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 Cheap and easy reflow soldering by Erich VK5HSE

 

Presented by Erich Heinzle VK5HSE at AHARS show and tell night

 

For step by step images look at the gallery or large images

 

This is a simple jig that can cheaply and effectively allow you to do reflow soldering at home.

The solder paste can be bought in a syringe from Jaycar and also at Aztronics.

This is not to be confused with a flux pen, which is used when manually soldering surface mount devices with a soldering iron, using the soldering iron to reflow solder already on the pads.

Solder paste has a limited shelf life. Keep it in the fridge.

A small needle or toothpick can be used to apply dabs of solder paste to the circuit board pads. This is easier than trying to use the nozzle on the solder paste syringe (which is easily unscrewed to allow needle or toothpick use).

Tweezers are pretty much essential for component placement.

The illuminated desk magnifier can be purchased from Aztronics. You'll need one.

The jig is a 3mm (1/8 inch) thick aluminium plate suspended halfway along a pair of aluminium angle arms. The plate has spacers between it and the arms (M3 nuts) to minimise heat conduction away from the plate into the arms. The plate should be flat.

The arms are attached to a board at the rear of the jig which extends downwards and forms a fulcrum at the same level as the base of the hot plate.

The 3mm plate is placed on the hot plate.

A handle joins the two arms at the front of the jig, to allow the metal plate to be lifted gently from the hot plate once the solder paste has melted and flowed. Dimensions are not critical, but the longer the arms, the more gently you can do the liftoff.

Jerking a circuit board full of components sitting on molten solder is not good, and is the major problem associated with doing reflow soldering in infrared toaster ovens, another common technique.

I have used the Tiffany brand, model HP1500, hot plate from Cunningham's warehouse, which was less than $40. Infrared or benchtop toaster ovens cost a lot more, and are arguably more difficult to use for reflow soldering because of the difficulties in removing the board without jarring it, and the risk of melting plastic components with the heat hitting the board from above.

A jeweller's loupe is useful when looking for shorted pads, and desoldering braid is used to clean up any shorts. Here are shots of a NUEPSK digital modem USB board, and an HPSDR Linear Power Unit board being reflowed.

 



 

To determine what hotplate setting and timings gave the best match to the recommended JEDEC heating and soaking profile, some trial runs were done using a K Type thermocouple (the yellow lead visible in the side view above).

It turns out that setting 4 gives repeatable results with a nominal 240V AC power supply. The hotplate can be started at room temperature on setting 4, turned off at 250 seconds, and then the jig gently lifted at 280 seconds to cool. Once lifted off the hotplate, the jig can have a chock gently put under it and the hotplate removed from underneath. This is the temperature profile for such a run, and is a good match to the recommended JEDEC profile:

 



To see if the circuit board could simply be left on the hotplate rather than lifted to cool after turning off the hotplate at 250 seconds, a run was done to see what would happen. It turns out that the thermal mass of the hotplate is far too great, and the board will remain too hot for too long, falling outside of the recommended JEDEC reflow soldering times:




Keen builders would:

1) add to the rear board a means by which the fulcrum could be adjusted to precisely level the 3mm plate on the hot plate – I used pieces of foam to get the level just right

2) add a mechanism at the front, such as a cam, to allow the 3mm plate to be lifted from the hot plate in a more controlled fashion – I was lazy and used some offcuts of wood to chock it after gently lifting it

3) use the front panel timer board from a gutted microwave oven to turn the hotplate on for the desired period.

Pitfalls:

You can't do double sided boards

You need to start with the hotplate at room temperature if you use these timings.

Hints:

Inspect the reflowed board to make sure all the matt grey solder paste has become bright and shiny, and that there are no shorts between fine pitched components.

Have a kettle with cold water nearby to sit on the hotplate once you are done to cool the hotplate down.