Computing stuff tied to the physical world

Soldering one-off SMD circuits

Now that we’re running up against the limitations of solderless breadboards, it’s time to find alternative ways of constructing electronics circuits. And in this day and age, there’s really only one mainstream approach worth pursuing: point-to-point soldered wires.

There are many ways to solder together components. It’s all about addressing the issue of mechanical stability, as well as taking care of the electrical connections, evidently:

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This impressive image comes from a Czech website. It shows how to make a Z80-based computer, as built in the 1980’s. Point-to-point soldering has been around for a while.

We’re not going to be quite as ambitious, but we can re-use the same technique today to construct a small board with an SPI dataflash chip on it, piggy-backed onto an LPC810.

The first step is to mechanically fix the chip in place – using the VCC (8) and GND (4) pins:

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The rest of the pins have been bent up a bit, to avoid touching the pads which are a bit too large for this SMD component. Then we use “Kynar wire-wrap” wire for all connections:

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This type of wire is available in different size spools at a very reasonable cost. The core is 0.01″ in diameter and has a resistance of about 0.3 Ω/m. Some important benefits are that the insulation is easy to remove, the wire solders very easily, and perhaps most important of all: the insulation can withstand brief contact with a hot soldering iron tip without immediately melting away.

Some links to Conrad’s Dutch web-shop, also available in most other European countries:

  • 15 m of wire-wrap wire, it’s available in many colours – 606632
  • side-strip pliers, suitable for these thin types of insulated wires – 1312388
  • another kind of wire stripper, mostly suitable for end-stripping – 821746

You might think that stripping such short bits of wire is difficult, but here is a video of an interesting approach. It strips the insulation in the middle and then shifts it around to make precisely controlled pieces of wire – while still being easy to hold and solder:

No need to watch all 10 minutes, the first 1:30 min at the time linked to above is enough.

As for the pcb material to use for this: in the above images, a low-cost single-sided paper-based “perf-board” was used, mainly because it’s very easy to break and cut into small pieces as needed. The trouble with single-sided copper plating on this type of material is that the pads will easily come loose when heated too much, or when pushed on.

A much better option is FR4 with “plated-through” holes and pads: first of all, it supports soldering on both sides, i.e. placing components on one side and soldering wires on the other side, but also such plated-through holes are a lot sturdier to solder on and to handle considerable amounts of mechanical force. This is most useful for headers, switches, etc.

A drawback of FR4, which is commonly used for PCBs, is that it’s much harder to cut. A small hacksaw could be used, or – with some patience – you can make a series of cuts with a Stanley knife on both sides, and then wiggle and break the board in half with flat pliers:

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This option is more durable, and will withstand lots of soldering and re-soldering.

For anotherastonishing – technique, check out this demo board and 10-minute video. This uses “magnet wire” with a clear insulation that melts and fluxes the wire at ≈ 400°C.

Who says one-off electronic designs have to end up as quick -and-dirty builds?

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