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Hot Plate + Aluminum Pie Pan = No Joy

I’m back with another episode of my DIY reflow soldering adventures! The first attempt using hot air didn’t go well, and the second attempt with a hot plate and half-inch thick aluminum block wasn’t great either. This time I used a lightweight disposable aluminum pie plate instead of the half-inch aluminum block. The idea was to reduce the thermal lag observed with the thicker block, and make it easier to remove the PCB from the hot plate after reflow was complete.

The lightweight pie plate proved to be a bad idea. Its bottom was embossed with small ridges, and was slightly warped and dented. The pie plate wouldn’t sit flat on the burner, and the PCB wouldn’t sit flat on the pie plate. To make matters worse, the pie plate also deformed as it heated. With such poor thermal contact between the burner, pie plate, and PCB, the PCB heated very slowly and unevenly. It did eventually reflow successfully after 10 minutes of heating, but I had to push down one side of the PCB with tweezers to improve the thermal contact.

The results of reflowing looked similar to my first reflow attempt using hot air: not good. There were solder bridges everywhere, but also clear evidence that many pads were dry, with only a portion of the pad area covered in solder. The photo above shows an example. The bare pads are ENIG plated and have a gold color. After reflow is finished it’s easy to see the areas that are still gold-colored, where solder never flowed. Ideally there should be no such areas.

Such poor flowing of solder makes me think the flux in my solder paste has gone bad. A few days ago I discovered that my newly-purchased paste was already beyond its shelf life, so I’ve got more solder paste on order. Hopefully that will result in better solder flow, with pads that are completely covered in solder, and few or no solder bridges.

 
It’s All About Time

My main goal with this whole reflow adventure is speed. I hope to assemble batches of a few dozen ROM-inator II PCBs faster than I can by drag-soldering with a standard iron. I can finish one PCB every 10 minutes with drag-soldering, and at least half of that time is consumed by fixing solder bridges. If I have to spend a similar amount of time fixing solder bridges on the reflowed PCBs as on the drag-soldered ones, then it’s unlikely reflow will provide the speed-up I’m looking for.

Even if new solder paste resolves my solder bridge problems, I’m also concerned by the setup time required to dispense the solder paste and position the chips. I timed these steps, and they took 6 minutes, all before the reflow process even started. With practice I could probably speed that up by a minute or two, but that still puts the total time including reflow close to 10 minutes, even assuming everything goes perfectly with no solder bridges. Only by reflowing multiple PCBs at once could I hope to save any appreciable time vs drag-soldering.

The video shows the entire process end to end, including dispensing the solder paste, placing the chips, reflow heating, and saying “woah” and “oh my gosh” about 20 times. I apologize for the poor quality of the video. My hand often blocks the view, and the camera isn’t positioned well to show the solder reflow in detail. Maybe you’ll find the “authentic amateur” style amusing!

Read 8 comments and join the conversation 

8 Comments so far

  1. Ferdinand Keil March 26th, 2017 11:35 pm

    A few things: the paste is the most critical component in reflow soldering. If one brand or type doesn’t work for you, try a different one. I had good experiences with Amtech, Multicore and Kester. Also, I found the shelf life to be much less a problem once I found the right paste.
    I got really good results using a cheap plastic stencil. It really makes a difference for fine-pitch parts. Had no problem soldering 0.4mm pitch parts.
    Finally, solder-mask resolution might also be an issue. Looking at your results, most of the bridging occurs between pins missing a solder-mask bar.

  2. Steve March 27th, 2017 6:46 am

    That’s a really good observation about the solder mask, and I wonder if you’re on to something there. There’s solder mask between most of the pads on a blank PCB, as you can see here: http://www.bigmessowires.com/wp-content/uploads/2017/03/20170327_072755.jpg It seems to be right at the thinness limit of what the PCB fab could do, though. I used DirtyPCBs for manufacturing these ones, which may not be the greatest fab. The reflowed PCB in the title photo appears to be missing solder mask between many more pads than the blank PCB, so I wonder if some of the solder mask was so marginal that it flaked off during the reflow heating.

