An Unanswered Question

I’ve been reading through my Virtual PCB chat session transcript from Tuesday. It was a fun session and I have a much better idea of how the virtual shows work now. I think I may just be getting it.

The chat session had a lot of interesting questions and dialog. I did notice, however, that I missed one question and thus didn’t answer it. Oops.

Owen asked if I am of the opinion that all footprints should have rounded pads (probably stencil cutouts, too) to help with paste release. Sorry I missed your question.

I’m not of that opinion. There are a lot of factors that come out of stencil decisions. Paste release is one of them. There are others, some more important. For example, the shape of a pad and stencil cut out can either encourage or discourage solder balls. The size of the opening can put too much or too little paste on the pad. Wide open cut-outs over heat slugs can cause float.Bad QFN paste w caption

The pads themselves, should follow the part manufacturers recommendation for shape and size. Most are rectangular. BGAs have round pads. Unless you have a very good and very specific reason, I would not deviate far from the part manufacturer’s recommended footprint.

Some of the factors that influence paste release are the stencil thickness, whether it’s polished or not, the angle of the cut, ratio of thickness to width and paste properties. How long the paste has been exposed to air as well as the room’s temperature and humidity can also have an impact. Lots of permutations.

If you’re reading this Owen, sorry I missed your question in the chat. I hope this answers it for you.

Duane Benson
If it’s going to the EU, make sure it’s peanut butter-free.

http://blog.screamingcircuits.com/

About Duane

Duane is the Web Marketing Manager for Screaming Circuits, an EMS company based in Canby, Oregon. He blogs regularly on matters ranging from circuit board design and assembly to general industry observations.
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6 Responses to An Unanswered Question

  1. Mike says:

    Duane,

    Thanks so much for moderating the chat!

    Any readers who missed the session but are interested in seeing the transcript may visit the on-demand version of the show (registration required, but it’s free) at http://www.virtual-pcb.com. Once inside the show, click on the link for Scheduled Chats and scroll to Duane’s.

  2. Jack Olson says:

    Duane, (prepare for rant…)

    I’ve noticed several times you’ve recommended the manufacturer’s land pattern dimensions as the way to go. I’m wondering how much in-depth study these manufacturer’s do to design the optimum dimensions, and any librarian will tell you they come across components from two different manufacturer’s with EXACTLY the same physical dimensions, yet the recommended land pattern sizes are different.

    I don’t think the writer’s of data sheets have the resources or interest to study assembly experience in depth. That’s why I think OCCASIONALLY you could give SOME credit to the IPC for trying to reach a consensus on these with the formulas and dimensions recommended in IPC-7351. at least for part types that have been around awhile (new package sizes are a different story, and of course the IPC publishing cycle has trouble keeping up with “the latest”)

    but… In my opinion the collective experience of many is often worth more than the opinion of a lone technical writer in some cubicle who may not have all the facts.

    Using your theory, we should change the land pattern dimensions every time we buy the part from a different mfg source, right?

    just a thought,
    Jack

  3. John Lambert says:

    good points Jack…
    When designing I always review the issue of alternate suppliers before defining land patterns.

    as to solder release issues / rounded pads…
    As stated there are MANY issues related to proper solder paste application.

    I do find:
    a) most rectangular opens don’t have a zero radius on their corners
    It may be very small, but there is generally some radius.

    Generally, I don’t leave it to chance (process variations).. I spec a modest radius on my rectangular openings.. (3-4 mil min.)
    There will always be some irregular shaped solder balls, that like to get stuck in sharp corners.
    and it is hard to really control the wall finish in zero radius corners.

    b) wall finish and radius of corners on solder stencil are often over looked for their
    influence on operators/machine need to clean/clear the stencil.
    This is closely related to the issue of paste release … but not the same.

    c) quality of ball formation (shape of solder pieces in solder paste).. is often overlooked
    when dealing with some of these same issues.
    The ball size (size of microscopic solder in paste) ….ditto.

  4. Joe says:

    Per Jack’s rant, I agree that believing that vendor recommended footprints can be taken as tried and true is dicey at best. That they often differ so much for the same package has to cause one to question the viewpoint and experience of the tech writers. Per PCB Matrix and their IPC-7351 footprint libraries, 3 classes of board density are supported with different footprints for each, but I can’t ever recall a vendor footprint recommendation mentioning which packaging density class their footprint should be considered as. In fact, since so many packages have “shrink” or “less space required” in their marketing sheets, I can see vendors providing “least” category footprints just to reinforce their reduced real estate claims whether one’s application is space constrained or not. I have also seen only slightly different packages (like inductors) seem to craft their footprint recommendations to preclude the use of other popular vendor’s similar parts as second sources, when a slightly larger pad size could allow more package dimensional variation.

    To more specifically address Owen’s original question, some of our CM’s process engineers have recommended rounded pads, especially for gull-wing leaded parts, specifically to address paste release issues. While Duane’s listing of other factors affecting paste release may be valid, it just stands to reason that elilminating square corners should allow paste bricks to be formed and deposited without dealing with paste left in the corners of stencil apertures.

  5. Duane Benson says:

    Thanks for the comments and commentary. Lot’s of good points. I guess the final answer on land patterns isn’t here yet, but in the mean time, probably a bit of an iterative process is in order.

    For newer, smaller packages, I’d check both the manufacturer’s recommendations and the IPC data. If the manufacture has done a good job of testing the layout and assembly process for their part and has some very specific requirements (e.g. Ti OMAP 0.4mm, Freescale ZigBee QFN), then that’s the route to take. If the data sheet is put together with little thought or without an specific requirements, then the IPC data is your route to go. For parts that are in pretty standard packages, just use the CAD packages library. Maybe customize a similar part. But, definitely check the datasheet to make sure that the leads all go to the right functions.

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