Homebrew, open source, repurposed, hacked, software defined, open hardware

Monday, 30 May 2011

Finishing off the SDR Cube transceiver

I finally got around to starting the final bits of the SDR cube transceiver I started a while back. It's a self contained SDR (software defined radio) transceiver made from a kit.

see: http://www.sdr-cube.com

I have already built the Softrock 6.3 which mounts inside the SDR cube, and now need to make the transmit power amplifier (TXPA)) board, and the low pass filter (LPF) board, and the receive amplifier (RXPA) board.

I have started with the TXPA board, and first of all, the surface mount components.

Surface mount soldering used to be a bit scary, but this technique using a sub $40 hot plate (skillet to those of you abroad) and solder paste, and a simple 3mm (1/8 inch) aluminium heat transfer and lift off jig makes it a 10 minute process to solder the whole board.

You can see the full write up of the settings used and the process at my local amateur radio club - Adelaide Hills Amateur Radio Society -  website:

http://ahars.com.au/htm/hb_reflowsoldering.html

I have done many runs with this hotplate starting at room temperature, and have found with the help of thermocouple readings that starting at ambient on setting 4 gives you a really good approximation of the recommended JEDEC heating profile if you turn it off at 250 seconds, then allow the thermal inertia to continue for another 30 seconds, and then lift off the board at 280 seconds.

A commonly asked question is why not leave the whole lot to cool, and skip the lift off. The answer is that the thermal mass of the hotplate is too great, and will cook the board and components after 280 seconds have elapsed, but lifting off gives you the ideal cooling down profile.

Some people use small toaster ovens, but these were more expensive when I went into Cunningham's warehouse, plus removing the hot board with molten solder is harder with a toaster oven, and some people report overheating plastic components with the heat coming from on top of the board.

First, you apply the solder paste with a toothpick or needle:


Then you place the components - tweezers work well, others use vacuum pickup devices bodged together with pen barrels, needles and aquarium pumps:


Then you turn on the hotplate. This is a cheap ~AUD$40 Tiffany branded hotplate which follows the recommended JEDEC heating profile on setting 4, and is then turned off at 250 seconds, and then with the boards lifted off at 280 seconds to cool, it works a treat.


The hotplate is unplugged at 250 seconds, and at 280 seconds, the boards are gently lifted with the simple jig made up of a 3mm (1/8 inch) aluminium carrier plate, and some support arms with a handle at the left and a fulcrum at the right. The jig can be gently lifted up and supported to get the boards away from the heat source.


The carrier plate spreads the heat nicely, allows the boards with the still molten solder to be gently lifted without dislodging the components, and allows the heat source to then be removed after it has been chocked, in this case, with the nearest thing handy - a tissue box.


After things have cooled enough to allow the boards to be handled, you can move onto the through hole components.

So, in summary, surface mount soldering need not be expensive, difficult or scary for the newbie. It really can be this quick, easy and cheap to do at home.

Update:

specs for the hotplate / skillet used for the SMD reflow. The unit weighs 2.4kg, and apart from the hot plate with embedded element, there is little else to it, other than the sheet metal enclosure, knob, and plastic feet. It is nominally rated at 1500W, and appears to use a bimetallic switch for heat regulation. I suspect my setting 4 is actually "always on" for the 250 second runs. The hot plate portion is 190mm round, and quite magnetic, suggesting that it is iron, as opposed to cast aluminium.




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