But I'm getting ahead of myself.
Starting the build:
As noted before, there are 480-ish parts - most of them being 0805-sized surface-mount resistors and capacitors - these being somewhat smaller than grains of rice - so I set about putting together my assembly area by buying a shallow baking pan.
The thing about these tiny parts is that no matter how cheap or expensive they are, they are equally invisible when they fly away accidentally - or just fall off the board, and even if the part costs less than a cent, if you don't have any more, it will then cost you several dollars to get more of them. (For the really inexpensive parts, just get several times the quantity that you need!)
I lined the bottom of the baking pan with white paper for both electrical and thermal insulation and started putting parts on the UI (User Interface) board.
At this point I should say that having put it together, I learned something that anyone putting one of these together should consider:
A few mistakes:
Assemble the RF Board FIRST, then assemble the UI board!
The reason for this is quite simple: There are no really tricky parts on the RF board and by the time you finish it, your SMD soldering skills - whatever state they might have been in when you started - will be much improved - and this will come in handy when you solder the fine-pitched 100 pin microcontroller on that main board to it!
I didn't do that.
In following the recommendation of someone else who'd put one of these together, one of the first things that I'd put on the board was, in fact, the microcontroller. Although I have soldered a fair amount of SMD before, it had been several months since I'd done it and I was a bit rusty.
Also, when I started soldering the microcontroller, I'd not noticed that the PC board was somewhat warped, probably a result of having been through the post from the UK, and that after I'd finished soldering the pins down a large percentage of them did not actually make contact to their pads because of the warpage! Had I my wits about me I would have removed the microcontroller, clamped the board down to flatten it and done it again, but I didn't do that: Instead, I went around, pin-by-pin with an extremely fine-tipped iron and tacked each one down, laboriously checking it and its destination for continuity until I'd finally gotten them all - a process that took over an hour and actually resulted in my ripping one of the pads off and a bit of microsurgery on the board to make the connection.
1) Build the RF board first to get practice at soldering SMD components
2) Before soldering the microcontoller, clamp the board down to assure that it is perfectly flat!
I employed several methods in populating the boards.
The first thing that I did was to wash the boards in soap and water, scrubbing fairly vigorously then patting them dry, followed by wiping them down with denatured alcohol to remove adhesive tape residue from the packing and residual oils - followed by another wash with soap and water and a good rinse and dry.
For soldering equipment, I would recommend, at a very minimum, the following:
- A TEMPERATURE-CONTROLLED iron. The iron should have a closed-loop feedback in it to maintain a constant temperature and not merely a dial to make it hotter or colder. DO NOT attempt to solder this sort of thing if your iron does not have the ability to maintain its temperature to within 10 degrees C (about 20 degrees F) or so of its setpoint or you can damage the board. Such irons are pretty cheap and plentiful these days so there's really no excuse for not having one if you are building these sorts of projects!
- Fine (small) solder braid. This is the small, thin solder braid (the trade name being "Solder Wick") that is roughly 2.5mm (1/8") or so wide or smaller. It really needs to be this small in order to fit between pins of SMD devices to suck up extra solder.
- Small-diameter solder. This would be the solder that is anywhere from the diameter of a hair to the diameter of #24 AWG wire. In the U.S. you can still get the 60/40 or 63/37 Tin/Lead solder - which melts at a lower temperature and has better longevity and chemical stability - but good-quality lead-free solder will also work. If you do not use rosin core solder, read the instructions with the solder carefully as some of the "organic" fluxes MUST be immediately washed off after soldering to prevent electrically conductive residue and/or corrosion from occurring!
- Good-quality tweezers. I have both metal and bamboo tweezers that are helpful for both picking up parts and holding them down during soldering. The bamboo tweezers, bought for less than $1.00 on EvilBay (including shipping), are nice in that they are insulating both electrically and heat-wise, the latter making it easier to solder down parts. Flat toothpicks also work very nicely for holding down components for soldering.
