20 mil qfp bridging

Hello

Having a problem with bridging on 20mil pitch qfp, only on the lead groups where squeegee goes over width of apertures. Using MPM sp-200 semi-auto, chem-etch stencil, 9-10mil width opening 100 durometer blade. Getting "tails" on deposits. Tried new blade, different head speed, snap-off, downstop, anyone got any other ideas. Thanks in advance

wayne

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| Hello | | Having a problem with bridging on 20mil pitch qfp, only on the lead groups where squeegee goes over width of apertures. Using MPM sp-200 semi-auto, chem-etch stencil, 9-10mil width opening | 100 durometer blade. Getting "tails" on deposits. Tried new blade, different head speed, snap-off, downstop, anyone got any other ideas. Thanks in advance | | wayne |

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| | Hello | | | | Having a problem with bridging on 20mil pitch qfp, only on the lead groups where squeegee goes over width of apertures. Using MPM sp-200 semi-auto, chem-etch stencil, 9-10mil width opening | | 100 durometer blade. Getting "tails" on deposits. Tried new blade, different head speed, snap-off, downstop, anyone got any other ideas. Thanks in advance | | | | wayne | | | | Wayne:

One thing to check.....

How's the squeegee cleaning off the paste as it travels across the stencil? If you're getting a thin film of paste left in any areas across the stencil, first check to be sure you have good support under your board. If it can flex away from the stencil as the squeegee travels across it, you will not get good clean release of paste from stencil and may get the "tails" you referred to.

After printing a PCB, examine the problem aperatures. Is there paste left sticking in the ends of some of the aperatures?

Slower speeds can help if you cannot get good support. Also, you may try slowing the separation speed of the stencil, and/or add delay time before the stencil begins to release. These "band-aids" may help but, will add cycle time.

As you work with narrower aperatures, support under your PCB becomes more critical.

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| | | Hello | | | | | | Having a problem with bridging on 20mil pitch qfp, only on the lead groups where squeegee goes over width of apertures. Using MPM sp-200 semi-auto, chem-etch stencil, 9-10mil width opening | | | 100 durometer blade. Getting "tails" on deposits. Tried new blade, different head speed, snap-off, downstop, anyone got any other ideas. Thanks in advance | | | | | | wayne | | | | | | | Wayne: | | One thing to check..... | | How's the squeegee cleaning off the paste as it travels across the stencil? If you're getting a thin film of paste left in any areas across the stencil, first check to be sure you have good support under your board. If it can flex away from the stencil as the squeegee travels across it, you will not get good clean release of paste from stencil and may get the "tails" you referred to. | | After printing a PCB, examine the problem aperatures. Is there paste left sticking in the ends of some of the aperatures? | | Slower speeds can help if you cannot get good support. Also, you may try slowing the separation speed of the stencil, and/or add delay time before the stencil begins to release. These "band-aids" may help but, will add cycle time. | | As you work with narrower aperatures, support under your PCB becomes more critical. | | | | | WAYNE, Another thing you might want to look into if the above suggestions fall short of a fix is your humidity levels. Not sure where you are located but being summer time this is known to happen. |

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| | | | Hello | | | | | | | | Having a problem with bridging on 20mil pitch qfp, only on the lead groups where squeegee goes over width of apertures. Using MPM sp-200 semi-auto, chem-etch stencil, 9-10mil width opening | | | | 100 durometer blade. Getting "tails" on deposits. Tried new blade, different head speed, snap-off, downstop, anyone got any other ideas. Thanks in advance | | | | | | | | wayne | | | | | | | | | | Wayne: | | | | One thing to check..... | | | | How's the squeegee cleaning off the paste as it travels across the stencil? If you're getting a thin film of paste left in any areas across the stencil, first check to be sure you have good support under your board. If it can flex away from the stencil as the squeegee travels across it, you will not get good clean release of paste from stencil and may get the "tails" you referred to. | | | | After printing a PCB, examine the problem aperatures. Is there paste left sticking in the ends of some of the aperatures? | | | | Slower speeds can help if you cannot get good support. Also, you may try slowing the separation speed of the stencil, and/or add delay time before the stencil begins to release. These "band-aids" may help but, will add cycle time. | | | | As you work with narrower aperatures, support under your PCB becomes more critical. | | | | | | | | | | WAYNE, | Another thing you might want to look into if the above suggestions fall short of a fix is your humidity levels. Not sure where you are located but being summer time this is known to happen. | | | | How often are you cleaning the bottom side of the stencil? Some say after every print but I clean after every third print with good success. Much more than that and you'll start getting those shorts.

