Here are some soldering tips according to part of the ZX81 Issue 4 page which I find generally useful. The information relates to a double sided DIP PCB.
Here are some pics of the not yet stripped and cut down to size PCB, so still 10x16cm in size:
ZX81 Issue 4 top layer - just etched
ZX81 Issue 4 bottom layer - just etched
As you can see, the photo sensitive layer is still on the PCB. Next is to cut off the sides, strip off the paint layer and start drilling the holes. After drilling the holes, you also could apply some flux spray which can protect your board and make soldering easier. I didn't do that in these examples, but I think for my next PCB I will. The spray can protect the copper against the acid on your fingers a little bit which prevents corrosion of the copper surface. Another tip would be if available to use tin covered wire which is easier to solder as well.
ZX81 Issue 4 - Bottom Layer - cut, drilled, via wires just soldered in
Here is a photo where you can see the small wires which I have 'sewed' in. This method of threading through the thin wire used for via connections back and forth saves us some time in inserting the via wires, as well as provides stably fastened wires which makes soldering easier. An advice here: try to anticipate the soldering job which is coming up next, and thread the wire through in such a way that the soldering job will not be so awkward and it will not be so easy to accidentally solder the wire to the wrong via or onto an adjacent trace.
a close-up of the thin via wires which are threaded in
From this close up you can see what I mean. I tried as much as possible to thread the wire in such a way that the soldering job would not be hindered too much. After this phase is completed, the next job is cutting away the via wires now still present. For this job you should use a pair of cutters which have very small and somewhat pointy jaws which have the cutting area close to the edge of the jaws so you can easily get as near as possible to the vias and cut away the wires which as shortly as possible. The cross section of the cutters should be like this:
Finally the components need to be soldered in. This job should be done strategically by planning the progress of components step by step across the PCB, because we need to solder the components with the bottom as well as the top side of the PCB. Reaching the top wires/pins of the components is the important thing to keep in mind. During this top side soldering job it is best to keep a bright TL light source below the PCB and another one above it, preferable a light source with a magnifying glass integrated, or a light source above the PCB while keeping a magnifying glass on a stand next to the component which needs to be soldered. The more light you use on all sides, the easier the job will be. Also, TL ligth is much better than normal lamps.
First heat the pad and wire or component pin, then add the thin(0.35mm) solder wire to the pad and feed more solder while the solder flows to the pad and component pin until sufficiently covered. Make sure the soldering wire is always made straight again before soldering and the soldering iron's tip is clean before each solder point. Otherwise when you touch a track next to the component it might catch the solder and become involved in the solder joint. When soldering the precision IC sockets, it's absolutely necessary to seperate the two strips from eachother with a pair of cutters so you can reach the pins at the top much better. Another tip: insert another strip on top of the one you are soldering to hold down the pins. Otherwise they might rise up during soldering because the plastic might melt a bit. Later on there are a few more cautions: make sure to insert the ICs with correct orientation because they are not all facing up. Also, make sure when ever pulling an IC to really pull it with an IC extractor, and not using a screwdriver to wedge the IC out, otherwise you might damage the PCB or the IC socket. Some care should be used in that case because the IC sockets are not normal ones. After this phase of adding the components, it will be wise to have another short inspection, though I should say that the inspection should best take place directly after soldering each part or IC strip, because then you can give it more attention. The power connector if possible is best soldered to bottom and top layers to improve it's connection to the PCB. For this purpose you can reach it from below and carefully solder the top tracks to it's side. For the EAR, MIC and expansion connectors it's not needed to solder the top pins. For the video output connector it is best to solder both top and bottom to ensure better mechanical strength. Remember, we will be plugging in the connector later on. Also at the bottom, the video connector's pins should be bent to hold it even more strongly to the PCB.
a close up of the first IC socket soldered in
From the photo you can see how I soldered the top pins of the socket onto the PCB pads. I used a needle tip soldering iron and thin 0.35mm solder wire, with bright TL light above and below the PCB to ensure no hidden short circuits. Remember to add the solder to the pad and pin and not onto the soldering iron's tip itself, or the solder might catch onto an adjacent track.
the second IC socket which is seperated into strips before soldering
Here you can see the first seperated strip soldered in, as you can see I started at the edge of the PCB, planning and working my way across the board, always anticipating to be able to reach the next parts to come. The strip here was a bit melted by the soldering iron's heat, which can be expected with such a delicate job, while still getting used to the type of soldering work needed there. Later on I got more used to this work and it looked much better as I went along. Any imperfections can be cleaned up a little using a sharp hobby knife and very carefully cutting off melted the plastic edge sticking out. Then the result looks more neat. Make sure not to touch the tracks of the PCB with the knife.
The resistor network RN3 of the keyboard connector as used on this particular board only needs 5 resistors. In case you can only get a 8 resistor type, the remaining 3 resistors can be cut off. Make sure to cut through the resistor network a little further away from the last resistor you plan to keep, or the pressure of the cutters can cause the nearest resistor to increase it's value. Just a little increase will not be a big problem though, it will still be usable. For this purpose I would advise using a bigger pair of cutters because these resistor networks are made from very strong material which could damage a small pair of cutters.
a close-up of the video connector just inserted into the PCB
The video output connector can be securely fastened by bending the connector's legs to a right angle as shown in the photo. It's best to cut off a little of the bottom leg as seen in the picture, so it won't reach the centre pad.
the original ZX81 connectors can be re-used for example on the ZX81 Issue 4 PCB
As shown in the photo above, the original ZX81 connectors for power, mic and ear can be soldered out from a ZX81 PCB and used for the ZX81 Issue 4. For making the holes as seen in the photo, I used a broken off hardmetal drill which still had a little bit of drilling area left on it. The drill is so strong (because it is very short now) that it can be used to cut larger areas out of a PCB. It is the type which has about 2,5-3mm diameter for most it's length and only the end is 0.8mm. This 0.8 mm end on my drill is only about 1.5mm long so it's not easy to break off and can be used in a sawing fashion.
Finally remains adding the ICs into the finished PCB. Remember the correct orientation according to the component placement diagram.