Introduction
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Put my caps where!?
The traditional place to put your components is on the top and bottom of the printed circuit board, but what if you placed some of them inside the board as it was being made? Can they even do that? Well, yes, they can, and do. With the push to create ever smaller and more densely populated PCB’s, printed circuit manufacturers have been pushed to develop better ways to manufacture and fabricate boards.
One of the techniques that has been developed to deal with higher frequencies and more densely packed boards is to embed some of the components in the board during manufacturing. Imagine placing some of the decoupling capacitors directly below the chip they are decoupling. Not only does this free up space on the outer layers of the board, but it also decreases the length of the trace connecting the device to the decoupling cap which reduces impedance and does a better job reducing noise. This is a great leap forward in improving signal integrity.
Fab house or Assembly house
It used to be that you sent your design to a PCB manufacturer or Fab house. Then you send the finished board and parts to an Assembly house. Today that line is blurred. Fab houses are increasingly having more to do with assembly. When you embed a component in the pcb, you create a cavity where a component can be placed. That component is then placed during pcb manufacturing. That component, and cavity, can be exposed to the outside when the board is done, or it can get covered up with the next layer of the board. As a result, the fab house is now an assembly house as well.
Not just for passives
While placing small passives inside of the board does not seem too far-fetched, how about placing a BGA on an inner layer? There are two types of active components that get placed inside of a board. A BGA is a good candidate for a component to be placed on an inner layer and then covered up with another layer. The other type of component is a bare die.
For various reasons, a designer might want to place a bare die on an inner layer, leaving the cavity open when the board is done. The die, in this case is not placed by the fab house but is placed by an assembly house after the board is completed. If you have not worked with bare die before, it is not for the hobbyist.
Bare Naked Silicon
Bare die is the silicon circuit that is inside of a packaged part. To use a die, it must first be adhered to the substrate. To make the connection you cannot use solder, but rather must use wire-bonds. A wire for bonding to a die will either be made of aluminum or of gold. Wire bonds are made on a wire-bonding machine and are very costly, require a great deal of maintenance and highly skilled operators. Never the less, using bare die does provide advantages to using a pre-packaged part in some instances.
Figure 1. Four Layer board with three cavities. Cavity extends from top layer to internal layer 1.
The board shown in figure 1 is a four-layer board I recently did for one of my customers (picture used with permission). You can see where wires can attach from the left and right with strain relief. There are some components on the top layer and there are three large cavities that extend from the inner layer 1 to the top layer. These cavities allow a bare die to be placed on the inner layer and wire-bonded. The stack-up was carefully chosen so that the die and the wire bonds do not break the plane of the top layer.
How do I do this?
Seeing this technology can be one of those creative moments when a designer feels she must have this new capability. Fortunately, my favorite CAD program, Altium, allows for designs with embedded components. They have some really good online tutorials, for example HERE. When I first heard about this ability I was almost as excited as when the Hobbits learned that beer comes in pints. Hopefully you will as well.
Final thoughts
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