    But none of the solder bridges are actually between the pads, where solder mask would make a difference. They’re all between the pins, in the angled area of the pin leg close to the chip body, so they’re elevated slightly above the surface of the PCB. Could that still be a solder mask issue somehow? I could play with the design files, and attempt to make the pads slightly thinner and the solder mask areas slightly thicker, if it were likely to help.

  3. Vincent March 27th, 2017 7:44 am

    I alway use spray on flux and then it is less crucial for the solder paste. Need to clean off afterwards thou.

    Vincent

  4. Ferdinand Keil March 27th, 2017 7:55 am

    The solder mask might very well have peeled off when you applied the paste or placed the component. Looking at the picture of the blank board, the mask seems to peel off quite easily (e.g. between the 5. and 6. pin from the right).

    However, the solder wetting on the pads on your pictures looks really awful. This might be either due to quality of the paste or the inappropriate temperature profile. My money is on the paste. In the past I got really bad results with one paste and quite good ones with another, while using the same setup (toaster oven with PID controller) and same temperature profile.

    So what I would do is 1) buy some new paste (e.g. Kester EP256) and 2) get a stencil (e.g. from OSH Stencils). The application of the paste makes a world of difference.

  5. Steve March 27th, 2017 8:47 am

    The solder paste I used was MG Chemicals leaded 63/37 no-clean. It was already 15 months old when I bought it new, and I doubt it was refrigerated, so its quality is suspect. I’ve ordered some Chip Quik SMD291AX leaded no-clean solder paste, which should arrive in a few days. Hopefully it will perform better. I was hoping to avoid stencils due to the extra hassle and time cost, but if I can’t achieve decent results with the new paste and a thin rigid aluminum plate, I’ll try with a stencil.

  6. Steve March 27th, 2017 9:49 am

    I tried one more variation with the questionable MG solder paste. 1) Apply liquid flux to the pads before applying solder paste 2) After applying solder paste, spread it around so it covers the entire pad area instead of lying in a thin bead from the syringe 3) heat the PCB directly on the hot plate burner without any aluminum heat diffuser. The extra flux and spread paste didn’t seem to help, and I still had the same poor solder wetting. But heating directly on the hot plate worked pretty well.

    I turned the burner on full and waited for the PCB to reach 100 C, which took about a minute. Then I turned the burner off, and the PCB temperature continued to rise to about 135 C. I waited 90 seconds, then turned the burner on full again and waited for the solder to melt, then immediately turned off the burner again. The temperature peaked around 225 C. The solder began melting when the IR thermometer showed 160 C, but the melting point should be 183 C, so my temperature readings are fishy. The only big drawback of heating PCBs directly on the burner is that it’s hard to get them off quickly without disturbing the chips, while the solder is still molten. I accidentally knocked off one chip while sliding the board off the burner.

  7. Bob March 28th, 2017 1:57 pm

    I’ll start by saying I have no experience with this sort of soldering. I’m just wondering how well at hot plate designed to cook food works with regards to temperature control? Does it cycle based on temperature? Or does is it a PWM duty cycle type thing? If it’s temperature, perhaps it is just a wide temperature swing between cycling off and back on, etc.

    Do you have a glass top stove in your kitchen? I wonder how it would work to lay a piece of aluminum foil over a burner and use that? Seems like it might distribute heat a little more evenly. You could also just gently slide the aluminum foil off to a cool area when it’s done. You also have to bonus of a vent fan right there!

    Also, isn’t the idea of a hot plate to get the temperature close to melting point and then use a hot air gun to finish the job? I also may just be showing my ignorance here!

  8. Steve March 28th, 2017 2:24 pm

    I’m starting to suspect that a stencil may be required to get decent results with this method, when using 0.5 mm pitch ICs like I am here. I see lots of examples on the web where people didn’t use a stencil and were still successful, but they all involve larger pin pitches like 1.0 mm and above, or else describe manually fixing soldering bridges with an iron after reflowing. Maybe I should have started with an “easier” PCB for my first reflow experiments!

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