In Part II we discussed the different characteristics of commonly used materials in rectifier assemblies. In this post, we'll look at using an alumina substrate to support a rectifier assembly using surface mount components.
Alumina substrates work great with surface mount or formed lead diodes.
Alumina substrates offer the advantage of higher thermal conductivity than typical FR-4 fiberglass board, the kind commonly found in computers, which helps keep the components running cool.
The rigidity of alumina means it can provide more support for components without flexing or breaking them. Wire bonds made to an alumina substrate are less subjected to thermal expansions and contractions compared to a printed circuit board. When the wire is less than half a mil in diameter, board flexing can be a concern.
Alumina’s low CTE also means it is compatible with many types of encapsulation materials, AND it provides greater voltage isolation over regular printed circuit boards. A properly selected encapsulation material will adhere well to the substrate, which improves isolation voltages between terminals, nodes, components, and the outside world.
Integrating passive components can be a cost saver. Capacitors and resistors can be screen-printed onto the substrate, thereby reducing real estate and interconnections where space is at a premium.
Besides wire bonds and soldering, conductive epoxy works well with alumina too.
Alumina can attached to a metal base plate by solder or conductive epoxy. This will help get the heat out of the assembly even faster, and because alumina’s CTE more closely matches that of aluminum, there is less chance for thermally induced mechanical stresses.
If that isn’t enough, direct-bond copper alumina substrate is a product that bonds copper directly to alumina, which increases the thermal conductity even more than regular alumina. The trade-off is cost (once again). And weight. Direct-bond copper (DBC) weighs more, thanks to the amount of copper traces present.
So why not use alumina for everything? I’ll give you a hint. It’s a four-letter word. C-O-S-T.
The disadvantage of using alumina over FR-4 board is that it costs more. Direct comparisons are hard to come by, but for about the same size, an alumina substrate can be four or more times the cost of a double-sided pcb. Cost driving factors in a substrate include thickness, length, and width. Drilling, plating, thick or thin films, and printing resistors or capacitors, are all cost adders.
When it comes down to it, one must weigh the cost-benefit of using alumina for each application. It works very well in high-rel applications, but is less suited for high volume consumer applications that are cost sensitive.
Up next are cleaning processes in rectifier designs, and what to look for.