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D-Pak misalignment Problem

Kantesh Doss


D-Pak misalignment Problem | 2 December, 1999

We have a serious misalignment problem of D-Pak components (Specifically Motorola's part number MJD-3055/MJD-2955)following reflow soldering process. We are using Alpha Metals UP78 solder paste. Our reflow oven is Nitrogen inerted and our oxygen level in the oven is around 500 ppm.

Do you have a reflow profile/process or solder paste stencil design that will minimize the movement of D-Pak's ? Is this problem very common in the industry ? I would welcome suggestions/recommendations to resolve this problem.

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Re: D-Pak misalignment Problem | 3 December, 1999

Usually the alignment problem comes from pad design. The ground pad (largest) is too small. If that is the case (and even if it isn't) try placing a dot of SMT adhesive before placement (it will cure before reflow and hold the part in place). Good Luck MDCox

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Re: D-Pak misalignment Problem | 4 December, 1999

Kantesh, Does this particular part have a heat sink across the bottom side of the component and does your pad size and stencil aperture match the component? I have just had a very similar problem with a DPAK. What we found was that the recomended pad size and stencil size was the same. Although you do want the component to form a solder bond between the heat sink and the PCB pad, the component must displace the solder that is put there in order to sit planar to the board. The problem we were having was the component was "floating" (for lack of a better term) in the solder. We remade the stencil and blocked off the center of the aperture to put solder only at the terminations and that solved the problem. This may not be the same in your particular situation but it is something to consider.


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Chris McDonald


Re: D-Pak misalignment Problem | 4 December, 1999

Just as explained in a follow-up you are getting too much paste on the pad. The pad is probably the right size but you cant have your aperature the same size as your pad. What we have done is put a Cross in the stencil. This will reduce the paste and you will have alot better results.

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Kantesh Doss


Re: D-Pak misalignment Problem | 6 December, 1999

Christopher: Thank you for sharing your experience about the DPAK. Yes, out part has a heat sink at the bottom. Our stencil opening was 7 mils (in both X and Y) smaller than the pad size excepting that the stencil opening consisted of a series of zebra stripes across the diagonal of the rectangular (or square) pad. This certainly worked better than solid stencil opening. The zebra striped opening still floated the component. Placing glue under the component lifted the component body so much that we ended up seeing very little solder at the heel fillets (at the two leads of the D-Pak). Since my message we have found a quick fix - this was to place columns of SMT Adhesive behind the D-PAK component body at the edge of the pad where the D-PAK body is placed. This has virtually eliminated the movement of the D-Pak component. We will try the cross suggested by Chris McDonald in the previous response to see whether it takes care of D-Pak misalignment or not.

Thank you everybody for your responses.

Dr. Kantesh Doss, SE&A, Johnson City, TN

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