Screaming Circuits: April 2008

Icky Via Near Pad

Here's a case where the board house didn't do an engineer any favors. There are issues with soldermask opening size, registration and masking between the via and the pad.

The pads are non-soldermask defined as we like them, however, the mask openings are much larger than we would like or than most manufacturers would recommend. Check the component datasheet for the actual number you need to use.


Then, the registration is way off. That alone wouldn't make the board unusable, but it does speak to the suitability of the fab process to this level of technology. Either it was a bad batch or this fab house shouldn't be making boards for BGAs or other fine pitch components.

The worst part is the direct solder path from many of the pads to their vias. It's quite likely that in reflow, the capillary action from the via will suck the solder paste and the solder ball off of the BGA leaving a complete open. Check out this post for a little more information.

Make sure your board house is capable of building to the technology you need. It's not worth the discount you might recieve. And, always give your boards a good look over before sending them out for assembly.

Duane Benson
In this case, BGA = Bad Green Area

ESC Show Design Features

Speaking of the Embedded Systems Conference, if you are attending and happen to have a cool circuit that you've been working on, drop by our booth and tell me about it. I like to show off interesting designs when I can and if yours looks like something our audience would be interested, I'll try and write something up about it.

Bring along an actual board, stuffed or not, and if you like, I'll take some pictures while on the floor.

Duane Benson
Solder is as solder does

Thank you!

We do our best to fill this blog with helpful information and minimize the glurgy stuff, but sometimes, we do talk about ourselves. We've been a little lean on posts lately anyway so I suppose a little self-serving filler is okay.

We recently received two Service Excellence awards from Circuits Assembly magazine. We say "Thank you" to all of you because the award winners were selected based on a survey with some of you and we really appreciate your kind words.

Service Excelence Awards

We won in the "Manufacturing Quality" and "Technology" categories. That makes us feel pretty good because we know both of those topics are very important to all of you. We do sometimes have problems, but we try our best to always keep what you folks need at the top of our minds. If we do have problems, we try our best to make it right for you and learn from it.

The awards are cool, but what we really care about is how we are doing for you guys every day. Please feel free, any time or place, to call or email us (or comment here) and let us know if you have any questions or concerns at all.

Duane Benson
Thank you from all of us here at Screaming Circuits.

Good QFN, Good QFN Paste Layer

Qfn_package_sm_2I write a lot here about things that go wrong with various layouts. Hopefully by doing that, I can help some of you avoid common pitfalls. But I should probably write about more good things though to balance it out.

So, here's a decent example. This is a good way to specify the solder paste layer for Good_qfn_stencil_c_2 your QFN parts. Notice that the center thermal area is segmented to give about 50% paste coverage. This will lead to a secure and reliable placement with minimal voiding underneath.

Good_qn_before_pasteThere are a lot of different ways to segment the stencil for good paste coverage. This is one is easy to do and will work well. Note that this QFN doesn't have any thermal vias so here we don't have to worry about any via in pad issues. If you do need thermal or grounding vias in the pad, check out our section on via-in-pad for guidelines to help properly deal with the.

Duane Benson
All we are is paste on the pcb
Just a drop of solder on an endless stencil

Upcoming Tradeshows

We're going to the Embedded Systems Conference in San Jose and we'll be in Booth 1944 on April 15-17. Escsv Come see us. It'll be fun. You can get a get a free strongly motivationalist Screaming Circuits poster. Maybe even a true geek shirt.

Follow this link to get a free admission to the exhibits. When you register, be sure and enter our priority code: "SC01".

Later in the month, we're also going to the Del Mar Electronics show April 30/May 1 in the Del Mar Del_mar_logo_150 fairgrounds, Booth 202.

Duane Benson
Hope to see you there!

Vacuum Tube Contamination Warning

Rca12ax7It has been brought to our attention that a certain number of assembled pc boards have been subject to accidental contamination with vacuum tubes. It is not known at this time if the tubes are original manufacturer devices from the 1950's or reproductions recently imported from Russia. (Ref: US DOCpet0820-492.45gh.99ac138). Clearly, however the suspect vacuum tubes are not wanted and were not specifed in the original electronic design.

As shown in the photograph in the left, vacuum tubes are glass envelopes with metal plates, grids and filaments. While mostly harmless on their own, when connected to an electronic circuit, such devices have been found to consume valuable coulombs that are needed in other areas of the electronic circuit.

The contamination tends to be more extreme and more prevalent with circuits that utilize compact packages such as 0201s, Micro BGAs and QFN parts. The US Department of Commerce has estimated that upwards of 82.3% of all circuits that utilize CSP (chip scale package) devices have some level of vacuum tube contamination. Further, it is not believed that these vacuum tubes are fully RoHS compliant.

In most cases, the contamination is clearly visible and quite easy to identify. However, in some of the more extreme infestations, you may need to dim the room lights and look for glowing filaments, as seen in the photo below.

Tubes_in_dark Mitigation: If you do find that your pc boards contain unwanted vacuum tubes, remediation involves a simple three step process. Fortunately, vacuum tubes tend to utilize sockets rather than thru-hole or surface mount solder. Step one, is to remove power from the circuit. Step two is to wait five minutes to allow the glass envelopes to cool. Step three is to carefully grasp the glass envelope and pull up to remove it from the circuit board.

Gordon Freeman

Continue reading "Vacuum Tube Contamination Warning" »

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