Here’s a case of close, but no cigar with the stencil opening. The pads are, in fact, covered by the openings, but as you can see, the openings are too big.
This stencil would end up laying way too much paste down. Some of it would be on the solder mask which might bubble up and turn into solder balls. All in all, the use of this stencil might just lead to something of a gloppy mess.
When you’re making your paste layer in the library component (presumably, this was custom-made), it’s sometimes appropriate to make the paste opening the same size as the pad and it’s sometimes appropriate to make the opening smaller, but it’s never appropriate to make the opening bigger then the pad size.
After writing this, I for some reason got curious as to the origin of the phrase: “close, but no cigar.” I know it’s been around a long time, but I couldn’t come up with any plausible meaning for it. Then I remembered this thing called the Internet, so I looked it up. According to a couple of different sites, carnival booths, like the big hammer, would give out cigars as prizes so if you almost made it, the Carney, would say “close, but no cigar.” Huh. Interesting, but much less interesting then I had expected.
Sorry. We don’t give out cigars if your stencil is good.
Exception to the rule is when Intrusive reflow is being used, it is usual to make the solder paste opening larger to get the correct volume of paste down.
There also seems to be an alignment issue. I realize this is an example of bad cad data. It happens all too often.
I’m a little confused that the stencil paste layer doesn’t match the pad layer. I can understand the oval to square but not the misalignment since the fab house uses the gerber file to create the fab and the paste layer comes from the copper layer.
With that said –
There are many exceptions to the rule. Sometimes the stencil aperture needs to be larger. Fab houses are allowed a certain amount of pad removal during manufacture –
Example:If a BGA pad is suppose to be 12mil and ends up 10mil that’s the way it is. IPC standards –
If you don’t catch this pad reduction by measuring an actual fab, then you are going to over print the pad.
If the pad isn’t large enough to get proper paste adhesion thru pad surface tension then paste release is effected. Most of the paste will stay in the stencil. This is one reason for the use of the larger square aperture with radius corners. This is a good case for Design for Manufacturabilty (DFM). No one has the time anymore for doing something twice.
I am one of those probably annoying people who think it doesn’t matter b/c “the CM will review it and touch it up if they don’t like the stencil openings.”