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QFN / Hot Plate Soldering

It is surprisingly easy to do DIY soldering of QFN and other surface mount (SMD/SMT) chips with no leads. The technique described here uses a hotplate instead of a soldering iron to tin and solder components. It yields exceptionally good results without extensive touching up afterwards. In fact it can be used for almost any single-sided SMD soldering.


DIY hot plate

Original and flattened solder wire

I have used two 15 ohm 25W 'HSA' aluminium clad resistors wired in parallel. I run these off a 30v supply set to around 28.5v. This draws a little over 100W of power which is about what you need. They are screwed to a 6mm aluminium block about 50x50mm in size. Note that you cannot solder any leads to them because the solder will just melt when it gets used. The wires brace it and I have a threaded bolt in the middle to tension them to suspend it about the wooden base. You can see a small board on the plate ready for cooking. Note that the end caps of HSA resistors can blow off when hot.


SOLDER PASTE


Paste

The easiest method is to use solder paste. It tends to dry out and has a limited life. However, I dilute the paste with liquid flux and apply it very moist with a pin. I try to avoid having paste between pads but as you can see you can dribble the paste over the tiny QFN pads and the solder-resist between them will ensure the paste 'pulls back' onto the pads when melted. Place the components on while the paste is wet. The paste attracts dust and fibres in the air so keep it covered when not applying it to the board.


TINNING WITH WIRE


Ready for tinning

Round vs flattened solder

Paste tends to be quite expensive so an alternative method is to tin the boards using solder wire. My technique is to flatten and stretch 0.27mm solder to get the right quantity of solder. I use a screwdriver as a roller. The thinner the screwdriver shaft (eg: 4 or 5mm) the more the solder stretches. I typically roll 40mm solder to 80 or 100mm. These examples are for a 6mm square QFN40 chip.

I clean and degrease the board then coat it with a flux pen. I then place small pieces of flattened solder on all pads while cold. The flux I use stays sticky for a long time and I find this helpful. When the solder melts it will be pulled to where it makes contact with the board. The sticky flux helps ensure it melts on all the pads I want tinned. The example with unstretched solder above deposits too much on the pads and is difficult to make 'adhere' to all the pads before melting.


Solder melted

Solder filed flat

I then put the board on the cooker. Mine takes about 2 minutes to melt the solder. It keep it on until it has flowed well and then remove it from the hotplate to cool. I then gently file the solder blobs flat with a fine diamond file. This removes the excees on some pads and helps identify which may need touching up. It also gives a flat surface for placing the components.

COMPONENTS


Solder on centre pad

As you saw above I leave the central ground pad of the QFN component untinned. I find this easier to control. Before placing components I clean and degrease the board again. I then re-flux and place flattened solder on the central pad of the QFN and then put the chip onto that without first melting it. Again the sticky flux helps keep the components on their pads while positioning the others. Because the central area has no gaps like we have between pads, and it has vias which tend to drain some of the solder away, I use flattened solder but try not to stretch it. You may need to experiment with how much solder you place on the central pad.


Components placed

Oven effect

The board with components is placed on the cold cooker and a vacuum-formed plastic 'fruit salad' container placed over it to create a simple oven. This helps the components heat up with the board. After the solder has melted I remove the container and usually prod the QFN chip to ensure it settles well. The solder will normally help the component snap into place but check that it does before removing the board from the hotplate. Here are some finished results with 6mm QFN40 and 0402 passive components...


Rx21

Rx30

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