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Minimizing the Head-in-Pillow Defect in SMT Assembly

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  • Phil Zarrow: This video is for those in the electronics assembly industry that want to minimize the head-in-pillow defect. It includes the solder paste properties that help overcome this challenge.
    Dr. Lasky, engineers that are working with the area arrays, a growing concern for them is a defect called head-in-pillow. How would you describe this defect?
    Dr. Ronald C. Lasky: The basic failure mechanism, as you pointed out, occurs in predominantly ball grid array packages and during the assembly process, the circuit board and ball grid array, as they go through the reflow oven, can often be moving because of the warping of the circuit board of the ball grid array package. Sometimes, what can happen is, they'll separate during the reflow process and the solder paste will reflow separate from the ball and you don't even get a solder connection.
    So, if you have this problem, one of the things you want to try to assure is that you have a nice tall stencil printed brick; so, you want to make sure you're getting good definition in your printing, not having hot slump, which can be an issue. You want to make sure that your pick-and-place machine is placing the ball squarely on the pad, not off to the side. It helps if you have a solder paste that has a little stringiness to it, so, if they do separate, it'll still stick together. And, something in the solder paste, also called oxidation barrier, that helps in the reflow process, too, in these type of situations.
    But, one of the things that can help, also, if you're having this problem – you spend a little time to solve it, you just can't seem to get it solved – call your solder paste supplier. They're dealing with these types of challenges on a daily basis. And, there's a case where one solder paste supplier customer called them, they tried everything to get rid of the head-in-pillow defect, and it ended up that the solder paste supplier had developed a new solder paste specifically to minimize the head-in-pillow defect, and all they had to do was change the solder paste, and they went from 7% yield loss to no yield loss.
    Phil Zarrow: This sounds like an episode from Patty and the Professor.
    Dr. Ronald C. Lasky: Yes, in Patty and the Professor, on page 51, has an episode of the head-in-pillow defect, also. If people would like a copy of Patty and the Professor, they can download it at And, also, I gave a talk at SMTA in Dallas that has a lot of slides on this; it goes into a lot more detail. And, you can also download that PowerPoint deck at
    Phil Zarrow: Excellent. Dr. Lasky, thank you.
    Dr. Ronald C. Lasky: Thank you.

    Keywords: indium, Indium Corporation, Dr. Ron Lasky,, Phil Zarrow,, ball grid array packages, reflow, solder paste, stencil printed brick, hot slump, pick and place machine, oxidation barrier, SMTA