Hope this helps, Scott

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| Hello | | Having a problem with bridging on 20mil pitch qfp, only on the lead groups where squeegee goes over width of apertures. Using MPM sp-200 semi-auto, chem-etch stencil, 9-10mil width opening | 100 durometer blade. Getting "tails" on deposits. Tried new blade, different head speed, snap-off, downstop, anyone got any other ideas. Thanks in advance | | wayne | Wayne: The squeegee goes over the width (and length) of all apertures, bud. ;-)

You probably have done quite a bit of work refining your process, but for 20 pitch we use:

* 6 mil electropolished stencils * metal squeegee blades * 9X50 mil 20 pitch apertures * aspect ratios around 1.5 * type 3 paste

As you say, the amount of paste deposited is dependent on the direction of the print stoke. This is because the paste roll only has 9 mil available to fill the aperture when its length that is perpendicular to the direction of the squeegee movement. And it just goes by so fast from paste perspective that it can't over-come inertia and the thixotropic "glue" that holds it to gether. I know that's what you said!!!! I was just bustin' ya. ;-) Where as the other apertures present 50 mils to the paste roll for filling.

When this becomes limiting, we:

* Use a looser type 4 paste * Print slower * Under duress, print with the part at 45� to the stroke.

Now on your getting tails, either:

* Squeegee is magnetizing the stencil and this magnetic field is ... * Poor release caused by solder paste properties, as others have said, or / and rough aperture walls of your chem-etch stencil.

Good luck

Dave F

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| Hello | | Having a problem with bridging on 20mil pitch qfp, only on the lead groups where squeegee goes over width of apertures. Using MPM sp-200 semi-auto, chem-etch stencil, 9-10mil width opening | 100 durometer blade. Getting "tails" on deposits. Tried new blade, different head speed, snap-off, downstop, anyone got any other ideas. Thanks in advance | | wayne |

For 20 mil pitch, I prefer to use a 6 mil band-etch stencil and a stainless steel blade. Laser cut will work as well but we get great results from band-etch and have found it to be cheaper than laser cut.

If you are only getting bridging on one axis, it may be a result of excess solder buildup from the HASL process. A lot of people are having good success with 20 mil using HASL boards, but I prefer the flatter surface you get from immersion gold, OSP or immersion tin processes.

Steve

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| | | | | Hello | | | | | | | | | | Having a problem with bridging on 20mil pitch qfp, only on the lead groups where squeegee goes over width of apertures. Using MPM sp-200 semi-auto, chem-etch stencil, 9-10mil width opening | | | | | 100 durometer blade. Getting "tails" on deposits. Tried new blade, different head speed, snap-off, downstop, anyone got any other ideas. Thanks in advance | | | | | | | | | | wayne | | | | | | | | | | | | | Wayne: | | | | | | One thing to check..... | | | | | | How's the squeegee cleaning off the paste as it travels across the stencil? If you're getting a thin film of paste left in any areas across the stencil, first check to be sure you have good support under your board. If it can flex away from the stencil as the squeegee travels across it, you will not get good clean release of paste from stencil and may get the "tails" you referred to. | | | | | | After printing a PCB, examine the problem aperatures. Is there paste left sticking in the ends of some of the aperatures? | | | | | | Slower speeds can help if you cannot get good support. Also, you may try slowing the separation speed of the stencil, and/or add delay time before the stencil begins to release. These "band-aids" may help but, will add cycle time. | | | | | | As you work with narrower aperatures, support under your PCB becomes more critical. | | | | | | | | | | | | | | | WAYNE, | | Another thing you might want to look into if the above suggestions fall short of a fix is your humidity levels. Not sure where you are located but being summer time this is known to happen. | | | | | | | | How often are you cleaning the bottom side of the stencil? Some say after every print but I clean after every third print with good success. Much more than that and you'll start getting those shorts. | | Hope this helps, | Scott | Couple of questions: 1- Are the apertures micro-reduced? 2- What is the stencil thickness? 3- what are the temperature and humidity in the plant? I would suggest trying a 6 mil thick laser-etched (and electro-polished)stencil, with 10-20% micro-reduction of the apertures for the fine pitch part. The laser etch gives a cleaner wall and a trapezoidal shape to the aperture which will facilitate a cleaner release from the stencil. Additionally, the laser etch, when combined with eletro-polishing, smooths the wall of the aperture out. The chem etch wall is typically rough, which can cause some deformation of the paste as it releases. Depending on the paste, temperature and humidity can cause the paste to slump, creating another possible cause of bridging.

