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Pin in Paste assistance

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#85697

Pin in Paste assistance | 29 September, 2020

Hi,

We are currently working on a board that has 3x 50-pin THT at 1.27mm pitch each. This is too small for us to be able to consistently do on our selective solder. We are instead considering pin in paste. The connector we are using can be spec'd at high-temp and withstands reflow, so we are good there. I originally was planning on using 0603 solder preforms, but there is no space under the connector, so that won't work.

It seems we have two options:

1) Use a step stencil to handle the extra paste

2) Use washer preforms.

3) This is a double-sided board. We could populate the three connectors in the bottom side process, gluing it down. (They would be upside down in the pick and place.) Then, place the 0603 preforms with the pick and place on the bottom side of the board, reflow the board with the connectors still upside down.

With option one, I use the indium calculator and find that we would need 63'000 cubic mils. This would require an 11 mil thick stencil, and an aperture of 128 mils x 45 mils. We have SMD parts that are near these connectors and are planning on using a 4 mil thick stencil for the rest of it. I fear that the step-up will make the parts near the THT have excess paste, as they won't be able to have full contact with the squeege. Of course, I could make the aperture larger, and the stencil thinner, but I don't know how far the aperture can be away from the pad and still wick solder in.

For option 2, I really don't want to have to hand place 150 washers on each pin. Indium's matrix array is too expensive, and I've heard that since they are connected, the solder will wick away to nearby pins.

Option 3 would work I think. I've just never heard of this method being done before.

Included is the board image, connector datasheet, and my current proposed aperture and pad sizing.

Thank you for any assistance

Attachments:

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#85699

Pin in Paste assistance | 30 September, 2020

I would prefer wave for this parts, but if it not an option, cut 6-8mil stencil and double stroke it on the printer. All the rest is adding additional steps/labor/cost that is not worth it.

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#85700

Pin in Paste assistance | 30 September, 2020

If you could change the design, a 2x25 1.27mm pitch surface mount connector is a standard part from Wurth. Samtec also do versions in any pin count you might desire, with locating pins, without, with picking clip etc.

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#85705

Pin in Paste assistance | 30 September, 2020

> If you could change the design, a 2x25 1.27mm
> pitch surface mount connector is a standard part
> from Wurth. Samtec also do versions in any pin
> count you might desire, with locating pins,
> without, with picking clip etc.

Oh, I would if I could! I've tried getting them to switch, but they don't want to due to mechanical stress concerns..

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#85706

Pin in Paste assistance | 1 October, 2020

Stress concerns, spread over 50 pins. Madness, but then people do still seem to have a level of paranoia with SMT. Here we'd suck it up and handsolder them, and the operatives would not be happy about it.

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