Introduction

While designing multi-layer Printed Circuit Boards (PCB’s), one of the most basic elements that the engineer must include is the requirement for interconnected traces/planes on one layer to traces/planes on another. The most efficient technique of achieving this is to use vias. These are small holes drilled into the layers of the PCB, and fitted with a copper tube connecting to pads on either end. The pads in turn connect to the required traces on respective layers.

Why use Vias?

With increasing use of high-density boards, and engineers must reduce trace widths and spacing to accommodate for applications. Vias are another technique of achieving higher density boards by making them in multiple layers. In turn, the design of vias has also been evolving, with designers and engineers trying out different types of vias such as ‘landless’ and ‘swing types’. One of the very effective methods of achieving increased layer density is by using ‘via-in-pad’ designs.

Example

Consider the plight of an engineer in the process of breaking out the connections from an FPGA or BGA package of, for example, 1760 pins with a 1mm pin-pitch. According to the application data sheet of such a package, 6 signal layers are necessary to breakout the connections from all the pins. However, with advanced via techniques, engineers can now accomplish this with only 2 signal layers, resulting in better interconnection as well as being a substantial cost saving option.

Fabricators using the high-density interconnect (HDI) techniques use advanced technology such as buried vias, blind vias, via-in-pad, and micro-via techniques to improve the density of their boards spectacularly. Micro-via techniques involve using lasers to drill holes of very small diameter. Together with the via technologies above, the use of micro-vias results in 24% increased routing density per layer over conventional design processes.

How do they work?

As the name suggests, via-in-pad is a via deliberately placed within the area of a solderable pad. Normally, conventional design practices prevent the use of a via very close to or within a solder pad. Most manufacturers also recommend not using a via this way. The main reason being the via often acts as a wick does during the reflow process, allowing all the solder paste to melt and flow into its hole, leaving the solder pad starved of solder and resulting in an unsoldered joint. This problem is solved by filling and capping the hole of the via-in-pad.

Therefore, just as with any other tool, via-in-pad technology can lead to spectacular results if used properly, or to disastrous consequences if misused. For instance, inadvertently leaving a via-in-pad uncapped under a BGA solder ball can result in the solder paste flowing down into the hole of the micro-via, leaving the joint open. Therefore, it is essential to have every via-in-pad filled and plated over. To be on the safe side, all vias on the board are filled and plated over, and this effectively takes the via out of consideration.

While filling and capping the via-in-hole does solve a major problem, it creates another one—lack of coplanarity. Unless care has been taken to achieve good planarity, there can be a tiny bump or an indent over the via. This can lead to a less reliable assembly, especially with chip scale and BGA packages.

Conclusion

To discuss whether integrating the use of via-in-pad technologies for your PCB specifications, please feel free to contact the team at PCB Global for the most efficient advice followed by a rapid quote.  Please email you design file to sales@pcbglobal.comfor a rapid and competitive quote.