via in pad as the name implies, is to hit the hole in the pad, with the increasing integration of electronic products, the components package is getting smaller and smaller, especially in complex PCB design pin spacing is too small, not hit the plate in the hole can not fan out, then you must hit the plate in the hole, from the next layer out of the line.
In the drawing of the PCB, the plate in the hole is nothing special, is over the hole in the pad, but there are several types of PCB production and processing processes, as hardware engineers must understand.
The most common hole in the plate
Is the hole in the pad, the production of PCB does not require a special process, the pad will leave a "hole eye", if the hole eye is too large may cause tin leakage problems, may also lead to false soldering problems, this process is the cheapest and most common, as follows QFN package on the heat sink hole.
Resin plugging hole + copper plating cap
This type of plate-in-the-hole process is a little more complex, first in the hole is filled with epoxy resin, and then electroplated copper seal, the surface can not see the hole eye, very flat, and there will be no tin leakage or false soldering problems, this process is generally used in the higher-end some HDI board, such as the board to go on high-precision devices, such as BGA and other pin devices, pin spacing is too small to fan out, the surface out of the line is difficult, need to go through the hole To the next layer out of the line, this plate in the hole has a great advantage, so that the alignment is smooth. Compared with the ordinary plate in the hole process is relatively complex, of course, the cost will also increase. Now the cell phone board is in the plate hole design.
Copper paste hole plugging + electroplated copper cap
This type of plate-in-the-hole process is generally used in need for rapid thermal conductivity of the location, such as the QFN under the GND PAD. over the hole in the middle of the plug filled with copper paste, and then an electric copper seal. Since the middle of the hole is filled with copper metal, the thermal conductivity is very good, and there is no difference between the resin plug hole from the surface. This process can be used when a large amount of heat dissipation is required for the QFN, which is so flat that the naked eye cannot see any traces of over-hole after production. Heat dissipation performance is particularly good. Of course, you get what you pay for, the process is quite good but more expensive.