Lots of dicussions lately on SMT so I thought I would throw out a topic for discussion.
How about reverse SMT?
1. You lay the compents down on a sticky sheet or a dab of hot glue and then fill a plastic channel around them.
2. You then quickly flow solder a bare wire with solder transfered off of a soldering itron tip/element and pull it to it's destination or around a peg to do dog legs.
3. Build a raft layer for insulation and continue on to the next layer of point to point wiring,etc.
4. Tell current PCB layout software that you are building a fourty layer board.
I've been thinking about this one for a while too. Reverse-building boards has a lot to recommend it. Including not worrying about pick & place.
The only concern I have is alignment. With fine pitch parts, it could be pretty bad to be off by 0.5mm. We need ridiculous accuracy or machine vision to make this work.
I'll be experimenting with conductive plastic pretty soon (just waiting for an account number from my carrier to get the plastic shipped to me) to see if it's a viable interconnect method for just this application.
Got the plastic. One 1/8" pellet had a resistance of about 900 Ohms between two multimeter probes. I hope that will come down when I make a better connection to the plastic. If not, then we have a logic interconnect, but it's still not right for power delivery.
- create wiring trace layers out of PLA/ABS or other direct printable material(with sprue for positioning/shape maintenance).
- cast plaster or other ceramic material around a layer,
- Repeat 1st 2 steps for multiple layers, letting the cast set
- thoroughly melt out printed traces
- fill with solder (a bit tricky, unless the solder happens to bind nicely to the cast material, in which case just running molten solder through would line the channels)
- skip the "remove from mold" step - the filled mold *is* the "cicuit board"
- glue the filled mold to a stiff backing to minimize the chance of cracking it, and to allow mounting it securely
That can be as 3D as you need - multiple planar levels with vertical interconnects, laid down in successive plaster casting layers. Cut off support sprue between layers.
The "top" layer would have solder-filled holes - heat the area and shove in a chip or deposit a surface mount chip (if you can make solder holes that finely spaced).
Not as straight-forward as milling copper clad PC boards, of course. But a bit more generic on the materials side.