Via in pad technology gets you intermediate density at a slightly higher cost compared to blind/buried vias. The extra dollars go into added manufacturing steps and material cost. But the benefits you get are:
- Fan out fine pitch (less than .75mm) BGAs
- Meets closely packed placement requirements
- Better thermal management
- Overcomes high speed design issues and constraints i.e. low inductance
- No via plugging is required at component locations
- Provides a flat, co-planar surface for component attachment
Don’t Leave Vias Open
The experienced contract manufacturer (CM) or EMS Provider knows that a significant aspect of that via in pad strategy is to create a sub-strategy dealing with via fill. The reason you want via fill is to avoid solder from being sucked through the hole and causing you reliability problems. However, if you want vias filled to prevent solder drain, it’s important for you to define the filling method.
Special Note: Maintaining effective communications between the PCB designer and fabrication house is particularly crucial when it comes to via in pad technology and ensuring there is a clear understanding of how a particular board is fabricated using via in pad technology.
You can use conductive or non-conductive fill material as your via fill (see figure below).
The conductive approach uses copper, silver, and epoxy, while the non-conductive fill uses a polymer combining a blend of copper and epoxy. In both cases, vias are drilled and copper plated followed by either the conductive or non-conductive fill.
As shown in figure below, a via cap is a secondary solder mask application that covers vias and via pads on top or bottom sides of the board (not both) after the conductor surface finish is applied. It only places a small amount of solder mask in the barrel of the hole. It is intended to cover the via pad, and most of the vias will have solder mask across the opening of the hole.
Flooded or tented vias, shown in figure below, provide no aperture in the primary solder mask for the vias on the top and/or bottom side of the board. The vias are neither capped nor encroached during primary solder mask application. Since the LPI solder mask has a relatively low percentage of solids, it will shrink and crack during the cure process providing a location to entrap chemistry and fluxes. However, this process has reliability and assembly issues for most applications.