PCB Assembly Services - Screaming Circuits: Thru-hole to SMT


Thru-hole to SMT

Thanks to a comment from Michael yesterday, I think everything is now cool with my Geiger counter. I had left the AT2313 default fuse setting at clock/8. That dropped the RS232 speed from 9600 to 1200 and it made the clicking sound into more of a tone, which just didn't sound right for a geiger counter. I still need a good radiation source though. I think I've picked up just a few clicks of background radiation, but that could just be wishful thinking.

WishfDFN-8ul thinking or not, that's not the point. The point is that this was an example of migrating from thru-hole parts to SMT. I managed to get virtually everything into SMT. The connectors, the power switch, the buzzer, batter holder and fuse clips for the tube stayed thru-hole. Although I'm sure I could have all but the battery holder and fuse clips into SMT had I wanted to. I tend to keep switches and connectors that will get a lot of use as thru-hole just for the extra staying power. If they aren't used frequently, then SMT is just fine.

There are a number of things to consider when switching from thru-hole to SMT:

  1. Everything is smaller, so you can fit more in the same space or the same in less space. I took advantage of the extra board area to add in a RS232 line driver so I could connect directly to a serial port. I also added in a power-on LED.
  2. Everything is smaller so your layout is more critical. Most PCB houses will build 8mil trace and space as standard process these days. That gives you a lot of flexibility in squeezing your routing into tight areas, but it doesn't give complete freedom. You have to be core careful because you frequently do have to route a bunch of traces into a pretty small area. When you get into the really fine pitch parts, like .5 or .4 mm center to center, you have to be extra careful.
  3. Some parts are dimensioned in metric and some in SAE units. If all are one way or the other, it's easy. But when you've got both, you may have to tweak with your grid spacing off and on to make sure your traces are centered in the SMT pads they connect to. It usually isn't a horrible problem, but it can make even spacing more difficult and can make you more likely to violate a design rule.
  4. You don't have automatic "vias" on each component leg so routing can be more difficult. You'll likely have to spend more time tweaking the part locations and the trace routing to get a decent layout. A lot of times everything's too close so it's not practical to just plant a lot of vias all over.
  5. Hand soldering is less or not practical. Some people do hand solder some pretty tiny parts, but it's not practical in more than isolated cases. If you're a hobbyist or on a tight budget, this might limit you to thru-hole or some of the largest SMT parts. For commercial work though, SMT is the way to go.

Some things to think about. But what do you get in return? Typically lower cost - especially if you want your design to go into volume manufacturing. You also get access to the newest parts that only come in SMT packages. And, many designs are space constrained, so you can cram more in while still keeping your board size down.

Duane Benson
I shot a neutrino into the air
And where it landed I already knew



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Hi Bode;

I mean that in most cases, not all though, as the components get smaller it becomes too difficult to hand place and or hand solder with an iron for most people - especially with more complex boards or more than just a few boards.

It's all relative. If you have more time than money, a lot of things become practical that otherwise aren't. Or, if you have the time and enjoy the process, again things become practical that may otherwise not be. It certainly can be done and plenty of people do.

With each smaller step in componentry, a few more people don't have the tools or steady enough hand. I've hand soldered done to 0603 and I've spoken with a few folks who've gone even smaller. It's not something I would want to do on a regular basis though. One guy claims to have hand soldered 01005s - it boggles the mind.

Just out of curiosity, what do you mean by "hand soldering is not practical." Do you mean, "soldering with a soldering iron is not practical with these SMT components?"

Or do you mean "making a board by placing components by hand / without a robot to place the parts is not practical?"

I agree with "hold solder = soldering iron" but my experience with my toaster oven reflow is very different. The spark fun / MAKE / world is full of people making amazing boards and I am pretty good at making prototypes. I have even done a PIC32 TQFP 100 a few times, although I changed to 64 and still like my board house to do it (they do my prototype boards by hand, no stencil, amazing).

Anyway, hand prototypes aren't really that hard with SMT, you just need to spend $50 on a toaster oven a few bucks on paste and syringes.

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