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Pin in Paste assistance

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Pin in Paste assistance | 29 September, 2020


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


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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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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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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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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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Pin in Paste assistance | 3 December, 2020

Hey Kyle - Good for you for pushing your horizons and considering pin-in-paste (aka "Intrusive Soldering"). Don't let the nay-sayers out there discourage you. When I encounter mixed tech PCBAs (SMT and through-hole) my first choice, if the design and components can support it is Pin-in-Paste: Much more controllable and repeatable than wave, wave with aperture fixtures, hand and even selective. Over the years, many of us have devloped Pin-in-Paste processes including myself, Joe Belmonte when he was at Motorola and Bob Willis -among others. You can find my paper on it on the ITM website ( and Bob Willis has published some good stuff on this. Of course you have to be sure the components on the PCBA can endure the thermal excursions experienced in your reflow profile for the PCBA. A lot more can than a few years ago. Beyond that, in a nutshell, you may have to change your lead-to-hole ratio. I like to have the inside hole diameter 12 mil over the lead diagonal measure for square/rectangular leads and 10 mil over the lead diameter for round leads. Go ahead an overprint, even beyond the annular ring. Sounds counter-logical but you should not get solder balling (due to this, anyway) since the solder is a continuous mass. If you find you do need to supplement the solder volume try using the "solder slug" preforms that are roughly the size of an 0805 and come on tape and reel. Your pick and place puts them in the solder deposit and treats it like a component - sure beats those donuts. Keep you below PCBA lead protrusion to less than 30 mil (to avoid icicles) So, read up, and have fun developing your process. Phil Z

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Pin in Paste assistance | 4 December, 2020

As a side note, you get to clean up all the paste that pukes on the bottom heaters of your reflow oven.

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Pin in Paste assistance | 11 December, 2020

You can run it on a carrier board to catch any paste that falls.

When we do Pin in Paste, we sometimes over print the apertures a bit to get the extra paste volume.

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Pin in Paste assistance | 15 December, 2020

Have you tried cutting the leads shorter (or lift the connector slightly) so that they barely protrude at the other side of the PCB? Just for testing purpose, for mass production you should find a connector with shorter leads then. This should help with solder bridging, not sure that's the main issue here though.

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Pin in Paste assistance | 17 December, 2020

for option 3, if you place the connectors on the first pass then flip the board, won't the protruding leads interfere with second pass stencil printing?

My vote would be to overprint onto the solder mask. you can print pretty far and still have it wick into the hole. It probably depends on the paste, mask and trace layout but I'd be you could print 2 or 3 times the diameter of the pad and still have it wick properly.

Maybe for a DOE, you could order a stencil with block of increasingly larger over prints and see what gives the best outcome.

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Pin in Paste assistance | 17 December, 2020

I think over printing is the best option, and I like the idea of ordering a test stencil with increasing sized apertures. Make sure you go beyond what you think will work, just so you know where line is.

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Pin in Paste assistance | 17 December, 2020

I have found that if you keep lead protrusion below the PCB at less than or equal to 0.030", you will not get icicling or "paste pukes"...

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Pin in Paste assistance | 20 January, 2021

Hi, Option 3 is quite often used, while it is better to glue with uv glue using uv lamps for curing. For this process, two inverters are needed in the line, a dispenser for UV glue and a UV lamp (eg metronelec UV CURE OVEN). It is better to use preforms to increase paste volume if over print not prefereble. Best regards, Denys


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