- Magnifiers! Unless you have extraordinarily good close-up vision, you will want some good magnifiers. These can be in the form of very strong glasses or binocular goggles - some of which have extra flip-down lenses to change magnification - for hands-free use as well as jeweler's loupes and strong magnifying glasses for extreme close-up examination of solder joints and circuit board traces.
- Volt-Ohm Meter. An absolute must for troubleshooting, measuring voltages and resistance to determine if something is amiss. It need not be fancy, but a cheap, $5 digital meter is probably better than a $10 automotive-type analog meter!
- Solder Paste. Using a dispenser - often in a small, hypodermic syringe with a large-ish gauge needle, this contains both paste and flux. This is applied to the pads and then the part applied into the paste and then heated with hot air - see below.
- Hot air rework tool. These are essentially a miniature heat gun, but with fairly precise temperature control and the ability to be fitted with different tips to heat various areas and shapes. If you need to remove a multi-pin chip such as the 100 pin microcontroller, this is going to be the ONLY way that you going to be able to do it without damaging either the chip or the board! This also allows you to solder many components at once if paste has been applied to their pads and then the components applied to them.
- Flux pen. This looks like a felt-tip pen, but filled with liquid solder flux. It is a good idea to apply this to each solder pad on the board to make it easier to solder, making better and more-reliable joints.
There are (literally) hundreds of web pages and YouTube videos on how to solder - specifically, how to do surface-mount soldering, so I won't explain the exact methods - just name them.
The "dab, place and solder" method:
For the individual, small, passive components such as resistors, capacitors and inductors I generally did the "dab, place and solder" method in which I would apply the solder paste to the pads of 4-8 components, place the components on those pads, and then, with the fine-tipped iron and either a toothpick or the bamboo tweezers, hold the component down while soldering.
When doing this it important to only place 4-8 components at a time because:
- It is best to place the component in paste when it is still "wet" so that it not only sticks.
- Doing a batch of 4-8 components is more efficient than doing one at a time, but not so many that you are likely to forget where each component that you just place is.
After soldering them down, I would then run my finger over the board to discover any "loose" components - that is, ones that were stuck to the board with paste, but not soldered, and hopefully I'd be able to quickly discover where it was supposed to go!
For soldering multi-pin ICs, there are many methods using soldering irons. Most of these involve tacking a corner pin or two and going from there, sometimes soldering each pin at a time if the pin pitch isn't too small.
One of the more popular methods involves "flooding" the pins with solder, often by dragging the tip across them all to distribute it to all of the connections and then sucking up the excess with solder braid: You can find web pages and YouTube video describing this "Drag" method in detail.
Of course, if you don't have solder paste, the use of very fine wire solder works fine as well with the appropriate adaptations of the above techniques.
The hot air method:
The above method works well if you are soldering same-value components scattered all over the board, or if you are attaching components with larger pin counts. If, however, you are able to populate a lot of components in a small area of a board it will be quicker and easier if you were to use the hot-air tool.
In that case, you would apply paste to the pads and then place the components into the paste - this time, for (possibly) many more components. Then, very gradually, heat up the area with the components until the solder flowed completely. In this case, with the components concentrated in one small area rather than all over the board, it makes sense to spend the time to bring up the temperature gradually.
One point of concern with the paste - particularly if it is heated too quickly - is that the liquids within it can flash-vaporize and bubble within: When these bubbles pop, it can cause the part being heated to jump off the board, or at least out of position, requiring you to stop what you are doing and relocate the part. If you are finding that the parts jump around when you are heating them, you are probably heating them too quickly!
For soldering larger devices such as multi-pin ICs, the typical methods include laying a thin line of solder paste along the line of pads rather than a dot on each pad: Methods for doing this may be found on many web pages and YouTube.
One advantage of having a hot air gun is that even if you do use a soldering iron, if the solder masks were applied properly, reheating components that were previously soldered with an iron will often make them re-align and straighten up and make their joints look more uniform. It also has the advantage that if you happened to miss soldering a joint to which you'd applied paste, it will now be soldered!
Next time on this topic: A few pictures and comments on the actual assembly!