Assembling PCBs With Surface Mount Components

Summary: it does work and isn’t too painful. Naturally I’m evolving methods and techniques as my skill improves.

This outlines how I placed solder paste, placed components, saw small components, soldered and reworked small PCBs. I didn’t want to be constrained by the time it takes to apply solder paste, position components, and reflow in the HackSpace oven, so I looked for ways I could do the whole process at home.

Also see my techniques for designing and making homebrew PCBs, and reference material.

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5cm * 5cm, Component View

Placing Solder Paste

I used lead-tin solder paste melting point 179C, so as to minimise the chance of scorching components, 25g, £20.

I tried three techniques.

Using a polyimide paste stencil from OSH stencils 5cm*5cm $14. Order placed to order received: 14 days (including their noticing I’d made a cockup and flipping the order at no extra cost).

  • works well for discrete components and large ics e.g. SOIC outline with 2.0*0.6mm pads separated by 0.7mm
  • just about works for “small” ics e.g. TSSOP outline with 1.5*0.3mm pads separated by 0.3mm
  • fails with the large FMC mezzanine connector with 100 3.0*0.5mm pads separated by 0.3mm; the film lifts and solder paste pools underneath some of the pads (other pads are perfect). Maybe it would work with more practice on my part (and I have an idea about how to improve my technique). But in the end I just wanted the damn board made; my remaining life is too short as it is!

N.B. I cannot fault the OSH Stencil – the problems are due to the PCB/component design and my poor technique. Their service is good as well. I would use them again.

Extruding paste through a 0.5mm needle just about works, but there’s difficulty

  • controlling the speed
  • “chopping” off the right amount
  • needle getting blocked too often (I used 30AWG wirewrap wire to clear the needle)

I’m sure it would work well with an automated machine, but I haven’t got one.

Using a jeweller’s screwdriver works surprisingly well. It is the best way for the FMC mezzanine connector: just puddle the solder along the edge of the contacts (i.e. not under) and don’t worry about bridges across the solder resist.

Also see the Bristol HackSpace wiki page on smt stencils, but I haven’t tried those techniques yet.

Placing Components

I tried several techniques:

Overall I find the tweezers most useful.

Vision

Placing components:

  • +2D to +3D £5 reading glasses from a local supermarket, optionally together and optionally at the same time as
  • a £10/£20 magnifying visor

Inspecting joints needs more magnification:

  • I sometimes use a *8 surface contact magnifier which also includes mm/thou rulers (0.2mm, 10thou resolution), or
  • a handheld *8 lens
  • I didn’t bother with a microscope (neither USB nor analogue), since I’m unconvinced they would offer me any advantages

Soldering

I principally use a technique inspired by SparkFun’s reflow skillet method for small SMD components including ics. Large components such as FMC connectors and through hole components are left until later.

I initially intended to use my slow cooker, but didn’t reach the necessary temperature quickly enough, so instead I used a £5 saucepan:

  • stainless steel not non-stick Teflon, since PTFE can be rather unpleasant if it gets too hot
  • with a glass lid so I can observe the solder paste melting. The idea is the lid probably keeps the air above a PCB warmer, thus potentially heating up components faster and minimising strain due to temperature differences
  • 2mm of sand in the bottom, to diffuse the heat and act as a buffer. I didn’t verify whether or not this was necessary, and the only downside is that sand might need to be brushed off the PCB’s underside

This method needs calibration using sacrificial trial boards, just as you would with any other method:

  • large SMD components (e.g. tantalum capacitors) that might move too readily can be restrained by roughly tacking them in place using a soldering iron with a 0.5mm tip
  • loosely hook 30AWG wirewrap wire into each corner mounting hole, and use the wires to lower the PCB into the saucepan. The wires are sufficiently flexible that pressing down the lid deforms the wires thus keeping a reasonable seal
  • warm pan (inc sand) on lowest gas setting, and lid on
  • every 30s remove the lid and use non-contact thermometer to measure sand’s surface temperature
  • when sand surface rises to >140C, put the PCB on the sand, replace lid, continue to monitor temperature
  • if necessary increase gas for 30s to rapidly increase temperature to just below the solder’s melting point
  • watch solder paste through lid, continue to monitor temperature
  • increasing the gas a notch causes the temperature to stabilise at around 220-230C
  • after appropriate time, remove PCB; the sand will stay warm for a long time – probably too long a time
  • through-hole components can now be hand-soldered

When the reflow skillet technique isn’t sufficient, I use conventional hand soldering using a temperature-controlled iron with a 0.5mm tip. I used the cheapest iron I could find, so I don’t know the temperature, but it wasn’t set at maximum. It is possible to solder all components by hand, which enables a board’s function to be tested bit-by-bit, but it would be tedious for a second board.

Soldering the large FMC connector works well with a technique inspired by SchmartBoards SOIC boards:

  • hold the connector in place with polyimide tape, which withstands the temperature
  • using a jewellers screwdriver, add solder paste around outside (not under) and across the solder resist
  • tack one corner pin in place with a soldering iron, then mechanically drag the opposite corner into the right place and tack it down
  • run soldering iron along solder pads and between two pins to drag paste/solder along solder pad against connector leads. Ensure both leads are heated both times so solder reflows under leads. Having extra-long solder pads helps
  • there is a visible flux residue, not apparent with the saucepan reflow technique

Soldering very fine-pitch ICs is similar except that

  • the paste is placed first, then the IC positioned
  • the IC is held in place by pressing down with a jeweller’s screwdriver while one corner is tacked
  • the IC is dragged and the opposite corner tacked; obviously “dragging” is only possible for ICs with flexible leads

A small number of individual SMD components can conveniently be placed using the hot-air gun techniques outlined in the “rework” section.

Rework

It is possible to reflow a second time, apparently without problems.

I remove excess solder using a copper braid that I impregnate with liquid flux just before use. That seems to work, but one pad/track did begin to lift after I heated it up too much.

Removing and replacing individual components with a hot-air gun is relatively easy, but a major issue is heating up the PCB under the component.

  • higher airflow, lower temperature will quickly get the PCB up to temperature but can blow the relevent component away. While desoldering and providing the component ends up in the bin, that’s not a problem, but the airflow may also dislodge neighbouring components.
  • lower airflow, higher temperature reduces the chance of blowing the component around, but might “cook” protruding components. This is mandatory when soldering components, since the airflow must not be allowed to interfere with surface tension pulling components into the correct position.

In both cases, covering neighbouring components with self-adhesive polyimide tape will deflect a useful amount of heat. In extreme cases with large components. e.g. electrolytic capacitors, it can be helpful to create a vertical “wind fence” from a throwaway cardboard tube.

Here’s an extraordinarily comprehensive professional guide to circuit board repair and rework.

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