Screaming Circuits: BGA via near pad


BGA via near pad

Using proper technique is important when putting vias in pads. It's also important when putting vias near pads. With BGAs, you pretty much always have to put vias either in or near the land pads. Obviously, we prefer near rather then in, but care must still be taken. First, the geometries are annoyingly small - especially if you have one of those .5mm pitch BGAs. Second, if your soldermask Bga_via_near_pad_wicking doesn't break the path from the pad to the via, you can have the same problem as a via in pad setting; the via can wick the solder away from the pad and down the via.

Bga_via_near_pad To prevent this from happening, make sure that the area around the land pad is completely masked. If the via touches the land pad, make sure that the solder mask either covers the entire via or at least the annular ring.

In these two board top-views, you can see actual boards illustrating the incorrect method (B) and correct method (A) for placing vias near BGA pads. I didn't have an example of the incorrect method lying around to photograph, so image "B" is a simulated case. But that's pretty Land_pad_merged_viamuch what it would look like.Land_pads_masked_vias

Sometimes you can get away with this when using lead-free solder, because it doesn't flow as much, but it's best to stick with the correct method, as in example "A".

Duane Benson

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Comments

Hi Fensty. Don't put any paste on the vias. That might actually increase the capillary action. We do use stainless steel stencils. If this is a lead-free design, it may be okay as is because lead-free solder doesn’t flow as much. For production, you will definitely want solder mask dams blocking the vias, but for a prototype, we may be able to get it to work for you. Your best bet is to give us a call and discuss some options. (866)784-5887

I have a design with LGA not BGA. The vendor, Linear Tech, wants vias between the device pads. I used PCB Express and did not know how to specify solder mask over the vias- so I didn't do it (oops). We want to assemble the first prototype so I am now specifying a stencil. Should the stencil permit solder paste to be applied to the vias ? When there are vias between pads, the pads are common to one another - so it is okay to short out. My thoughts are that allowing solder on the vias will help prevent the vias from robbing the pads of all of their solder. This board will eventually be coming to Screaming Circuits to be populated - so your suggestions will help yourself eventually. Also, do you use metal stencils ? THe LT part is LTM4606 - figure 17 on page 17 of the datasheet shows what they asked for. Thanks.

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