Electronics > Manufacturing & Assembly

Modified LQFP land pattern to solve bridging problems (reflow only)

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Georgy.Moshkin:
Hello, recently I came up with alternate pin arrangement that allows to use wider stencil openings and minimize solder bridging possibility. Standard 0.5mm pitch LQFP stencil paste openings are too thin and paste is easily smeared during stencil removal and/or when components are placed by hand. No solder bridges so far after switching to this land pattern. Mechanical strength to be investigated. Additionally, 0.2 mm thin paste-less area seem to act as solder thief. Distance between pad centers are the same as standard LQFP layout of STM32 chips. Paste shrink: 0mm , mask swell: 0.05mm.

LQFP100 pattern in Diptrace format: diptrace-lqfp100.zip
PDF dimensions + photos: Modified LQFP land pattern for solder bridging prevention.pdf

ataradov:
This looks interesting. But I would expect board houses to have issues with solder mask bridges that are this narrow. Cheaper ones would not be able to manufacture then for sure. At the same time it may not matter this much with this design.

Georgy.Moshkin:
Good point! But actually the gap is the same as for standard LQFP layout (Diptrace part libraries, stm32 datasheet)
Standard: pad width 0.3mm / pitch 0.5mm / gap between pads 0.2mm; If solder mask swell is 0.05mm then solder mask thinnest area would be 0.1mm
Proposed: alternated pad widths 0.2mm / 0.4mm / gap = 0.5 - (0.2/2+ 0.4/2) = 0.5 - 0.3 = 0.2 [mm]. Gap is the same because thinnest zone reduced to 0.2mm whereas adjacent zone increased to 0.4mm.
Maybe different mask swell or thinner pad areas of 0.16mm will perform better.
Evenly numbered pins have hidden solder joints, so no visual inspection for those. It looks good from the sides, solder climbed up the second and fourth pin from below. For guaranteed results need more experimentation and x-ray inspection.

T3sl4co1l:
Interesting practically, but how applicable is it in principle?  That is, given min and max dimensions of typical footprints (let alone of of ancient JEDEC types..), how can you guarantee a full heel AND toe (alternately) fillet?  Often the difference between accumulated min/max dimensions exceeds this, so you can't guarantee meeting both.

I mean, the pads can always be longer, without preventing the reservoir effect.  But if the intent is for the blob to touch the pin, that would seem hard to achieve given the specs of most parts.  And you don't have much space under the part (between plastic body and lead heel) for the inner blobs.

*Shrug*, it's neat at least; just, probably one of those things that's somewhere between "neat but not neat enough" and "too clever for its own good".  Which, I'm not sure where this lands, really? :-//

For my part, I've had no problem with traditional footprints, I've also not had a ton of feedback from assembly so that maybe doesn't mean much.

Tim

mikeselectricstuff:
Might be helpful for hand assembly and less-than-stellar pick/place machines, as IME what causes shorts is when the part is mis-placed and needs sliding in to position, dragging paste with it

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