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Are You Screaming For Help With Prototypes?

May 8, 2008
At last month's Embedded Systems Conference in San Jose, I met with Duane Benson, the Web marketing manager for Screaming Circuits. The company assembles prototypes in as little as 24 hours for one or more boards. You simply sen

At last month's Embedded Systems Conference in San Jose, I met with Duane Benson, the Web marketing manager for Screaming Circuits. The company assembles prototypes in as little as 24 hours for one or more boards. You simply send it a package of parts along with the unpopulated printed-circuit board (PCB), and Screaming Circuits will assemble it for you.

Screaming Circuits’ assembly capabilities include machine-placed surface-mount technology down to 0201 components, fine-pitch parts, and ball-grid array (BGA) and leadless parts, including quad flat no-lead (QFN) packages. The company also offers a variety of board finishes and board types such as rigid, flex, rigid-flex, ceramic, and metal core and virtually any layer count or board size. It can place parts off of cut strips of any length, which means it doesn’t need reels, but it can use full reels and Digi-Key’s digi-reels.

I asked about the typical users of this service, thinking they would be designers who didn’t have in-house prototyping capability. But this is not the case, or more precisely, they aren’t the only designers who might take advantage of the service. Benson pointed out that turnaround times for in-house prototypes can be 30 days or more. Screaming Circuits, on the other hand, has a 24-hour turnaround time as one of its options.

IN THE BLOGOSPHERE But the news at the show was that the Screaming Circuits Blog for design engineers had passed the 200-article mark (blog.screamingcircuits.com). The blog is meant to provide knowledge that can be helpful to all engineers, whether or not they actually use the service.

As Benson explains it, the company sees many PCB and component technologies every day and can share lessons about advanced packages like 0201 passives, tiny QFN parts, and micro BGAs; PCB finishes such as electroless nickel immersion gold (ENIG), immersion silver, and organic solderability preservative (OSP); the European Union’s Restrictions on Hazardous Substances (RoHS); and layout techniques such as via-in-pad, solder paste layer design, and package selection.

Like any blog, comments and posts from guest bloggers are welcome. It’s organized by categories, such as Via in Pad and PCB Finishes, and by date. The entries Benson showed me were quite interesting.

In the Via in Pad section, an entry he wrote called “Icky Via Near Pad” explained that the board house used by one design engineer that contracted with Screaming Circuits didn’t do him any favors. There were issues with soldermask opening size, registration, and masking between via and pad (see the figure).

“The pads are non-soldermask defined as we like them; however, the mask openings are much larger than we would like or that most manufacturers would recommend,” Benson wrote. “Check the component datasheet for the actual number you need to use.” Also, the registration was way off and was clearly shown in the photo.

“The worst part is the direct solder path from many of the pads to their vias,” Benson added in the blog. “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.” This was another indication to me that these guys know their stuff.

ALSO ON THE WEB In addition to the advice and announcements from the blog, the site offers design guides in pdf format. One of them, “QFN Design Guidelines,” includes information about a phenomenon called QFN float.

QFN packages have a metal contact pad on their underside. It may be there for grounding or heat conduction, depending on the specific part. However, float happens when you lay too much solder paste on the PCB for that center pad. The paper goes on to explain the problems that may occur with the signal pins due to the float, and it offers a solution.

Finally, Benson showed me how easy it is to order a prototype from the site and to get a price quote as well. Though prices can run upward of $2000 for a 24-hour turnaround time for a typical order, you can get a significant discount if you can wait longer, from 48 hours to 10 days. To give this service a try, go to www.screamingcircuits.com.

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