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SMT Soldering Tools and Process

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Tools

Shown above is my reflow soldering station for surface mount printed circuit boards. The newest addition is the beverage warmer, which I purchased at Wal-Mart for $1.87, it has only a 9Watt heating element which means it can't overheat. The target temperature would be 100 deg. C but this beverage warmer only gets to 90 deg. C after about ten minutes warmup.

The long blue thing behind the beverage warmer is the hot air embossing tool. I purchased this in the stamp section of the local crafts store (Hobby Lobby), the everyday price is $19.99 .

The glasses are just drugstore reading glasses, I need +3 power, it represents a compromise to working even closer and some depth perception. You might need higher power, but I have not had good luck with the lamp / magnifying glass combination.

The syringe is one that I buy at the local drug store and transfer a small amount of Kester 256 solder paste. The needle (21 ga.) is cut off to a length of approximately 3/8", and deburred. The solder paste is expensive when purchased in the minimum quantity (500 grams for about $80) and must be stored in the refrigerator.

And a good pair of stainless steel tweezers, the tips are slightly blunt. Don't buy the very sharp type, they are likely to fling a tiny part to a place where it will never be seen again.

The total cost is less than the cost of even the least expensive toaster oven.

 

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Process


Click on Bold Text

The paste is applied to the board using one of two basic techniques: 1) the pads of resistors, capacitors, and devices with broadly spaced leads (transistors and opamps in SOT-23 packages) you place a daub of solder paste of each pad. The daub of paste should look a bit like the Hershey's Kiss or a less peaked like a cow pie. 2) Integrated circuits with two rows of pads or four rows of pads are arranged in a quadrangle, the paste is laid across the pads and like a small continuous rope of paste.

The components are placed onto the paste and then pressed into the paste. The paste will securely hold the component by surface tension, you can even turn the board over and the components won't fall off. You don't need toothpicks with bee's wax or any other tool aides.

About 10-15 minutes before you have finished placing all of the surface mount components, turn on the beverage warmer and use the high position. When all of the components are placed and the beverage warmer is up to temperature, place the circuit board on the beverage warmer, wait for a couple of minutes , the paste will start to get glossy and rounded, then will start to have some wispy smoke, then become dull gray. At this the point that I like to start adding heat with the heat embossing gun.

Take your time, start by moving the hot air around the board, don't hold it on one place for more than a few seconds. The purpose is to slowly (approximately three (3) minutes) to get the board and components up to temperature. View long movie, 27Mbytes which shows a little more than a minute of the reflow soldering process.

 

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At this point, the paste should be starting to flow (view short movie, 3.7Mbytes), you'll recognize this point where the solder turns to silver. You will notice that the components squirm a little and settle into their optimum position. Use only enough heat to get the solder to flow completely. When all of the solder has flowed, remove the heat from the heat embossing gun, but leave the board on the beverage warmer for several additional minutes. This cool down period is very important. Finally, turn off the beverage warmer and remove the board.

Results

Inspect your board for solder bridges, incomplete soldered components, etc., I do not recommend trying to reheat the board with the hot air gun, although it is perfectly alright to put the board back on the warmer. Use solder wick and a conventional soldering iron to make any minor repairs.

The board shown above was soldered using these tools and as you can see looks quite professional. The board is the QSD board for the Softrock 5.0 Software Defined Radio.

From another mans perspective I suggest you also check out the well documented SMT Board Fabrication Project By Cecil Bayona KD5NWA

In the March 2010 issue of CQ Magazine, Joe Eisenberg K0NEB, Scratching the Surface of Surface-Mount, Pg. 64