On Mar 26, 2009, at 2:06 PM, solarwind wrote: > I recently read an article on the Sparkfun website regarding reflow > soldering for surface mount components that can be done at home on any > form of hot plate (stove top frying pan, skillet, etc...). Yes. Even a toaster oven can be made to work for SMT soldering. A convection oven with internal fan is preferable for better heat distribution. While a hotplate will work, most boards I've encountered are not perfectly flat, thereby greatly affecting the heat transfer. > I have several questions regarding this (that were unanswered in the > article, and for which I was unable to find answers on the Internet). > > * I thought heat damages components. I was consistently cautioned not > to overheat components during hand soldering. If the idea is to heat > up the solder just enough to melt (and maybe a bit more so as not to > form a "cold soldered" joint), then wouldn't reflow soldering do the > same damage to components? Yes, heat damages components. It's the way the heat is applied, however. A solder iron may have a tip temperature of over 400c. At this temperature, you need to complete your connection quickly or else rish damaging the component. If you use an oven with paste solder for SMT components, you will typically limit the temperature to around 200c. Paste solder turns liquid at around 170c. I have hand assembled many boards with SMT components and have never seen a component damaged in a 200c oven. In my job (that was recently eliminated) we used a standard convection oven. On the food tray, we placed a 1/4" thick aluminum sheet - quite substantial. This sheet had a small hole drilled in a corner to allow placement of a K-type thermistor to monitor temperature. Do not trust the dial setting on the oven. The aluminum sheet was allowed to preheat to around 100c, where we would then place the board to be soldered. Paste and components have obviously already been placed on the board. Temperature would then be monitored on the aluminum plate till 200c is reached. The oven would then be turned off, but leave the door closed to maintain the heat. Watch your board around the most substantial sized components - inductors, power regulators, etc. Once the solder liquifies, open the oven door and allow plate and board to cool. You'll happily find that there is very little touchup work required. We used this method with .4mm pitch chips and resistors as small as 0402. It works surprisingly well, and for a small shop that can't afford SMD ovens, it is perfectly adequate. > > * One of my DIP chips came with a notice saying that it should be > "cooked" at 125 degrees Celsius prior to reflow soldering. I would > never reflow solder a DIP chip, but this is interesting. Why do they > have such requirements? Is it because of any possibility of trapped > moisture in the chip that could rapidly expand in high heat and damage > internals? What's the worst case for not obeying this notice? If you're hand soldering the components, don't worry. It is exactly because of trapped moisture that can otherwise affect the quality of the soldering. When flowed, it is possible for the joint to crystalize if the moisture content is too high. > * I've also heard of reflow soldering "profiles". What exactly are > these profiles? Is it a requirement that I adhere to them for homebrew > reflow soldering projects? Don't worry for homebrew. A profile is an EXACT temperature control for an EXACT period of time. A SMD oven may heat the bottom of the board at 140c for 30 seconds while heating the top side to 200c within the same period. This may be the specification to perfectly solder the part on a particular grade of PCB, where the temp would then be reduced in precise increments. > * How do I apply solder paste on pads? Do I just squeeze it out of the > tube and rub it consistently over the pads so that all of it is > connected? Does a very precise amount need to be applied discretely on > each individual pad? How is this done? If you are placing SMD parts by hand, the solder is applied via a blunt hypodermic needle. There are several sizes depending on the pitch of component you are working with, affecting the flow out of the needle. Typically, the dispenser is the paste solder, and the needle screws right on the end. There is a air operated foot pedal to pressurize the dispenser, allowing the solder to flow. It can be done with a plunger applicator as well. When doing an IC, it is not necessary to exactly apply the paste to each pad. You can draw a small bead across the length of the pads, place the chip, and through practice, you will slide your soldering iron across the pads and perfectly soldering your component. You can also use a hot air machine if you have access to one. > * Where to buy solder paste, are there different types, what brand, > what type? Should I use flux? Should I pre-flux the pads or apply flux > and then solder paste? I just buy the past from DigiKey. It isn't cheap, and be advised they must ship it overnight since storage requires it be refrigerated. Kestler is typically the brand, and it is typically 63/37 not unlike wire solder. The paste contains a no-clean flux, and if you bake your board in an oven, you may be surprised at how clean the board looks (no flux deposits) when done. If you hand solder with paste solder, you'll likely need to clean up with some isopropyl alcohol. > > * I've also heard that it is wise to calibrate or know where your > skillet/stove reaches a certain temperature. What temperature is good > for reflow soldering? How do I measure this temperature? That's where the thermistor comes in. 200c was where we would typically find very good results. > > * How long should the PCB be on the skillet/frying pan during the > reflow soldering process? At your adjusted temperature - ie 200c - till it melts. Unfortunately, since we are not talking about thousands of dollars worth of equipment, that is about the best answer. 200c will not typically damage your parts. My experience is about 3 minutes from the time the board is placed on that aluminum sheet. Most generic SMD parts are very inexpensive. If you can afford to practice on a PCB, you'll gain a lot of confidence. It actually is pretty easy once you've done it a few times. > > I know these are a lot of questions, but I am very interested in this > cost-effective do-it-yourself technique to solder surface mount and > fine pitch components on PCBs with ease. > It really does work. Applying the paste and a steady hand with tweezers to place the parts may be the most difficult step. Joe -- http://www.piclist.com PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist