Tips on QFP breakout board soldering

in techniques by the machinegeek | 9 comments

QFPboard
Joe Desbonnet writes in with two tips he learned when using the DP QFP breakout board.

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Comments

  1. Jay Wilkinson says:

    Lot of great articles there. But be warned there is excessive use of the word “dandy”. lol

  2. Jay Wilkinson says:

    Oh a third unmentioned tip seen in the pictures is to back light to board to look for solder bridges and such.

    And someone should redesign that board with a second pitch size on the back. With 3 boards you could have each variation of the possible combinations.

  3. Drone says:

    Man that site is really sluggish. blog.blog on top of blogspot? It’s like blog bloat on top of blot bloat. And scripting galore too. But the content is interesting enough to suffer through the experience.

  4. vimark says:

    QFP breakout with second pitch on the back coming up. Very nice demo you got there

  5. arhi says:

    Using wooden peg instead of plastic one is lot better.

  6. Joe Desbonnet says:

    @Drone: Thanks for the feedback on that Blogger template. I switched it recently from an older template to one of the newer ones. I initially though it was an improvement, but I’m having second thoughts now.

    Also update on the solder mask observation: that worked perfectly for an Analog Devices ADAS 1000 64 pin LQFP, but now trying a 32 pin LQFP TI ADS1292R and the pads are shorter… so that tip doesn’t work for all packages.

  7. ericwertz says:

    I’m not sure that I love the clothes pin trick. The problem is that by forcing the package’s pin down hard to the surface of the pad, you essentially end up with a giant void between the pin and the pad because there’s neither solder or flux between the two.

    If one were to do this, I’d suggest going back and reflowing the initial tacked-down pin(s) with at least extra flux before any of the other pins were soldered. A better strategy would be to pin the chip down and only tack down one pin, then remove the clothes pin, tack down the opposite one, and then reflow the first (perhaps after tacking down one of his neighbor’s if you’re paranoid).

    I do think that it’s highly desirable to float the package’s pin’s bottom on solder. IPC quite possibly wouldn’t pass the joint otherwise (if you’re being particularly anal about the quality of your solder joints).

    Your observation about the solder mask not always working as a great guide is likely to be even more true with breakout boards designed for hand-soldering. Such breakouts are often laid-out with longer pads (and correspondingly less solder mask) to make it easier to hand-solder. This extra play will work against you when trying to rely on using the mask to hold the part into place.

  8. Joe Desbonnet says:

    I have no clue about PCB manufacturing: but if you could make the solder mask thicker… could that be a physical alignment guide (ie the solder mask ridge in between each IC pin pad). Probably not commercially viable even if it was possible.

    • ericwertz says:

      I thought that that was half the point of your blog post – using the soldermask as an alignment guide. The short answer is that it *can* act as a guide as it is, if you take care to lay down the solder mask exactly where you want it (subject to the tolerances in the PCB house’s process). Getting a thicker solder mask would be possible, but I’m guessing that if you have to ask how much it would cost, you probably can’t afford it (as the saying goes). In reality, I suspect that what you’d be asking-for/getting would be multiple solder mask layers, not a thicker one.

      For example, the first Google ADK Accessory Board had a 5-6 layer solder mask (one for each of the colors in the Google logo on the board), and it was said to have cost them a bunch to have them done that way. It may actually be the case that you can’t have the layers stacked, but rather non-overlapping — I don’t have a clue.

      Ideally what you might be thinking that you want is something like how Schmartboards work, where channels (that are probably milled into the board) hold the pins in place in recess.

      The practical alternative would be to do/add your own solder mask as is done with the Fab-In-A-Box (pcbfx.com) if you really had your heart set on it.

      Personally, I don’t think that it’s worth any extra effort as I think that it really only takes an extra 5-20 seconds to get the chip aligned well. If you needed to do 100, then Plan B is probably getting a $20+ kapton/mylar stencil and reflowing it yourself. You might even consider hand-dispensing solder paste to the opposing pins/pads, reflowing it (and having it self-align), then soldering the rest by hand, if you were so inclined.

      Just place a $10 5x5cm order with SeeedStudio with as many like footprints on it with varying soldermask dimensions and see if any of them work well enough to your choosing. I’m guessing what you’ll find is that the snugger your solder mask, the more *difficult* hand-soldering will be if you put a premium on getting a “proper” solder fillet from pin to pad.

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