By Wagner Lipnharski
- UST Research Inc. - January 2003
Please
DO NOT COPY the text or Pictures below - Link it.
Baking SMT on a cheap kitchen toaster oven is not difficult. You just need some patience and exploration good will. Each toaster oven react in a different way, each one, cheap as it is, will produce a different temperature heating and cooling profile curve. Soldering SMT components to a PCB requires "some" temperature profile, that is, certain temperature for certain time, then other temperature for other time, and so on. This is NOT so much critical, as I experienced. The soldering process happens in one way or another when the everything reaches around 450°F, but critical is the sake of the components on board. Some components can crack immediately if the temperature goes up or down very fast, others don't show any damage immediately, but they will fail in a month or two (capacitors for example). As a rule, average temperature profile should be:
This cool down process is important to avoid damage to components. We need to call the attention to the fact that a cheap toaster oven, is a little piece of garbage, the thermostat is ridiculous and it is there to control temperature in a 30 minutes average, not a 2 or 4 minutes period. We need to understand how slow temperature travels from the interior of the oven to the thermostat that normally (on the cheap ovens) are located in the lateral compartment. When you set the temperature to 160°F, the thermostat is cold, so it turns on the thermal elements into the main chamber, and they got really RED. The inner temperature start to travel by the inner chamber metal plates and propagate to the thermostat at the other compartment. It can takes almost 45 seconds or more to the temperature reaches the thermostat. In 45 seconds, a RED thermal element can increase the inner chamber temperature from 100 to 300°F. So, when the thermostat senses 160°F, and turn off the thermal heaters, the temperature into the main chamber is about 350°F. In a regular cake baking or sausage roasting it does not change much the results. At this moment, the thermal elements will stay off during several minutes, until the thermostat travels from 160 to close to 350°F then back to lower than 160°F when it turn on the elements again. This process will be repeating and the cake will be ok, but not the circuit board and components. This is a very wrong profile to bake the SMT board. This mostly happens for low temperature, as the initial 4 minutes of 160°F. As a bypass to this problem, turn on the oven with the SMT boards inside for 160°F, for no more than 90 seconds, then turn it off. It will avoid the temperature to increase so much into the main chamber. If you have a way to measure the inner temperature, great, if not, just guess. Keep the boards inside and wait extra 2.5 minutes. Then turn the knob to 320°F for two minutes, turn to 450°F (full power in most toasters) for 60 to 90 seconds, turn all off. Below some pictures of solder paste
applied, and some results explained. |
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Below pictures after the toaster oven baking process. |
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Note the upper chip, the situation is worse, the lead+pad are at a corner, cooler, and worse, they are connected to a ground copper wide area, cooler yet. But both leads were soldered with zero ohms contact. It is not totally repeatable, a little bit more of solder paste fixes it. Check FIGURE 5 above, you see a solder bridge, but not lacking of solder at the corner leads |
To avoid those corner problems, I started to make the solder paste strip longer, a little bit beyond the corner lead+pad, as you can see at FIGURE 4 at this page. It means that when the melting process starts, the adjacent pin will steal some solder paste, but even so, there will be more than enough solder paste to solder the corner lead. This gave me a little better success, and peace of mind. |
As you can see, there are more catches in this technique than you can think about. Only dedication and observation can lead you to a result close to the professional ones. After the solder process, the board requires an alcohol 91° wash and brush, following by hot water and soap brush to finish cleaning and removal of the extra garbage from the solder paste. 5 minutes into another warm box at 120°F will finish dry the board perfectly. The board ends up clean and shinning. If you have anything to comment or improve this page (mostly the English grammar), please contact me at wagner@ustr.net The above text and pictures are property of Wagner Lipnharski or UST Research Inc, they are not free to copy. If you want, you can link this page to your own pages, but please do not copy them. I intend to keep updating and improving this page, so copying is not smart. Linking will keep the readers with the up-to-date information. This was done with my time and dedication, just to help other people that can be wanting to do home made SMT baking. |
Sent by Peka Siiskonen My experimenting w. SMD: |
Last edit Feb/03/2003