Screaming Circuits: February 2009

ESC Here We Come

ESC med logo Soon. Very soon, we'll be down in San Jose again for the Embedded Systems Conference. We'll be in booth number 235 - a 10x20 over on the left side.

We'll be announcing an excitng new service at the show too!

Drop in and see us and our friends from Sunstone Circuits. It's March 31 - April 2 at the McHenery Convention Center in beautiful downtown San Jose.

Duane Benson
Do you know the way to San Jose?
Turn left at the second light. You can't miss it.

And Another Reason...

Another reason to inspect your PCBs before sending them on to the assembly house.

Missing barrel 

At first glance, these boards looked fine. But with a little closer inspection, you can see that the middle barrel isn't plated through. Bummer.

There are a number of possible causes for this.

  1. It could simply have been a goof at the board house. Sometimes a process will slip or someone in CAM will introduce an error.
  2. It could have been caused by improperly creating a library part. Maybe the symbol was built up by hand and ended up with a non-plated hole in that spot.
  3. It could have been built without a library part. Sometimes designers will just create a place for a thru-hole part using vias and traces instead of creating the library part. If that's the case, the center could have ended up with a non-plated hole instead of a plated via.

Myself, I'm betting this was a board fab problem. In any case - another good reason to check out those boards before sending them on for assembly.

And even better then just looking at them - also have them electrical tested at the fab house. Our fab partners at Sunstone Circuits can do that for you.

Duane Benson
Roll out the barrel...
and have a barrel of open solder joints

Paste Dipping Isn't So Bad After All

I've been chatting a bit with Jim Hisert of Indium Corporation about various issues, challenges and changes we in the assembly industry face. Sometimes it's us service providers, sometimes it's OEM manufacturers and sometimes it's material suppliers like Indium that face the challenges.

POP (Package on Package) is becoming a more common practice these days. In general, when we see them here at Screaming Circuits, the sandwich has already been assembled so we can just treat it as a single part. Myself, I'm not that familiar with the techniques for building it up in-process, but my understanding is that the bottom part is placed on the pasted PCB like any other BGA. Then the top part is dipped in dipping paste and placed on top of the bottom part. That way, you don't have to try and screen paste on to the bottom BGA.

Here's Jim's comment on that:

"When I started working with dipping paste quite some time ago, I hated it.  Why couldn’t everyone just use flux anyway?  Flux dipping is easy.  Easy to apply, easy to clean up - easy to make work.  Why mess with a paste that can cause bridging, insufficient transfer, and has a shorter shelf life than flux alone?  A lot has changed since then. 

Warpage is a problem we can’t escape - we have to deal with it.  Sometimes added solder volume PoP Paste on Spheres-Jim Hisertis a necessity for PoP components and BGA rework.  PoP components often need solder volume to compensate for the thinned-down intermediate packages that tend to warp.  BGA rework needs extra solder to mimic the volume that is present when the initial BGA was attach in the SMT printing process.  During rework, we can’t fit that 29” x 29” stencil over the board anymore, but it’s not too difficult to place a paste-loaded component down in between the other components on the board.

The good news is that solder suppliers are designing solder paste more like flux.  Some of the new pastes actually look and feel like gray flux.  With low viscosity and high tack, modern PoP pastes can transfer up to 300% - 400% more solder than modified SMT dipping pastes.

Paste still isn’t as easy to clean up as flux alone, but 2 out of 3’s not bad, right?


PCB123 ZigBee Robots, Part Four

This is the third or fourth in this series. I paused for a while and just picked it back up again. As I eat my soup and write this, it occurs to me that I've given each post a different name so if anyone actually wants to follow my progress, I've made it quite difficult to do so. I'll recap first and then later, try to be more consistent with post titles.

  1. It all started at the ESC show back in Boston with this post called "Easy Zigbee" about some ZigBee modules I found in the Microchip booth. You'll note that I'm using Microchip components in this series, but my sister company, MEC Innovation, uses a lot of Atmel chips. We like both company's chips here. Especially with good salsa.
  2. Next, I wrote about my plan in "ZigBee, Part two".
  3. The most recent post in this series was "PCB123 QFN Footprint". That's where I started with the CAD package and I got stalled with the parts library.

From now on, I'll identify this series as "PCB123 ZigBee Robots, Part X".

Anyway, enough of that rubbish. I've picked it up again and this morning created the library part for the QFN28 PIC18F2321 microcontroller. I'm lousy at building footprints so I consider that a major accomplishment.

I have a couple more footprints to make - a DFN8 regulator and a CSP BGA RS232 chip. I muddled through the microcontroller but after I do those other two chips, I should be clear enough to able to post some hints on how to make your library components in PCB123.

Duane Benson
Later - I'm going to finish my soup now.

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