print process (alignment, repeatability, etc) or P&P process. | | | | Dean | | | I agree with Dean. Solder mask between fine pitch pads, or any others for that matter do little to prevent bridging. Paste application issue resolution should be first targeted for prevention. | | Earl Moon | Another influencing factor may be your board finish. If you are having to accept HASL boards for fine pitch (Yes it does happen!) then the solder levelling process can rip the thin webs of resist off the PCA and deposit them all over the fine pitch pads!

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Peter Barton

#10144

Re: solder mask between qfp pads | 16 August, 1999

| | | | One of the DFM suggestions I've seen is to remove the solder mask web, i.e. the mask between pads on 20-mil QFP devices. The rational is that if the mask is thicker than the pad the mask can prevent the stencil from making intimate contact with the pad and allowing solder paste to squeeze under the stencil and encouraging solder bridging. Another school of thought seems to be that the solder mask prevents bridging between pads. Anyone have experience/preference in solder mask or lack of solder mask between pads? | | | | | | | | Gerry | | | | | | | I have enciountered both techniques in CM and have heard very little of performance enhancements other than better gasketing. As far as preventing bridging, I think its a draw. Both solder mask coatings and glass / epoxy act as solder resist. win - win situation. If you're solder is bridge, maby there are issues in your print process (alignment, repeatability, etc) or P&P process. | | | | | | Dean | | | | | I agree with Dean. Solder mask between fine pitch pads, or any others for that matter do little to prevent bridging. Paste application issue resolution should be first targeted for prevention. | | | | Earl Moon | | | Another influencing factor may be your board finish. If you are having to accept HASL boards for fine pitch (Yes it does happen!) then the solder levelling process can rip the thin webs of resist off the PCA and deposit them all over the fine pitch pads! | | Gerry,

I agree with all the replies so far and would like to add that having a block window in the resist around the pads rather than having individual webs between also makes the PCB easier to make in the first instance. Less possibility of resist encroachment onto pads due to any misalignment when imaging the resist. I,m pretty sure the PCB manufacturer would prefer the block approach.

Pete B.

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Scott Cook

#10145

Re: solder mask between qfp pads | 16 August, 1999

| One of the DFM suggestions I've seen is to remove the solder mask web, i.e. the mask between pads on 20-mil QFP devices. The rational is that if the mask is thicker than the pad the mask can prevent the stencil from making intimate contact with the pad and allowing solder paste to squeeze under the stencil and encouraging solder bridging. Another school of thought seems to be that the solder mask prevents bridging between pads. Anyone have experience/preference in solder mask or lack of solder mask between pads? | Gerry,

MHO.....

It's all relative. Let's go to root cause; all else is workaround. Develop a solid printing process (including apertures in the foils), and go with a solid board house. FLAT PADS. However you accomplish this is really a biz decision. Once accomplished, you are done. No bridging on 20 mil parts. I've used step foils, keyhole apertures, reduced apertures, and mask between pads. It all boils down to a good printing process, a good placement process, and a good board house. Sorry, but I believe in simplicity.

Scott Cook scook@nettally.com

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#10146

Re: solder mask between qfp pads | 18 August, 1999

| | | | | One of the DFM suggestions I've seen is to remove the solder mask web, i.e. the mask between pads on 20-mil QFP devices. The rational is that if the mask is thicker than the pad the mask can prevent the stencil from making intimate contact with the pad and allowing solder paste to squeeze under the stencil and encouraging solder bridging. Another school of thought seems to be that the solder mask prevents bridging between pads. Anyone have experience/preference in solder mask or lack of solder mask between pads? | | | | | | | | | | Gerry | | | | | | | | | I have enciountered both techniques in CM and have heard very little of performance enhancements other than better gasketing. As far as preventing bridging, I think its a draw. Both solder mask coatings and glass / epoxy act as solder resist. win - win situation. If you're solder is bridge, maby there are issues in your print process (alignment, repeatability, etc) or P&P process. | | | | | | | | Dean | | | | | | | I agree with Dean. Solder mask between fine pitch pads, or any others for that matter do little to prevent bridging. Paste application issue resolution should be first targeted for prevention. | | | | | | Earl Moon | | | | | Another influencing factor may be your board finish. If you are having to accept HASL boards for fine pitch (Yes it does happen!) then the solder levelling process can rip the thin webs of resist off the PCA and deposit them all over the fine pitch pads! | | | | Gerry, | | I agree with all the replies so far and would like to add that having a block window in the resist around the pads rather than having individual webs between also makes the PCB easier to make in the first instance. Less possibility of resist encroachment onto pads due to any misalignment when imaging the resist. I,m pretty sure the PCB manufacturer would prefer the block approach. | | Pete B. | | Peter: Help us understand a "block window." Thanks. Dave F

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PeterB

#10147

Re: solder mask between qfp pads | 20 August, 1999

| | | | | | One of the DFM suggestions I've seen is to remove the solder mask web, i.e. the mask between pads on 20-mil QFP devices. The rational is that if the mask is thicker than the pad the mask can prevent the stencil from making intimate contact with the pad and allowing solder paste to squeeze under the stencil and encouraging solder bridging. Another school of thought seems to be that the solder mask prevents bridging between pads. Anyone have experience/preference in solder mask or lack of solder mask between pads? | | | | | | | | | | | | Gerry | | | | | | | | | | | I have enciountered both techniques in CM and have heard very little of performance enhancements other than better gasketing. As far as preventing bridging, I think its a draw. Both solder mask coatings and glass / epoxy act as solder resist. win - win situation. If you're solder is bridge, maby there are issues in your print process (alignment, repeatability, etc) or P&P process. | | | | | | | | | | Dean | | | | | | | | | I agree with Dean. Solder mask between fine pitch pads, or any others for that matter do little to prevent bridging. Paste application issue resolution should be first targeted for prevention. | | | | | | | | Earl Moon | | | | | | | Another influencing factor may be your board finish. If you are having to accept HASL boards for fine pitch (Yes it does happen!) then the solder levelling process can rip the thin webs of resist off the PCA and deposit them all over the fine pitch pads! | | | | | | Gerry, | | | | I agree with all the replies so far and would like to add that having a block window in the resist around the pads rather than having individual webs between also makes the PCB easier to make in the first instance. Less possibility of resist encroachment onto pads due to any misalignment when imaging the resist. I,m pretty sure the PCB manufacturer would prefer the block approach. | | | | Pete B. | | | | | Peter: Help us understand a "block window." Thanks. Dave F | Dave

Sorry, maybe my description of this style style of resist aperture is open to interpretation. What I meant by this is instead of having individual apertures around each of the component pads a single rectangular aperture can be applied along each row. Imagine apertures around all of the individual pads. Remove all of the webs in between and you are left with one long rectangular aperture. On a QFP240 for instance, instead of having 240 apertures there would be 4 rectangular apertures, one along each row of pads.

Hope this clarifies matters.

Pete B.

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