Extreme Surface Mount Soldering

Step 7 The Surface Mount Design Process

With your hot plate controller ready to go all you need is a surface mount circuit board. That means you need a design that you want to realize using surface mount components. I'm not going to tell you what you should design. That's obviously up to you. If you haven't already given this some thought, you probably wouldn't be reading this Instructable. I'll assume you've already got a design in mind and were just waiting until you could figure out how to solder it before going ahead.

So while I'm not about to tell you what to design, I'm quite willing to offer some guidance on how to go about turning your design into a circuit board. Of course, I'll also complete the detailed instruction on soldering all the components onto it. As I stated at the beginning, most of this is pretty well worked out and the information is available. If it weren't for the fact that I see so many questions about how to do this and see so many expensive, complicated, or downright misleading answers, I wouldn't bother. But I do, so I am.

Great sources for new ideas and design guidance are IC manufacturer's web sites. Check out especially their application notes for the circuits you want to design. (That's where I got the circuit for the AC Control portion of the Hot Plate Control System.) There's also a ton of information available from other web sites. So getting good ideas is not a problem, but you still may not be sure your particular design will work. One way to help ease your mind is to simulate your circuit, and the best simulation tool I know of is this one. Although it was originally intended to help with switching power supply design, it's really a rather complete implementation of SPICE and provides powerful simulation capabilities for analog circuits. You can learn a lot more about SPICE using Google. A nice tutorial is here.

Whether or not you create your own circuit designs, you'll still need to draw a schematic and use it to guide your circuit board layout. As you do your circuit board, you'll need to be mindful of where you will have your actual circuit boards made. There are many circuit board manufacturers that cater to the hobbiest, but I'm going to recommend just one based on the combination of very reasonable prices and very solid support. That's BatchPCB which is the service that SparkFun uses and recommends (actually use Gold Phoenix to fab the boards). Besides price and support, there is another key reason for this recommendation. The circuit board manufacturing at BacthPCB is based on industry-standard Gerber files and is not tied to a proprietary design software. This means you can use whatever tools you like to design your boards and if you want to go to another circuit board vendor, you're free to do that without having to redesign your circuit boards. You just send the same Gerber files to the new vendor and you're good to go. BatchPCB also gives you solder mask on both sides and stencil (labeling) on the top side of your boards. Solder mask is essential for successful surface mount soldering - especially with very small components. I mention these things because not all circuit board manufacturers offer these at hobbiest prices.

The choice of a circuit board manufacturer that uses Gerber files is important because it means we should choose schematic and board design software that produces design output in the Gerber format. Again, I'm going to offer one solution based on price (free) and support (excellent). The free version of the EAGLE package will meet the needs of nearly every hobbiest. There are many excellent tutorials available from SparkFun and also as part of the EAGLE package itself. The software is mature and dependable with a large user base. EAGLE provides all the flexibility we need to create designs including the soldermask and stencil layers that are so important to successful surface mount design. EAGLE has extensive part libraries, including a growing one from SparkFun, to speed your circuit board design.

A possible alternative to EAGLE is KiCAD. While this package looks to be very capable and has a growing user base, it has nowhere near the support that EAGLE has. The main advantage of KiCAD is that there's no restriction on board size or number of layers. There's an independent evaluation of KiCAD here and atutorial here (courtesy of Scott Driscoll). Unless you are limited by EAGLE's restriction to just two signal layers, I'd suggest sticking to EAGLE. Multi-layer boards are significantly more expensive.

The SparkFun tutorial points to this Gerber viewer. Using the Gerber viewer to view each of our Gerber files is very important because we will see exactly what our circuit board vendor will use to make our boards. Errors can be easily overlooked when we're designing our circuits and using a Gerber viewer can help make sure we catch as many as possible.

