Surface Mount for the Hobbyist

Surface mount components are a daunting thought at first, but they open up several possibilities:

• Some components are unavailable in leaded packages
• Some components are more expensive in leaded packages
• Leads bring parasitic inductance with them at high frequency
• Components can be smaller

However, it does come with a few downsides:

• More planning is necessary
• You'll need more tools
• Breadboarding isn't really possible
• Components can be much smaller

I recently found that I couldn't avoid designing a surface mount board because the components I wanted couldn't be had in through-hole configurations, so I jumped right in and tried to avoid using through-hole components. Go big or go home, right?

After you've designed your circuit, there are a few things to consider. First, consider this chart. The smallest surface mount part you should use in your layout is essentially determined by how much you're willing to spend on tweezers and optics--at some point you'll need a microscope to place parts. The next thing to consider is the spacing between solder pads. In the image below, you can see that there is some risk of a short between the right and middle pads in the bottom row. I didn't anticipate this until I was placing parts on the board, and it happened to work out fine, but it could have caused trouble. Finally, get your circuit board printed with a solder mask, this will dramatically speed up the soldering process.

Closely-spaced 0603 SMD pads

Bearing those things in mind, design your board and send it off for printing (or print it yourself). Possibly with your components, order a heat gun and solder paste. I used this heat gun and some leaded solder paste. The lead-free stuff only had a shelf life of 6 months, refrigerated, and I wasn't willing to put it in the fridge to begin with. While you're waiting for everything to ship plan how you're going to test the board during assembly. Testing before and during assembly is important because you cannot see underneath the surface mounted components, so you'll have a hard time visually checking for short circuits. I mostly followed this arrangement:

1. Check connectivity of ground planes.
2. Check for shorts to ground from non-ground traces.
3. Check for catastrophic shorts, like a 5V rail to an RF line.
4. Solder on capacitors and check for shorts through/around the caps.
5. Do resistors next, and check for correct point-to-point resistances.
6. Solder on any voltage regulators, but hold off on power connectors.
7. Apply power to the voltage regulator inputs, check the output voltage.
8. Mount cheap silicon, check diode voltage drops where possible. If possible, apply power to regulator inputs and test.
9. Solder on expensive parts.
10. Solder on connectors with plastic cases last.

This arrangement should help avoid overheating anything pricey and avoid melting the plastic bodies of connectors. It also allows you to check the power supply voltages before they have a chance to cost you money. Finally, by checking for shorts in capacitors and resistors early, it avoids damaging voltage regulators.

Before the heat gun and solder paste arrive in the mail, you'll want to scrounge up some scrap circuit board and a large flat piece of sheet steel. The circuit board is for setting the temperature of the heat gun, and the steel is to protect whatever you'll be doing your soldering on (workbench, coffee table, whatever). Any time you solder, use the sheet metal to protect your work surface. The metal itself will get pretty hot, so consider using something insulating to keep it off your work bench/coffee table. Better yet, consider doing this outdoors.

Once you've got your solder paste and heat gun, you'll need to set the temperature of the heat gun (if it's adjustable). Apply a small blob of solder paste to a clean piece of scrap circuit board and heat it up. Adjust the temperature of the heat gun just high enough to get the solder to flow. Keep that setting for all your surface mount soldering (unless you change solder, then re-calibrate the heat gun).

When you have the parts, clean the circuit board with IPA. For each component you wish to solder, apply a small blob of solder paste to each contact pad and stick the component on top of the solder. Clean up any excess which sneaks off the pads. Soldering multiple parts at once saves some time, but increases the potential consequences of screwing up. The rest is pretty straight forward; turn on the heat gun and warm up the solder until it melts and flows. The solder mask will ensure that any excess flows back under the contacts.

Build your board up, step by step, and check it along the way. Eventually, you'll end up with a working board like the one below.

Complete surface mount project

Oscillator

I'm blogging to help me keep track of a project to build a HF radio for my Summits on the Air outings. I've been drawing on the experiences of others that I can find on Google, so I feel that I should post some things I've learned to share in kind.

First, I considered building around some passive double-balanced mixers, but this requires either purchasing monolithic units or purchasing two transformers per mixer, along with diodes. In either case, I could hardly justify, to myself, paying $10 in shipping for $5 worth of parts. So, I started on a Gilbert cell. Inspired by this work, I'm pretty dead set on a single-ended Gilbert cell mixer.

There's one problem. All of the radio bands worth considering for SOTA are above 7 MHz, probably realistically above 14 MHz and my cheap, ancient signal generator only works up to 1 MHz, so testing my mixer will require a new oscillator.

It's worth mentioning, before putting my first circuit diagram up, that I was inspired by the story behind the NorCal 2N2/XX transceiver and a sale on Newark to use mostly PN2222A transistors in my designs.

The oscillator is as follows:

Oscillator Circuit Diagram

Q1 and Q2 are both PN2222A transistors. L1 is a hand-wound coil, C1 is derived from observed circuit behavior. This circuit was an adaptation of this work.

The simulated output is:

Simulated Oscillator Behavior

The blue trace is the voltage at the Q2 collector. The red trace is the voltage on the Q1 base.

I measured the voltage on the Q2 collector in the constructed circuit:

Measured Voltage on Q2 Collector

I'm not sure how to reconcile this terrible photograph of the scope screen with the simulated behavior. To make matters worse, connecting the oscilloscope to the tank causes the resonant frequency to jump to ~25 MHz, so I've got to buffer the tank voltage to measure it. This will undoubtedly load the tank with more capacitance and reduce the maximum frequency of the oscillator. I hope that the output will be much more sinusoidal, and maybe it will be worthwhile. If the maximum frequency is too low, I can wind a new inductor.