Introduction
In my previous posts here and here, I made a small circuit with the prescribed parts from Texas Instruments and made a PCB for it. I recently received the passives and went on the assemble the board. This is the story of how I failed!
Setting things up
I started very enthusiastically and the video below is me setting up for the assembly...
Results
No I did not make a video. I tried assembly of three boards and failed when it came to the QFN BQ25504. One of my boards is shown in the image below.
There is obvious excessive solder paste on this one...
Board Assembled partly.
The basic problem I faced was the repeated bridging of pins on the QFN. I tried varying the quantity of solder paste but the pitch is so fine that it almost always failed. I tried it on three different boards and the problem is the same. My solder paste is a bit old but I use just enough flux to make it creamy but since my hot air station has no temperature indicator, I am flying blind. The air control is tweaked to a minimum but still the parts get blown here and there.
Lessons learned
Here is a list of lesson s learned for now so I don't forget.
1. Get extra samples! - I requested just enough samples and that means I will only be able experiment so much.
2. Just enough solder paste. If the paste is more then pins will bridge and paste should only be applied to the pads. Clean up the extra since it may bubble up and move the parts.
3. Have a temperature reading on the hot air station. Unless you are really practiced, you need to control the temperature of the hot air.
4. Practice practice practice. I am going to take apart every smd circuit I have in my scrap bin and practice before I jump back.
5. Build a Reflow Oven. Ben-Heck has some tutorials on making a reflow oven and I think that will make one in the near future.
Status
I am short of silicons and with the deadline this coming week, I will tread carefully. Lucky for me I made jumpers so I can make the three sections separately and join them together. Finger crossed...
Update 1
I got some new solder paste from a friend and it has a lot less flux in it. Used that and the QFNs seem to get soldered pretty well. After a few tries, the bridging stopped happening. Just reduced the amount of solder and it worked. Unfortunately, I am out of BQ25507s so I am putting up another request to TI. It will take a while but I will get it working in the long run.
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