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Best way to ensure max solder load on SOIC w Thermal Pad

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#63092

Best way to ensure max solder load on SOIC w Thermal Pad | 9 November, 2010

Hi SMT World,

I am facing a technical challenge related to an SOIC 14lds with thermal pad on it's belly that needs to be soldered onto PCB, together with leads. It is a 50 per board, expected 80% solder load on every one.

We have some units without solder contact at the belly thermal pad. Stencil apperture is 85x125mils and pad size is 80x120mils.

Due to the gullwing design of the SOIC, there is always a 5mil step from lowest point of leads to the belly. This is the air gap that we "hoped" to somehow covered when solder paste melted during reflow. But we can't put a control or proper process in place ensuring repeatability.

What's the proper way to overcome this challenge and ensuring repeatability?

Thanks for your advice and experience sharing.

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#63094

Best way to ensure max solder load on SOIC w Thermal Pad | 9 November, 2010

What's your stencil thickness?

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#63097

Best way to ensure max solder load on SOIC w Thermal Pad | 9 November, 2010

Stencils are 5mils thickness, SS material. Printer is Accela.

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#63101

Best way to ensure max solder load on SOIC w Thermal Pad | 10 November, 2010

The 5 thou standoff of your SOIC is probably the minimum. We wouldn't be surprised to see 10 thou. With this, you could have maybe a 6 thou gap between printed solder paste and the bottom of the heat slug.

So, you're expecting the rabbit ears of paste that often occur after printing and the solder swell during reflow to fill the standoff gap, right?

Can you increase your placement down pressure to flex the SOIC leads and better press the slug into the paste?

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#63119

Best way to ensure max solder load on SOIC w Thermal Pad | 12 November, 2010

I wouldn't count on solder bridging that kind of gap. I've never used solder preforms, but I wonder if that would be worthwhile to try?

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#63126

Best way to ensure max solder load on SOIC w Thermal Pad | 14 November, 2010

Hopefully you have a dispenser on your machine so that you can add a few large dots of paste to the thermal pad.

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