Good decoupling is nearly always essential in a world of mixed-signal designs that exhibit increasing speed, performance, and component density. There are cases, though, where proper decoupling is absolutely critical if the circuit is to deliver its intended performance. That's where the concept of buried decoupling using ac vias comes into play. Examples of such circuits include RF power amplifiers, ultra-fast analog-to-digital converters (ADCs), and laser-diode drivers.
In such cases, each millimeter of unwanted path length translates into a nanohenry of inductance, which can degrade circuit performance by tenths of dB. (Remember that at 1 GHz, 1 nH amounts to a reactance of more than 6 (omega)).
The parasitic inductance of a chip capacitor is generally very small, little more than a piece of wire of the same physical length as the chip. But unfortunately, you also have to consider the pc-board tracks and vias, which is where the degradation occurs.
To get the ultimate performance from the bare capacitor, you must get rid of all the intermediary conductors. That can be achieved by "burying" the capacitor chip vertically in the thickness of the pc-board material—directly beneath the pin to be decoupled. The other end of the capacitor is soldered to the ground plane. This results in a dc-blocking connection between the top and bottom layers—hence the name "ac via."
The figure shows the cross-section of such an ac via, along with a conventional, surface-mounted capacitor for comparison. It's immediately obvious that the latter suffers from a significantly longer path length than the ac via. In a typical case, this could amount to 4 to 5 mm. This method also has the advantage of using up no pc-board area.
To successfully implement this method, some constraints must be respected:
- The capacitor case should have an approximately square section with an overall length equal to, or slightly greater than, the thickness of the pc-board material.
- The bottom layer must be attributed to the ground plane.
- The hole in which the capacitor will be placed should have a diameter slightly larger than the diagonal of the chip, and its walls must be free of metallization. If the pc-board process doesn't allow for it, the metallization has to be removed by drilling.
- The ac vias should be laid before any other operation on the pc board.
Place the circuit board on a flat surface, ground plane upwards, and insert the capacitors in the holes. Solder the capacitors to the ground plane. Normally, this is sufficient and the board is ready to receive the other components.
Alternatively, you may also solder the capacitors on the top side. Yet in some cases, the resulting solder bumps might upset the placement of other components. You have to judge which solution suits you best. Note that even if only one side is soldered, there's little risk of the capacitor drifting astray in the subsequent operations of wave or reflow soldering. The superficial tension of the molten solder is usually high enough to keep the chip in its hole.
Although this method requires some manual reworking, it delivers the ultimate performance when near-ideal bypassing is needed. For some really difficult cases, this price is certainly worth paying. Plus, it conserves the board real estate.
Of course, this technique isn't restricted to decoupling or bypassing. Most two-terminal, small-size components can be buried in this fashion. But in the absence of an automatic placement capability, the benefits aren't so obvious.