Brian

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| | | | | | Hello | | | | | | | | | | | | Having a problem with bridging on 20mil pitch qfp, only on the lead groups where squeegee goes over width of apertures. Using MPM sp-200 semi-auto, chem-etch stencil, 9-10mil width opening | | | | | | 100 durometer blade. Getting "tails" on deposits. Tried new blade, different head speed, snap-off, downstop, anyone got any other ideas. Thanks in advance | | | | | | | | | | | | wayne | | | | | | | | | | | | | | | | Wayne: | | | | | | | | One thing to check..... | | | | | | | | How's the squeegee cleaning off the paste as it travels across the stencil? If you're getting a thin film of paste left in any areas across the stencil, first check to be sure you have good support under your board. If it can flex away from the stencil as the squeegee travels across it, you will not get good clean release of paste from stencil and may get the "tails" you referred to. | | | | | | | | After printing a PCB, examine the problem aperatures. Is there paste left sticking in the ends of some of the aperatures? | | | | | | | | Slower speeds can help if you cannot get good support. Also, you may try slowing the separation speed of the stencil, and/or add delay time before the stencil begins to release. These "band-aids" may help but, will add cycle time. | | | | | | | | As you work with narrower aperatures, support under your PCB becomes more critical. | | | | | | | | | | | | | | | | | | | | WAYNE, | | | Another thing you might want to look into if the above suggestions fall short of a fix is your humidity levels. Not sure where you are located but being summer time this is known to happen. | | | | | | | | | | | | How often are you cleaning the bottom side of the stencil? Some say after every print but I clean after every third print with good success. Much more than that and you'll start getting those shorts. | | | | Hope this helps, | | Scott | | | Couple of questions: | 1- Are the apertures micro-reduced? | 2- What is the stencil thickness? | 3- what are the temperature and humidity in the plant? | I would suggest trying a 6 mil thick laser-etched (and electro-polished)stencil, with 10-20% micro-reduction of the apertures for the fine pitch part. | The laser etch gives a cleaner wall and a trapezoidal shape to the aperture which will facilitate a cleaner release from the stencil. Additionally, the laser etch, when combined with eletro-polishing, smooths the wall of the aperture out. The chem etch wall is typically rough, which can cause some deformation of the paste as it releases. | Depending on the paste, temperature and humidity can cause the paste to slump, creating another possible cause of bridging. | | Brian P.S. One other thing. Make sure the stencil is being cleaned on a routine basis during production. Solder residue left on the bottom of the stencil can also be a cause of the bridging.

Brian

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Wayne,

Sounds like a gasketing problem. When the horizontal apertures are bleeding but not the vertical apertures. Could be poor underside support, HASL miniscus issues, masking or plugging issues, but what many people do not account for is that typically a PCB shop will etch the apertures on the lower spec limit. If the aperture is supposed to be .012" +/-10%, check to make sure the aperture is not .0108". If you went 1:1 assuming .012", you have lost your gasket. If you went chem etch, then there is even a possibility the apertures are oversized. When the process is not working, assume nothing, verify everything. Hope this helps.

Steve A

| Hello | | Having a problem with bridging on 20mil pitch qfp, only on the lead groups where squeegee goes over width of apertures. Using MPM sp-200 semi-auto, chem-etch stencil, 9-10mil width opening | 100 durometer blade. Getting "tails" on deposits. Tried new blade, different head speed, snap-off, downstop, anyone got any other ideas. Thanks in advance | | wayne |

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