Notes on electronics


A list of tools any well-equipped electronics lab should have. While this list is possibly a bit daunting for the beginner, it shouldn't discourage you too much. You don't need all of the items in this list. I'm just saying they're very useful in certain situations and very good to know they exist.

Construction techniques

Solderless breadboards are very popular these days. They are very useful for small circuits, especially the slow analog kind. It really pays off to buy "jumper wires" on ebay from china, they are very cheap and take out a lot of tedium. However you have to keep in mind they are fundamentally non-permanent because the contacts are too loose and will oxidize over time. They are also rather limited to low frequency operation and the big wires limit the complexity of the circuit. You're limited to 2.54 mm pitch parts though you can buy "breakout boards" for other footprints.

Making a printed circuit board is the ultimate technique. It's extremely cheap in large quantities (which is often as little as 10) and readily allows you to use parts of any footprint. High frequency characteristics are excellent. Assembling PCBs is far faster than any other technique. However, it takes a long time to have a PCB made at acceptable cost (3-6 weeks, including international shipping). My recommended suppliers are Oshpark and Hackvana. Price varies from $10-$100 for "beginner" boards and can easily reach several hundred dollars for more advanced work. This is the only real option with advanced technology (BGA packages, lots of small pitch SMD stuff).

Once the PCB has been made you have two options for assembling it. You can hand solder (put component on board, apply soldering iron and solid solder) and reflow solder (apply solder paste, place component on paste with tweezers, apply heat). I recommend reflow soldering for SMD as it scales very well to boards with lots of components (aijuboard has well over 400 components) and you're virtually unlimited in your choice of components. There is also wave soldering and vapor phase soldering, however they are all but irrelevant for the small lab.

SMD components are definitely worth using on a PCB. A lot of people underestimate their own ability to solder surface-mount components due to lack of experience. Even if your fine motor skills are poor you should be able to solder 1206 -- in fact I find it more straightforward than soldering leaded components. My fine motor skills are probably below average (for a person of my age) and I can still solder 0402 by hand (although it is tedious). Hand soldering parts with pads below the package (QFN, BGA, QFP with thermal pad) is probably impossible; although i've seen crazy hacks I highly recommend reflow soldering. Soldering 0.5mm qfp and smaller is a pain in the ass with any technique. 1.27 mm SOIC is pretty easy to solder on the other hand and definitely worth considering.

A cheap but tedious alternative to PCBs is perfboard, which is basically a circuit board that has a regular 2.54 mm grid of holes and solder pads in it (there is a variant called "stripboard" that connects them in rows that you cut with a knife but I find it useless and annoying). You put components on the top and wires on either the top or bottom -- your choice. DIP components are definitely preferred, although with fine enough wire and some practice you can do 1.27 mm pitch leaded SMD (if you're crazy you can go down to 0.4 mm QFP under a microscope). This method eliminates laying out a PCB and the 4 week PCB manufacturing turnaround. The cost is $5-$10 per 16x10 cm perfboard. You can get cheap perfboard made from FR2 but it sucks, don't buy it (the pads stick poorly to the base material and are very easily lifted off); look for FR4. (FR2 is literally made from paper whereas FR4 is made from glass fibres in an epoxy resin. FR4 has the downside (?) of being mechanically very rigid and hard, which makes machining it difficult. FR2 can be cut by scoring with a knife and breaking it on a table edge, this will definitely not work with FR4. A hacksaw works well. Cutting FR4 by machine releases glass fibre dust that is very nasty, wear breathing protection).

Using normal wire with plastic insulation is very annoying because you have to strip it and can only do one point-to-point connection at a time. Wires are also bulky. Pen wiring uses fine enamelled wire instead. It also uses a tool called a wiring pen that lets you wind the wire around pins. You can route buses in a daisy chain fashion by going along each pin, then cutting it off using a scalpel at the last pin. You then solder them at a high temperature (400 °C) and you have to wait for the insulation to melt and "bubble off". I highly recommend this technique, it's so much faster than using ordinary wire. Cost is $20 for a wiring pen and $20 for 120 meters of wire, which is plenty. Do note that the resistance is significant and the wires can only carry 100 mA or so of current and should not be used for power supplies. It also releases toxic compounds when soldered and you should invest in fume extraction if you plan on doing this on a daily basis (I'm happy doing it in front of an open window).

