My Indium test report:
I always have problem soldering my MCPCB especially the VR21SP4. I designed this one with thinner and brighter solder mask to get better beam and better high current capability (thinner solder joint). The closer you cram E21As, the hotter the LED will be. The VR16SP4 has 0,02mm solder mask thickness, 0,4mm spacing, and less bright white color. VR21SP4 has 0,01mm solder mask thickness, 0,25mm spacing, and very bright reflective white color. But the thin reflective masking is very very fragile.
Soldering VR16SP4 and VR16S1 are easy as long as we use Sn63Pb37 solder paste for the LED pads. The flux residue easily cleaned by hot IPA and ultrasonic cleaner. The problem with VR21SP4 is its so fragile that even normal flux, ultrasonic cleaning, and hot IPA bath will destroy the masking. Reflective white masking ink contains special filler (higher content of Barium sulphate I guess) and less epoxy binder, reducing the toughness significantly.
One of the solution is to use low temp solder, compatible flux, and perhaps, no ultrasonic cleaning at all. Most low temp solders have worse thermal conductivity than Sn63Pb37. As bad as only 19W/MK for Sn42Bi58. And I can’t omit ultrasonic cleaning process, especially for high current applications. Without UC, it’s almost impossible to get the flux residue cleaned from underneath the LED. At high current the residue will thins and seep up to the LED surface and then gets burnt. Water based aluminum flux proved to be very good as long as soldering temperature does not exceeds 200°C. Aluminum fluxes are usually super aggressive and requires high temperature to work on aluminum. It easily destroy solder masks and eats the aluminum at normal soldering temperature (220°C - 260°C). But used at medium temperature (100°C - 150°C) its a very effective flux to clean any oxides on solder joints and copper traces. Being water based, warm water is all it takes to clean it. Dissolves readily even without brushing. Remember, always clean anything fluxed with aluminum flux!! At warm temperature it starts to be HIGHLY corrosive, I used Zinc chloride based aluminum flux.
So today I had a great result combining Indium and aluminum flux to solder VR21SP4. This is also the first time I can solder E21A with minimal manual LED positioning (with my sharpened bamboo stick). All those E21As align themselves just like “normal” LED. Solder dosing is now very easy to do without the need to use solder paste stencil. I just wet the pads with aluminum flux, heat it to 160°C. Then, put a tiny (really tiny) amount of Indium (the size of an ant head for the entire 8 LED pads). I moved the molten solder to distribute them evenly using flat shaped bamboo.
I let the whole MCPCB cool to room temperature, put a small blob of flux on the center. Then carefully placed the E21As at their approximate locations. Place the MCPCB back to the hot plate and slowly heat it to 160°C max. All the E21A, moved effortlessly to designated locations. If there’s slight misalignment, all I had to do was wiggle the LED from the side. After the MCPCB cooled down to about 40°C I soak them in 50°C ultrasonic bath for several seconds. I usually do the UC cleaning 3 times, replacing the water each time.
Only the LED pads soldered with Indium, the rest of the wire pads soldered with Sn42Bi58.
I haven’t test the performance with Indium solder yet, but it should be at least, slightly better than Sn63Pb37. As for now, Indium already saves me a lot of time in soldering department. At almost $250 (including $62 duty tax) per 250gr, this is the most expensive solder wire I have ever used. FeDex forced me to pay ridiculous duty tax because its classified as “noble material”, crazy!! I bought 5N Indium which supposed to be 99,999% pure. That’s why I couldn’t get any smaller than 250gr. The factory asked me for 1kg minimum order at first. IMHO, cheaper Indium with less purity should be OK for our uses. 3N purity and up are already enough and can be bought much cheaper via ebay or AE.
Note: you can see from the picture, I even salvaged the Indium balls - Cheapo!
- Clemence