Screaming Circuits: An Unanswered Question


An Unanswered Question

I've been reading through my Virtual-PCB chat session transcript from yesterday. It was a fun session and I have a much better idea of how the virtual shows work now. I think I may just be getting it.

The chat session had a lot of interesting questions and dialog. I did notice, however, that I missed one question and thus didn't answer it. Oops.

Owen asked if I am of the opinion that all footprints should have rounded pads (probably stencil cutouts too) to help with paste release. Sorry I missed your question.

I'm not of that opinion. There are a lot of factors that come out of stencil decisions. Paste release is one of them. There are others, some more important. For example, the shape of a pad and stencil cut out can either encourage or discourage solder balls. The size of the opening can put too much or too little paste on the pad. Wide open cut-outs over heat slugs can cause float.Bad QFN paste w caption

The pads themselves, should follow the part manufacturers recommendation for shape and size. Most  are rectangular. BGAs have round pads. Unless you have a very good and very specific reason, I would not deviate far from the part manufacturer's recommended footprint.

Some of the factors that influence paste release are the stencil thickness, whether it's polished or not, the angle of the cut, ratio  of thickness to width and paste properties. How long the paste has been exposed to air as well as the room's temperature and humidity can also have an impact. Lot's of permutations.

If you're reading this Owen, Sorry I missed your question in the chat. I hope this answers it for you.

Duane Benson
If it's going to the EU, make sure it's peanut butter free.

TrackBack

TrackBack URL for this entry:
http://www.typepad.com/services/trackback/6a00d8341c008a53ef014e5fc5bab5970c

Listed below are links to weblogs that reference An Unanswered Question:

Comments

Rants are always welcome. I tend to learn something from pretty much every rant.

re: "Using your theory, we should change the land pattern dimensions every time we buy the part from a different mfg source, right?"

Sometimes it certainly seems like that should be the case - at least for the newest packages.

Duane, (prepare for rant...)

I've noticed several times you've recommended the manufacturer's land pattern dimensions as the way to go. I'm wondering how much in-depth study these manufacturer's do to design the optimum dimensions, and any librarian will tell you they come across components from two different manufacturer's with EXACTLY the same physical dimensions, yet the recommended land pattern sizes are different.

I don't think the writer's of data sheets have the resources or interest to study assembly experience in depth. That's why I think OCCASIONALLY you could give SOME credit to the IPC for trying to reach a consensus on these with the formulas and dimensions recommended in IPC-7351. at least for part types that have been around awhile (new package sizes are a different story, and of course the IPC publishing cycle has trouble keeping up with "the latest")

but... In my opinion the collective experience of many is often worth more than the opinion of a lone technical writer in some cubicle who may not have all the facts.

Using your theory, we should change the land pattern dimensions every time we buy the part from a different mfg source, right?

just a thought,
Jack

Duane,

Thanks so much for moderating the chat! Any readers who missed the session but are interested in seeing the transcript may visit the on-demand version of the show (registration required, but it's free) at www.virtual-pcb.com. Once inside the show, click on the link for Scheduled Chats and scroll to Duane's.

Post a comment

If you have a TypeKey or TypePad account, please Sign In.

« Virtual-PCB | Main | Who's Right? »