For those who are intending on purchasing a DIY Kit, here is the assembly drawing - shows which part goes where. All of this information is on the Silk-Screen of the board, but I find it much easier to build when I can look at a reference (and possibly even make notes on it).
Gold plating for solder pads can be a bad thing! Gold tends to form intermetallic compounds with lead/tin/aluminum (google "purple plague" for a well known example). These can be brittle and poorly conductive. Components have been known to fall of circuit boards...
looking great Pilot, can't wait to see these in action.
About the programming - can the controller chips be supplied programmed off the board for self-assemblers or can they only be programmed after assembly using the pads on the board?
U2 Cool is going to be a bit brighter than a Neutral, but we are talking about 3 FULLY driven XM-L's. It's a lot of light no matter which exact emitter you choose.
The ENIG (Electroless Nickel, Immersion Gold) process has been perfected over the years, and solder joint quality on ENIG is excellent. "Purple Plague" is really a reference to brittle joints between aluminum and gold and is seen when gold is used inside of a chip package where a wire-bond takes place. Wire bonding and soldering are very different processes. "Black-Pad" can be a problem if electro-plated Nickel is used as the base, but circuit board finishes use electroless nickel to avoid that problem. Even NASA and the U.S. Military approve of the use of ENIG PCBs - And I promise that neither of those agencies would tolerate random components falling off of their assemblies. Components won't be falling off of these boards.
Purple plague is the aluminum-gold intermetallic. It is the most common/notorious. There are also gold-lead and and gold-tin intermetallics.
See the wikipedia article on gold plating. last section on soldering issues:
You are probably better off masking the gold from the component pads. There are also some indications that, under the right conditions, gold may help catalyze the formation of tin whiskers from lead-free solder. And gold can grow its own whiskers. But it sure is pretty...
These are the conclusions after testing the way that solder joints fail on a variety of different component packages and a variety of different PCB finishes.
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Conclusions
For plastic packages, crack initiation, propagation, and failure occurred at either the package or
board interfaces for sections with or without voids. This was true for A or B cycle conditions.
Generally, voids were concentrated near the package interfaces. There appeared to be no crack
propagation among the voids, except for the voids interconnected at the interface.
For a ceramic assembly failure (See Comment Below), brittle failure was observed for Nickel-Gold surface finishes. OSP and
HASL showed ductile failure through eutectic solder joints. For plastic packages, there was no
distinction between the three surfaces
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The ceramic packages tested were very odd - from the document:
Ceramic packages with 625 I/Os and 361 I/Os were also included in our evaluation. Ceramic
packages had high melting solder balls (90Pb/lOSn) with 0.035 inch diameters. These balls were
attached to the 'ceramic substrate with eutectic solder (63Sd37Pb). At reflow, package side eutectic
solder and the PWB side eutectic paste was reflowed to provide the electro-mechanical interconnects.
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The high melting point solder is nothing like standard leaded or lead-free. It's VERY heavy in gold content. Nothing like that is used on the 3 XM-L Board
I recently built some boards for the military and was expressly forbidden to use solder-on-gold. Also forbidden to use lead-free solder for that matter. In their environment, every little detail matters immensely. Things that don't matter in a benign, short life consumer environment can quickly turn into nasty problems. I can also show you some gold plated circuit boards out of some HP test equipment that failed because of the problem.