DDS and PIC soldering on TRX2A DDS board

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DDS and PIC soldering on TRX2A DDS board

Postby F4EEF » 13 Jun 2017 17:20

:o
Hello,

When soldering DDS an PIC on DDS board I encountered many difficulties even using flux, small gauge solder (Sn62Pb36Ag2 0.3 mm) and a soldering station with a small tip so I made many solder bridges and was unable to remove them using solder wick and bent IC legs and broke board tracks.

So I threw the board into waste !

How did you succeed this step during building the TRX2A ?

It seems very, very difficult for me !

Did you use solder and soldering iron or solder paste and hot gun ?

Thanks in advance for your answers and advices.

73 de Alain F4EEF
F4EEF
 
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Joined: 14 Dec 2015 15:26
Location: France

Re: DDS and PIC soldering on TRX2A DDS board

Postby 5B4AIY » 14 Jun 2017 05:57

Hi, Alain,

I am sorry that you encountered such difficulty in mounting the DDS and PIC chips. I would agree that these are the two most difficult aspects of the assembly, and, as you have discovered, there is no room for error. I can only speak for myself, but having made a similar mistake as you, fortunately not with the Juma TRX-2, I eventually adopted the following technique.

I first tinned the pads by wiping them with flux, and using only a minimal amount of solder. I then removed any excess with solder wick. Next, I put flux on the pads again. Then I prepared a small amount of epoxy and placed a small spot in the centre of the location of the chip, and carefully placed the chip in position. After pressing the chip onto the board I could gently move it until it was perfectly aligned with the pads.

With the chip held in place with the epoxy, it was now possible to solder the legs to the pads using a 1mm tip on my temperature controlled soldering iron. Inevitably there will be some solder bridges, but these were easily removed with fine solderwick. When I checked with a multimeter I still found that there were shorts, and these were caused by very fine 'whiskers' of solder between the leads - I removed them by gently stroking between the legs of the chip with a sharp sewing needle.

Finally, using a very bright LED flashlight illuminating the backside of the board I carefully examined on the front side with a magnifying glass to verify that all the legs were soldered, and that there were no bridges or whiskers remaining.

Many commercially manufactured board also use a spot of epoxy to hold the chips in position whilst they are being either wave soldered or hot-air gun soldered with solder paste.

I should also add that all of the assembly and soldering was done whilst wearing a head mounted magnifier, and in my case I used a 2.5x lens. If you can get one with a built-in LED flashlight, this is also a great help.

I hope this is of some assistance.

73, Adrian, 5B4AIY
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Postby F4EEF » 14 Jun 2017 06:30

Hi Adrian,

Thanks a lot for your answer to my post.

I have some other questions : what kind of flux and solder did you used and what was the temperature of your soldering station when you soldered the DDS and PIC ?

Thanks again for your answers.

73 de Alain F4EEF
F4EEF
 
Posts: 5
Joined: 14 Dec 2015 15:26
Location: France

Re: DDS and PIC soldering on TRX2A DDS board

Postby 5B4AIY » 14 Jun 2017 14:00

Hi, Alain,

I used RMA (Rosin Mildly Activated) flux that I obtained on eBay. It has the brand name of Powerfix, and is to DIN EN 29454-1, and is manufactured by Abraham Diederichs GmbH & Co, Oberkamper Strasse 37-39, D-42349, Wuppertal, Germany. This flux is a soft, light honey coloured paste, and is easily removed with de-natured alcohol. After finishing all the soldering, I generally clean the board first with a soft cloth soaked in alcohol, finally removing any remaining traces with a Q-tip moistened with alcohol.

As for the temperature, I tend to use a fairly high setting, typically around 320C - 330C. I have found that this very quickly melts the flux and solder and makes a perfect joint within 1 to 2 seconds, thus minimising the thermal exposure. At lower temperatures it takes somewhat longer to melt the solder, and the pads and components are then subjected to a greater thermal input than with a short time with the soldering iron tip at a higher temperature.

For soldering the leads of the ICs, with the chip in position I apply the tip of the soldering iron to the lead along with the solder, and then 'stroke' the iron along the lead to the board, and this results in a perfect joint. For the other ICs, I find that they are sufficiently large that I can hold them in place with the tip of my finger and then tack solder one of the leads to hold it in place. Then I carefully examine it to ensure that it is the correct device, correctly oriented and aligned. The next step is to tack solder the opposite corner lead in place, and make one final check. With only the two corner leads 'tacked' onto the pads, it is an easy matter to remove and re-position the device if it is not perfectly oriented or aligned. If everything is OK, then I can place the solder on each lead in turn and touch it with the tip of the soldering iron, remove the solder, then the tip of the iron, and, that's it!

The solder I used was obtained from the same supplier as the Juma kit. They marketed at that time a solder pack containing not only suitable solder, but also tweezers and a small bottle of liquid flux, which I did not use, preferring the paste. The solder was normal 60/40 rosin-cored tin-lead solder, not the non-lead solder that is now being used as this requires even higher temperatures to melt.

Perhaps I should add, that if you do not like the idea of using a spot of epoxy to hold the DDS/PIC chips in position, then a small blob of something like Blu-Tak, or Faber-Castell 'Tack-It' creative removable adhesive that you can get in stationery stores, craft shops, etc, also works. The beauty of this method is that the chip is not permanently attached to the PCB, and thus if you should ever have to remove it, then after clipping all the leads, it will simply lift off, whereas with epoxy it is more difficult, and you may also remove any tracks that were under the epoxy as well.

73, Adrian, 5B4AIY
5B4AIY
 
Posts: 160
Joined: 13 Nov 2011 09:22
Location: Cyprus

Re: DDS and PIC soldering on TRX2A DDS board

Postby F4EEF » 14 Jun 2017 19:39

Hi Adrian,

Thanks again for all your tips and tricks about soldering the SMD.

I think I will be able to build the DDS board successfuly now !

I wrote to Kari from RF Device to buy a new DDS board with resistors, ferite beads, inductors, capacitors, diodes, transistors and all the IC's including DDS and PIC but I have to wait for some time because Kari is very busy before the midnight sun festival in Finland.

I hope I will be able to receive the new parts before the end of june.

Best regards.

73 de Alain, F4EEF
F4EEF
 
Posts: 5
Joined: 14 Dec 2015 15:26
Location: France


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