标题: DIY Pizza Reflow Soldering [打印本页] 作者: liyf 时间: 2012-1-8 09:52 标题: DIY Pizza Reflow Soldering DIY Pizza Reflow SolderingThis tutorial explains how to reflow-solder PCBs in a standard pizza oven. The soldering profile as well as the initial instructions are from Sergei Lupashin (ETH IDSC). This tutorial was written with delicate ICs like gyroscopes, accelemeters and magnetometers in mind. We had good success and zero failure so far with the temperature curves below.
Materials
“Satrap fomello” (Coop) or similar toaster oven (should go up to at least 230° C, have top+bottom heating, some kind of convection/internal fan a plus
Thermometer (good for up to 250 C. We use a cooking thermometer from Farnell)
The thermometer is there simply as a reference. In general it appears to lag the actual PCB temperature, but this depends on PCB size, etc.
ESD-safe SMD tweezers. These tweezers are so fine that they shouldn't be used for anything else than populating parts.
A cheap stereoscope is a plus. It significantly helps populating the parts and it helps checking/correcting the solder paste.
Solder paste. We use leaded paste, as it is significantly easier to handle. Lead is however toxic, so make sure to wear gloves, do not eat or drink during the assembly and clean carefully the workbench once you're finished. If you assemble more than just a few prototypes per year, consider using lead-free paste.
CalibrationBefore actually conducting the reflow, set the oven to the temperatures listed below (each for about 15 minutes) and try to match the temperature using the thermometer you put inside. The markings on the temperature knob will not match the real temperatures, so we used a pen to mark the “real” knob positions for the desired temperatures.
Applying Solder PasteWhen ordering the PCB, buy the matching stencil along with it. You don't need a professional stencil frame, just put other PCBs of the same height around your work piece and then align it manually. If you do it right, the stencil “snaps” to the right position, as the tinned pads are slightly higher than the solder stop layer.
Align the stencil with the PCB
Put plenty of paste on one edge of the stencil
Press down the stencil and distribute the paste in one or few moves with something with a clean edge, e.g. another PCB
Do NOT MOVE the stencil, lift it firmly in a single straight move
Check the results. It should look quite clean with sharp edges, more or less just like a PCB with pads. If it doesn't look that way, remove all paste, clean the PCB and start over
Solder paste applied cleanlyThe solder paste is applied cleanly on one of the footprints of pxIMU: Autopilot and IMU.
Populating the PCBIt significantly helps if you have sorted your parts with line numbers and you have a matching list. We also always have the PCB design software open.
Start with larger ICs like sensors, microcontrollers, etc. in the center of the PCB
Continue with capacitors, best start with 100n (very, very, very often used)
Try to populate one type all over the PCB, then continue with the next type
Align all parts in a way that they're at least that good aligned that the right PCB pad is closer to the pin than the next wrong one. They will “flow” into place, but only if they're closer to the right one
Gently push down parts (really only gently!) after they reached their final position. Most of the parts will slightly stick there
Finish up by populating parts / connectors on the board edges
Check the whole PCB / the critical ICs again for alignment as a last step
Population ExampleThis photo shows two pxIMUs partly populated. Most notably the sensors and some capacitors are populated, but not yet parts on the board edges like larger connectors.
Placement help for LEDsThis image shows the schematic symbol of the leds on top of the real led. This helps when populating with a microscope and saves the led-test with a multimeter.
Heating up / SolderingThe actual soldering step is quite easy. Just remember to move the PCB VERY GENTLY from your desk into the oven. One slight vibration and your parts are misaligned. Edit this diagram
Profile When setting a new temperature, turn the knob further than needed and then turn it back. Else the temperature controller might not respond at all.
Put the PCB into the oven, close the door gently (T+0)
Start at 80° C (wait for 10m or so for oven to reach that temp, this pre-baking will also dry humidity-sensitive parts like leds)
Set oven to 160° C, start timer (T+10)
At 3 min go to 180° C (T+13)
At 4 min go to 210° C (T+14)
At 6-7 min turn off oven (set temperature to 0), carefully open door. DO NOT TOUCH THE BOARD AT THIS POINT (T+16)
Use 1-3 min for smaller boards (30 x 40 mm), longer for larger boards (5 min 50 x 50 mm). Basically all of the solder paste on the board must reach melting temperature. Boards that have large ground planes, etc, will in general take longer to heat up.
After ~15min it should be safe to take out the board. (T+30)
References and similar GuidesPlease find below a list of similar guides and references that might be helpful as well.
