Hacking footprints

[matseng]

Sometimes the routing of a PCB can be easier by tweaking the footprints a bit....

Here I didn't need the Q3 and Q4 outputs on a latch so I shrunk down the size of the pads to minimum size and then removed the stop mask on them so they are covered. This makes it easy for me to route a wire between the other pads - something that can't be done on a TSSOP package since the pads are too close together to fit a normal wire between them.

Hack the footprint - why not?

[JuKu]

For prototypes and one-offs, clever. For production, there are risks. You are relying on the solder mask for isolation. That is a safe assumption, unless the board sees vibration. In that case, the unsoldered pins could rub off the mask. Also, the soldered pins are lifted by the solder mask under non-soldered pins. That probably helps solder flow between the pads and pins you want to solder, but it is still something the process is not calibrated for. I would show the detail to manufacturer and ask their opinion before ordering ten thousand of those.

But a very clever trick that I keep in mind. Tricks like that sometimes make the difference of four amd two layers or even one or two layers, which is crucial for diy boards. Thanks for sharing!

[Sleepwalker3]

JuKu makes a good point, though for small runs it should be fine, as long as your aware of the potential problems. You could always put a strip of kapton over the track, though again it raises the chip a little and may create soldering problems. Still for small runs...
Great idea Mats!

[Alex555]

Couldn't you just cut off the unneeded pins so you don't have to deal with solder mask thickness/rubbing it off?

[matseng]

Yes, cutting the legs is an option, but on a tssop the they are really tiny and tightand you need to count then and pick the correct ones. This is more of an use a standard part, plunk it down on the board, solder and forget.

The extra thickness should not affect the solderability of the package that much. The soldermask is usually 20 microns = 0.8 mil and the hasl layer on the exposed pads should have some thickness them as well so I'd hazard a guess that the height difference will be less than 10 microns.

If hand soldered this would have no effect at all. And for automated reflow it should be as before if the apertures in the stencil is increased with a percent or two.

I have a hard time believing that board vibrations could make a pin that is so short and sturdy as a tssop pin to vibrate enough to rub off the soldermask. You almost need a chisel and hammer to make a dent in it. Also the pin would need to bend down more and more over time as well. When it have rubbed off the first micron of the mask won't reach the next micron layer - unless of course the chip is pressed down flexing the springy legs during the soldering...


Still, with all this being said this definitely is a hack that os good for prototypes and one-offs and possibly in some cheapass consumer electronics. I wouldn't dream of doing this in a spaceship, the abs brakes in a car or in your pacemaker. :-) No way Jose. :-)

[Bertho]

Yes, cutting the legs is an option, but on a tssop the they are really tiny and tightand you need to count then and pick the correct ones. This is more of an use a standard part, plunk it down on the board, solder and forget.

In this case I suggest you just solder it and wiggle a knife under the pins bending the legs up a bit. I do not think this will be a high-volume production design...

[bearmos]

a hack that os good for prototypes and one-offs and possibly in some cheapass consumer electronics

hilarious:) nice trick for one-off's!

[presslab]

I took apart a thumb drive and it made me think of this thread. :)

[matseng]

LOL, It seems like whatever you do or "invent" by yourself there's always someone else that have done the same thing before :-)

[arakis]

LOL, It seems like whatever you do or "invent" by yourself there's always someone else that have done the same thing before :-)

This unfortunately is the prevailing opinion of many engineers, and it's what stops many of them from innovating... I on the other hand disregard it, and enjoy reinventing the wheel :)

[matseng]

Yes absolutely, doing things just for the sake of "doing things yourself" is a perfectly valid reason to keep on hacking and tinkering and inventing...

[presslab]

Didn't mean my pics in a bad way, just saying that it's not a bad idea after all, and it can be used in very high volume production.

[matseng]

No offense whatsoever taken. It's nice to see that the idea was not too stupid and/or unusable and that it actually is used out there in the real world.

[ian]

It's nice to see that the idea was not too stupid and/or unusable and that it actually is used out there in the real world.

Now, let's not go too far ;D Just cause they did it, doesn't mean it was a good idea ;)

[matseng]

And this is what it looks like in my version of the "real world"....