“Where is your umbrella?” he asked, friendly eyes fixed on me from beneath a giant canopy emblazoned with the 7/11 logo. He’s shirtless and soaked, and he has a point. A ruthless series of cyclones has dumped rain on Manila for three weeks straight. Half of the city is underwater. Rainwater is trickling down the insides of my jeans. “I don’t have an umbrella,” I tell him. ”I don’t like them.” He squints at me, and I try to imagine what he sees. A barefoot guy dripping rain in the middle of the street, heavy sports duffel slung over my shoulder. Yiddish mothers take a look at me and bring me a bowl of soup. Fashionable girls cross to the other side of the street, avoiding eye contact. ”You don’t like umbrellas,” he repeats, eyeing my rainsoaked clothes. ”That’s too bad.”
But honestly, I don’t care about my clothes. I’ll dry out soon enough. I care about the forty pounds of aluminum framing, linear actuators, electronics and solar cells inside the shoulder bag. It’s the latest iteration of the pocket factory. The guys in Hong Kong designed and built it, and using a special soldering technique we worked out in the mantis lab, I just got the first soldered, working panels coming off it thirty minutes ago. In an hour, I’ll be boarding a plane to Hong Kong to bring the factory back to the Haddock team (plane tickets between manila and Hong Kong are cheaper and faster than sending a fedex, and we’ve quickly grown accustomed to bouncing between cities and crashing on each other’s couches). But right now, it’s 4:00 in the morning and I’m very wet. Everything in the factory is waterproof, I tell myself, or it can dry out. I turn towards the main road to catch a taxi, and a fountain of water pours down my collar from a neaby gutter. I say that bit about how waterproof everything is, again, and whisper a quick prayer to the invention gods.
The trick to getting this version of the pocket factory to work was a technique that we call Twice-Baked Potatos. In Hong Kong, Shawn, Samtim and Angus built a new, simplified mechanism to place and build up solar cells. Our first mechanism is a traditional placement machine, using a vacuum to pick and place solettes onto a backing and pressurized syringes to add little dabs of solderpaste between the solettes. This new design holds the backing upside-down, covers it with a spray-on glue, and sticks the solettes to the board upside-down. The design is great–it’s much simpler and smaller than the pocket factory we’re working on at the Mantis lab, but until an hour ago, it wasn’t complete. The problem with this upside-down placement is that it’s tricky to get solderpaste onto the solettes. Solderpaste is…well, what it sounds like. It’s a thick mixture of solder particles and flux, and when you heat it up, it melts and holds the pads of different electronic components together. We use a special lead-free, low-temperature solderpaste to electrically connect the top of one solette to the bottom of the next.
In our main machine, we dispense the solderpaste with a pneumatic syringe, pushing solderpaste out a needle with a timed burst of high-pressure air. To do that with this machine, we’d have to build a special mechanism that can move the needle over the solettes to squirt on the solderpaste, and then get back out of the way so we can push the next solette onto the backing. It’s not a simple design–in fact, it would be much more complicated than the rest of the design.So we tried something different. A few weeks ago, we worked out the Twice Baked Potato method. Solderpaste has three states: paste, cracker and reflowed. When it’s a paste, it’s smushy and sticky. If you put paste on solettes and then stack them up, they’ll stick to one another in an unfortunate, lead-free menage-a-toi. If you heat the solderpaste up to 150 degrees C, it reflows–the flux evaporates and the solder melts and flows together, forming a small spherical lump of solder. The thing is, once you’ve reflowed a solette, all its flux is gone, and if you try to reflow the solder again, it doesn’t make electrical connections as reliably. The trick is to hit it halfway. At ~100C, the solderpaste hardens to the touch, but there’s still some flux trapped inside. We can stencil on a thin layer of solderpaste onto a solette, harden it by baking it briefly in a toaster oven, and we’re left with a stack of thin solettes than, when you heat them up, will form a solder joint to anything that’s touching them. Bley Joel made fifty of these once-baked solettes today, and tonight I worked through a bunch of them, loading them into the hopper, placing panels and reflowing the panels in the toaster to solder everything together. I’m starting to get complete, working panels where twelve solettes solder together nicely, but I’m stuck on the connection between the solettes and the fiberglass circuit board I use as a backing. I’m just not getting a 100% reliable connection–in fact, I’m lucky to get one good connection between a the solettes and circuit board.
I ruminate on this during the flight, flipping through reflow profiles and thinking about how to get the perfect reflow. Waiting for my baggage at the Hong Kong airport, there’s no doubt which bag is mine. It’s the one sitting on the conveyor belt in a growing puddle of rainwater. The one holding an itty bitty factory that makes solar panels.