Wirewrapping is a venerable permanent solderless technique that wraps wire very tightly around special square wire wrapping pins. This sounds dodgy at first but a proper wirewrap connection is in fact more reliable than a soldered connection (solder is pretty brittle). I've never tried it because it requires special wirewrapping sockets which are expensive ($5 per IC). Also cheap wirewrapping tools allegedly sucks, and good "semi-automatic" equipment comes at a cost ($200-ish on ebay). Might be worth it if you can score a lot of wirewrapping equipment at an auction, otherwise not worth bothering.

Whatever method you use, for any hand-wired board I highly recommend printing out a "netlist" listing the pins each signal connects to and then ticking them off as you go along.

Notes on reflow soldering

The three steps are: 1. Apply solder paste 2. Place components 3. Reflow. Rinse and repeat if components on both sides. (Solder surface tension will hold the components on the bottom up even the solder is liquid — as long as the components are not too heavy.)

Solder paste can be applied via either a syringe (for very simple pcbs with large pads, takes practice and is tedious, don't recommend it) or a solder paste stencil (my method of choice). The stencil is either plastic or steel, steel is better (easier to use, lasts longer) but more expensive. Oshstencil provides plastic stencils at $5-$10 and Hackvana supplies metal stenciles for $30-$50 ("Framed" metal stencils are fancy ones for production use, don't bother unless you have too much money).

To apply solder paste you place the stencil on the pcb, align it well, secure it, put a blob of solder paste on one side, spread it using a credit card, razor blade etc and very carefully lift off the solder stencil. The securing step is absolutely crucial. If the stencil moves horizontally or vertically or flexes by even a single millimeter your results will often be abysmal with fine-pitched stuff. You must also be very careful lifting the stencil in order not to smear the paste. (It doesn't really matter too much if, e.g., 0.5 mm qfp is smeared a bit, but it will reduce yield and require rework with a soldering iron later). If you're lucky you end up with a nicely pasted board. If not, wash the board and stencil using pure isopropanol and try again.

One simple technique to secure the board is placing unpopulated boards of the same height around the board (keep some random boards around for that purpose) and then taping it down with lots of tape. You then remove all tape but one strip so as to leave a "hinge" that lets you lift off the stencil relatively cleanly.

Placing components is easy. Use a very fine pair of antistatic tweezers and just place them on top of the solder paste. Pushing them down a little bit will effectively glue them to the board. It's amazing how much abuse boards with components secured only by solder paste can handle. The one thing they REALLY don't like is your elbow or some junk on the table scraping components and solder paste off the board -- be extremely careful and keep a tidy working area (NO unneccessary items on your desk) for this reason.

Reflowing is the moment of truth. You can use either hot air (at 250-350 °C), which is tedious and requires practice, or an oven. The best ovens in terms of value-for-money are cheap toaster ovens you can get at discounters or the like. I paid 50 EUR for mine and it has worked a treat. Needless to say any oven you used for reflowing should never be used for food preparation. Use a thermocouple secured to the board with Kapton tape to measure temperature. You want to heat it to at least 200 °C, maybe 220 °C just to make sure all parts of the board have reflowed. (Temperatures for leadfree solder are higher). You also want to optically inspect the board for cold spots -- the solder part turns shiny when it melts making them reasonably easy to spot. Contrary to popular opinion you don't need to follow any precise curve (does your soldering iron follow a curve?), just heat it up and let it cool slowly. As soon as you're happy with the result turn off the oven and let it gently cool to 100 °C without touching anything. You can then open the oven and wait for it to cool to temperature you feel comfortable handling. You should be left with beautiful solder joints everywhere.

Chinese reflow ovens are tempting but worse than useless. Not only is their performance abysmal (they tend to be infrared ovens which is a very difficult technique the industry has mostly abandoned), they are often poorly constructed and pose a fire and electrocution hazard.

Finally you are probably going to find a few defects. Bridging between fine-pitched leaded components is very common in my experience. Use a soldering iron, a flux pen and solder wick to clean it up (takes some practice). Another common defect is "tombstoning" of passive components -- only one pad is soldered properly and as a result of solder tension the component becomes a tiny tombstone. Using tweezers and a soldering iron, remove the component, then handsolder it.

You can not really reflow pin-through-hole components, do them by hand as a final step.