The aimed source eliminates the glancing angle problem, resulting in better adhesion, more uniform
growth structure, denser films, and significantly better metals distribution. We have improved the ratio of metal thickness
on the face to the thickness on the edges from 3:1 to 2:1. So, that 3000Å to 1000Å angstrom situation that we
talked about above is now 3000Å to 1500A. So what does this all mean? It means that windows coated with a 3000Å
thickness of Platinum on the face perimeter, and 1500Å on the vertical edge, will survive a 240°C solder pot test
(Sn63) for over 2 minutes without dewetting. That is a very aggressive test.
What metals do I need?
Typically, the solderable metal
stack consists of three layers,
- The adhesion, or base
layer. This layer is usually Chromium, or Titanium, and is the foundation of the stack.
The solderable layer is next. This layer is often Nickel, or Platinum.
The protective layer is almost always Gold.
Platinum or Nickel, which is better for your application?
Both metals solder quite well, but Platinum tends to be somewhat more user friendly, in our opinion.
A major difference between the two metals is the dissolution rate by the solder. Tin rich solders will immediately
consume, or scavenge, the gold layer, and will next begin to dissolve the underlying layer of Ni or Pt. Nickel dissolves in
molten tin at a rate of around three times that of Platinum. If your process requires relatively long dwell times in the liquidus
phase, then Platinum is a better choice as your solderable layer. Or, if you think that you may want or need to reflow the
solder to correct for a misaligned optic or component, then platinum lends itself much better to this situation than nickel,
at least in our experience.
Most certainly there is
a cost difference. Platinum does come at a premium, but it is not prohibitively more costly than nickel.
Our high volume, commercial use customers generally use nickel. Our military and special use customers use platinum.