SMT Assembly on the cheap

Projects /  Reflow  Original post date:???

This is what I do to make SMT assembly at home on the cheap for my projects.

If you are ordering eval board or PCB precut, have a stencil and solder paste, this is not for you. It is easy to spend money to get things done.

I try to build things on the cheap. I order PCB and sometime put multiple design on the same PCB just to get as much as I can from the deal. The cheap board house do not like to make you life easy by V scoring or cutting sub board out for free even though that's how they cut copies of your board from a panel in the first place.

So I am going to show you how to cut your PCB the old fashion way. FR4 (Fibreglass) material like are pretty strong up to a point. It can develop a sudden failure at the maximum stress point e.g. a deep scratch/score. The material will crack along the stress line and it will spread. We can take advantage of that to cut a PCB without fancy tools.

I live in an apartment, so do not have a separate fridge for storing solder paste away from my food. I don't have a fancy laser cutter at my disposal to make my own stencil either. So I was faced with trying to solder 0.5mm pitched parts and QFN for the first time last year with my FPGA project. Starting something big and hard is a good way to fail, but sometimes you walk out alive and learn something.

I was inspired by this video. Now this guy is a master.

I started my own experiments:
At the time I didn't have the hot air rework tool, so I use a butane hot air tool. I still find the latter easier to use than a hot air tools as it is less bulky and the air flow doesn't try to blow my tiny parts away. The draw backs are: hard to light, replacement parts are expensive and hard to find, no idea what the temperature is, sometimes it splutters because I have it pointing down, expensive hard to find butane refills. The smoke shops here are disappearing and those tiny refills are at $7-8 a bottle. So I got myself a hot air rework tools which is a better long term investment.

What I ended up doing on the FPGA board was pre-tin all the pads and to hand reflow each of the pins individually with a soldering iron. It was tedious, but since I wasn't adding additional solder, there was ZERO solder bridges for that entire PCB! Actually I had issues with not having enough solder on my QFN parts. Hence I now also tin the part. The 0402, QFN parts are reflowed one at a time with my butane hot air tool.

I have since done much smaller parts and even use chip scale BGA. 0.4mm QFN are pretty easy now.

Manual V Score 

(Optional Step - for Proto PCB that don't come with V Score or pre-cut)

Tools: Box cutter, ruler, cutting surface, C clamp, piers, large piece of wood etc.

1. Cut along line - deeply on top and bottom of PCB

(Picture above shows what can go wrong - see left edge.)

2. Pray and snap PCB along a nice solid edge. 

Use pliers to hold the other end and snap PCB

3. Repeat steps 1-2 if needed.

Reflow with regular solder

1. Tin the pads

2. Now tin the QFN - Do not tin the Thermal Pad

I also have some success without doing this.

2. Position it over foot print

3. Set reflow temperature to 250C for lead solder, Here is my tool.

I use low air flow (I even let air to leak through the slot on the tip for really light parts)

Position over the part to heat the board and the part. Then slowly heat up the sides until all the solder melts. I gently tap the part. Let it cool.

I have a bit too much solder for a few of the pads. Clean up with an iron.

Test for continuity. 

This is easy if you are building trivial stuff like a breakout board.

1. Use diode test on the multimeter.

2. Connect the Positive terminal of the meter to the ground of the PCB.

3. Probe each of the pins to the chip for the parasitic diode. Huh!? What!?

D1 is what I am probing for. That's my easy way of testing the connectivity into the chip that I came up with last year on my FPGA project. Beyond that you would need to X-ray the solder joints, JTAG, or some test code to toggle the pins on the chip.

Reference: "ESD Protection Circuits for Advanced CMOS Technology",PHD Dissertation, Jung-Hoon Chun, June 2006, Stanford University. (.pdf)

See the Min Column: GND - 0.3V. That's the diode I am testing for. It pretty much covers for most if not all pins on a modern chip. It is present even in modern chips that support 5V tolerant or hotswap.

4. If test fails, try to fix the connection by soldering iron and/or reflow with hot air tool.