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LMH6517: solder paste pattern as many square sections

Frank Eory2
Prodigy 200 points
Part Number: LMH6517


In the LMH6517 datasheet, it says to put the solder paste as 4 squares instead of one big square for the thermal pad. I put a picture below. 

In PowerPad layout guidelines http://www.ti.com/lit/pdf/sloa120, it says to use the full area under the thermal pad. The PowerPad looks like a trademark, so I suppose that the instruction is not necessarily good for all shapes. Is there a general guideline for the kind of footprint similar to RTV0032A? I have seen many times large thermal pads divided into many sections. I would like to know the pros and cons for using the full area vs not.

Frank

  • 0
    TI__Genius 15565 points

    Hello Frank,

    Please follow the datasheet recommendation, you can find more details geared specifically towards QFN packages here:

    Pleas refer to section 4.4 which mentions, "It is good practice to minimize the presence of voids within the exposed pad interconnection. Total elimination is difficult, but the design of the exposed pad stencil is crucial. The proposed stencil design enables out-gassing of the solder paste during reflow and also regulates the finished solder thickness. Typically, the solder-paste coverage is approximately 50% to 70% of the pad area (see Figure 11). Designing an aperture that prints solder 1:1 with the exposed pad results in excessive metal volume that can "float" the part and cause opens and other manufacturing defects. In addition, the amount of voiding post reflow in the thermal pad solder joint should not exceed 50% in high-power applications (to be verified using an x-ray). Based on a JEDEC High K board stackup, 25% has been determined to be a point of diminishing thermal performance returns, but TI prefers a limit of 50% to be set (reference JESD51-7)."

    Best,

    Hasan Babiker