The size and shape of the copper pads that surface mount devices are soldered to, the corresponding openings in the solder mask, and the openings in the solder paste stencil are collectively referred to as the footprints. Designing footprints for surface mount parts used to be pretty simple. For example, there are standard rules of thumb built into EAGLE that usually create satisfactory footprints based just on the copper pad dimensions and these work fine for most surface mount devices. Creating footprints using these is discussed in the SparkFun tutorials. However, when we want to use really tiny devices, such as QFN, DFN, and MLF, the footprint design becomes more complicated. Fortunately, the information you will need is readily available. Every IC manufacturer seems to have an applications note explaining the details for their parts. You should find these for the parts you use. If you can't find one for the specific parts you're using, then I recommend this one from Intersil. It seems to contain a very complete set of design guidelines along with detailed footprints for many standard parts. This one from Atmel has some pictures of stencils which show ways to do what Intersil recommends. Note the combination of paste type, size of opening, vias in pads, and thickness of stencil. These all interact - that's why you have to experiment. BTW, you'll have to use the EAGLE tutorials to learn how to create the solder mask and solder paste footprints since the SparkFun tutorials don't really get into those details. There are many other sources of information about proper footprints and soldering technique from Atmel, ON Semi, and Freescale.

While I highly recommend careful design of footprints for the copper pads, the solder mask, and the solder paste stencil, I'll also go out on a bit of a limb here and say everything doesn't have to be perfect. One particular area I've strayed is in the solder paste mask. Most recommendations are to use a metal stencil with etched openings having a prescribed side slope to it. If you do this, you'll find that your stencil will cost as much as or more than your boards and your parts all together!. A much cheaper alternative is to get your stencil from Pololu. The stencil will be made of 3 mil thick mylar with laser cut openings. For tiny openings (like on QFN devices), Pololu only guarantees that you'll get some paste on each pad. In my experience this works just fine and makes stenciling on solder paste a totally acceptable process for the hobbiest. Pololu just needs a Gerber file of your solder stencil design. Since EAGLE outputs that industry standard format, it's no problem!

You'll almost certainly have to order your solder paste since there don't seem to be local sources for it - even in fairly large cities. Stencils Unlimited seems to be very reasonably priced for solder paste. They'll have it shipped directly to you from their supplier. As far as solder paste itself, I recommend using lead-free, no clean flux. You can get this in a syringe and that should be about as much as you'll need for several small boards. Keep it sealed in two zip-lock bags in the refrigerator. Why not regular lead solder you ask. Well, aside from the toxic fumes, you'll find the shipping much more expensive and the refrigerator life much sorter. Since our Hot Plate Soldering System puts out plenty of heat, just use lead-free. (BTW, Stencils Unlimited also supplies prototype stencils. You'll find these to be much more expensive than the ones from Pololu, but I'm sure they're very nice and will work great if you want to spend the money.)

The Intersil apps note referred to above also discusses the soldering process and describes temperature profiles. While the documentation for the exact solder paste you chose will give more details, the Intersil explanation is very thorough and will tell you what you need to know. The Hot Plate Solder System you've built can come very close to the desired profiles. Certainly, it comes close enough to produce very satisfactory results based on my experience. We'll delve into solder profiles in a moment.

Here are a few additional hints that didn't seem to fit anywhere else:

Buy your components first and have them handy as you design the board. Make sure the footprints you use or create match your parts.

Print out the board design and place parts on it to be sure it all fits, that clearances are adequate, etc. Do this before you actually order the board.

Learn the limitations and restrictions of your circuit board manufacturer. These are known as Design Rules and you must know and observe them. The ones for BatchPCB are here. SparkFun provides an in-depth set and a great discussion of theirs here.

Professional PCB designers use checklists to be sure they don't miss any steps. You will find it helpful to create your own as you go along.

Read as much as you can - don't just use one info source. (Not even a great one like this Instructable. ;)}

Successful surface mount design requires some time and effort to learn. Start small, be patient, and practice. But it's not rocket science. Is it complex? Yes. Is it difficult? No, not really. Is it beyond a resourceful hobbiest? Not at all!