Tag Archives: assembly

Done and Done

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LED scroller 005 trimmed And … (drum roll, please) … it works.

I put in a couple of batteries, programmed the MCU and turned it on. It works.

I’m always surprised when something I design works on the first try. This being such a simple design, I probably shouldn’t be surprised, though. I should at least give myself a little more credit.

The unpopulated lands on the board in the photo are supposed to be unpopulated. I left a few things out because they aren’t needed for what I’m doing with this piece now and leaving them off keeps the cost down.

So, what did I learn from the process?

  • If you have a lot of different parts laying around, it’s pretty easy to grab the wrong one.
  • I ran into some variability in the “zero rotation” position in the CAD library land patterns.
  • The whole process is pretty easy, but start to finish, there are quiet a few steps.
  • It’s a nerve-racking wait after sending off a box of parts.
  • Good communications between designer and assembler are very important.
  • Clear documentation from the designer is very important.
  • This was a WHOLE LOT easier than hand soldering all the SMT parts (I’ve done that before).

That’s a good set of educational results. Next time, I think it will be easier.

Note the large diode polarity indicators on either side of the long row of LEDs and by LED D25. D1, the Schottky on the upper right has the same polarity indicator, but it’s in between the pads, under the part. In case you’re interested, I have a 3V supply. The LEDs drop 1.8V and I’ve got a 150 ohm resistor for each. That gives me a theoretical 8 mA per port for a total maximum of 176 mA with all 22 lit up. That’s within spec in the -40C to 85C temperature range but too much when above 85C. I’m not sticking this in an engine compartment or anything, so no worries there.

Duane Benson
0x45 0x53 0x43 0x20 0x62 0x6F 0x6F 0x74
0x68 0x20 0x38 0x32 0x33 0x20 0x20 0x20

http://blog.screamingcircuits.com/

Virtual Show, Real Value

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We are eagerly looking forward to next week’s Virtual PCB trade show. It’s the fourth year we’ve produced the Web-based event, and we’ve learned a few things along the way.

1. Although the attendees are online, they usually act as if they are in the flesh. There’s plenty of the “how are you doing,” “great to see you,” and gentle ribbing that takes place when we run into each other at PCB West, SMTAI, Apex or one of the other “bricks and mortar” shows. It’s social. (Perhaps that’s why they call it “social” media.)
2. People are polite to the point of near invisibility. Just like a physical show, some attendees do lots of talking, while others never utter a peep. That’s OK. Lurk away. Everyone learns in their own way.
3. Speaking of learning, it’s almost impossible to attend Virtual PCB and not take away something. Nearly 3,000 people registered last year! These are your peers across the entire electronics manufacturing spectrum, from design to assembly to test. The same experts you might see at a physical show — folks like signal integrity expert Dr. Eric Bogatin or reliability guru Werner Engelmaier, will be there, holding court and sharing their wisdom. More than that, it’s a chance to meet folks from all over the world. These are potential future colleagues and employers. Insofar as networking is concerned, it’s tough to beat.

We hope you take a moment to register (it’s free!) at www.virtual-pcb.com and log-on to Virtual PCB, March 8-9. It’s a fresh way to stay up on our industry — without ever leaving your desk.

Narcissistic Parts

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Maybe not completely narcissistic, but at least self-centered.  Or, self-centering. Okay, are you lost now? Am I making any sense at  all? Well, I’m going to say that it doesn’t matter, because the  world-revolves around me.

But what I am talking about is parts that will more or less center themselves during the reflow process. Some parts like BGAs and QFNs tend  to follow the surface tension of the melted solder and tweak themselves  into a more centered position on the land. That’s a good thing.

It’s not always a good thing though. Sometimes that same surface  tension action can work against you. Take this TO-263 (pictured). When it was placed on the land, before reflow, the leads were centered  right in their pads like they should be. The big land for the thermal  pad is set up a little too high though and once melted, the surface tension from the big thermal pad sucked the part up, nearly dragging it  off of the lands for the leads. Bummer days. Here’s another example.

You probably shouldn’t leave the part like this, so here’s a few suggestions.

You could make the thermal pad smaller so that when the metal tab of  the part is centered, the leads will be, too. Cooling needs might dictate  that you don’t reduce the size of the pad, though. If that’s the case,  you could make the bad bigger by extending it down toward the leads, again so the leads will be centered when the body of the part is. You could also mask off the top part of the pad, or put a thin strip of mask  as a solder dam. What you’re doing is making sure that if and when surface tension moves the part, the leads will end up where they are supposed to.

<em>Duane Benson
It is all about me, you know</em>

http://blog.screamingcircuits.com