'Have you seen my new soldering Iron?'

Transcription

1 'Have you seen my new soldering Iron?' Kenneth Maxon Here some month's back I heard from a guy who knew a guy who knew how to solder surface mount parts using his toaster oven. Being quite a bit of a skeptic, a guy who knows a guy is a bit of a reach for me and I tossed the idea out as ridiculous. Today I'm here to share with you, my own first hand experiences in using a toaster oven to put down fine pitch surface mount parts. I owe great thanks to Mark C. and his friend Dana O. who introduced me to the technology and to Jeff B and Jim C for helping work through the learning phase. It is the intention of the author to present a pictorial documentation of the process with this article and hopefully pass along the Art. I hope that other members of the Seattle Robotics Society (and beyond) will pick up this skill and add it to their toolbox in dealing with board design and stuffing in the future. The process starts with the use of a 'Magic Ingredient', a water-soluble solder paste. This 'Magic Ingredient' is available though DigiKey and is made by Kester Solders. Buying of the product in itself is interesting as it must be kept refrigerated and will only be delivered if ordered for overnight delivery. Since the material is water based it must be stored in a cold place (refrigerator) and only 'keeps' for a few months. Starting with a clean board, the solder paste is applied from the syringe. The paste comes pre-loaded into a syringe however, the reader must order the plunger for the syringe and the applicator tips separately. I found it easiest to hold the syringe with my left hand depressing the plunger with my thumb, while slowly pulling the board along underneath with my right hand. My first try came, only after watching several successful tries by Mark who seemed to be a natural! The two photos below show the application of solder past to two different PCB's. One with a high-density fine pitch surface mount part and the other with 0.050" pitch surface mount parts.

2 When applying paste to 0.050" pitch (and above) boards, dab paste onto each individual pad Vs fine pitch where a bead of past is laid down across all the pads. So, how much paste should the reader use for their board? As this is very much an art I'd suggest try it and see. The part can always be removed and re-applied. (More on that later) The following photograph shows the amount of solder paste used on the author's board. One step not shown is the first attempt to smoothly lay down a bead of paste which 'clumped' and 'gooped' all over the place. A damp paper towel from the kitchen quickly removed the mess and left the board clean and ready for a second try. The next step in the process involves placing the parts. This is not as critical as it may seem. The reader can appreciate the surprise we went through when learning that in the re-flow process, the part will partially center itself over the pads on the PCB. This is due to the surface tension of each little bubble of solder under each of the 132 pins in our fine pitch part. Knowing this, the author still spent an extra few minutes making sure that the processor lined up exactly with the pads below it. The photos below depict the parts placement process. Notice the use of tweezers to place small surface mount parts, while fingers seemed the ultimate tool for the larger microprocessor parts.

3 The next picture shows the 'mess' left by all that solder paste squished between the hundreds of little surface mount pins.

4 Off to the oven with you! The next step in the process is to cook the boards. We used a toaster oven that the landlady of the author was kind enough to donate to the project. There is nothing special about this oven, a plain old proctor-silex from target will suffice for the job at hand. Looking to industry for answers, Mark tracked down a thermal profile used when infrared re-flow is done commercially. Matching closely with the author's own experiences in dealing with board stuffing houses we reached our own modified 'cooking' profile. 4 min. 2 min. 30 sec deg. 325 deg. 450 deg. Warm up board and allow temperatures to equalize. Bring temperature up to saturation. Temperature raised until solder melts and beads at individual pins, then held for 30 additional seconds. Tap the oven before cool down... The steps above are pretty self explanatory. There isn't any way through an article to share the excitement as the solder begins to 'pop up' into little beads around the individual pins. Small shiny silver beads appear starting from the back of the IC and moving slowly foreword. Once the solder has melted the oven is tapped a few times to help the IC to self-center over the pads. Care should be taken when doing this with smaller boards. The board in the second photo below fell between the rack rails when being 'tapped' shortly after the picture was taken and the process had to be started over.

5 A word of warning: As the solder paste warms up (during application to the board) it becomes soft. This allows the part to slide all over the board before finally being placed into the oven. Additionally, until the solder has cooled, the part can slide all over the board with melted solder all over it. We found it best to turn the oven off, open the door, and let to board sit until mostly cool before pulling it out so as to not risk destroying our masterpieces. In a number of places it was necessary to clean up a few of the traces. The board used for this article had about 3 no-connect pins and four solder bridges. Bridges are located by eye inspection in front of a bright light. Unconnected pins are discovered by wiggling individual pins with an x-acto knife. On previous boards our group has had better luck including one board with only two disconnected pins and no solder bridges! Fixing misconnects is as simple as touching the pin with a hot iron as there is a perfectly formed solder ball right underneath it! Fixing bridges is a little harder and requires some fine mesh solder wick. The photo below shows the process of using solder wick to remove a bridge.

6 Finally, the photo's that follow show a number of close up shots of processors put down using this method by the group of roboticists that included the author. As is evident these are processors in 132 pin fine pitch packages.

7 They say that too many cooks spoil the meal, but at times we had four people crammed in looking through the front window of the oven, each with our own ideas and suggestions. Thanks for the help guys...