Via in Pad

Increasing signal speeds, PCB functional density and PCB layer, thickness are increasing the challenges associated with PCB design, especially with respect to the signal integrity. These constrains have forced chip manufacturers to produce fine pitch chips (BGA, QFN, flip chips…), and PCB designers to use Via in Pad (VIP) Plated Over (VIPPO) structures in conjunction with traditional designs, such as dog bone traces, microvias, skip vias and pad-with-trace along with incorporating VIPPO in order to achieve routability and signal integrity requirements.

 

What’s Via in Pad?

 

The final type of via filling in IPC-4761 is known as Via in Pad (VIP), and as it implies, VIP refers primarily to cases where the via is placed entirely within the pad of a surface mount device (SMD). Many PCB designers are finding taking advantage of this Via in Pad fabrication process more than ever before, as BGA packages get smaller and smaller, so that escaping traces becomes more and more difficult.

When microvias are placed in pads intended to be soldered (Via in Pad), the holes should be filled. Vias can be filled and plated over or filled with copper to yield an acceptable solderable surface. If the vias are not filled, the volume of the microvia will “steal” solder from the intended solder joint. The air, which is entrapped after solder paste has been applied, is also likely to out gas during the reflow assembly process and create voids in the solder joints.

In PCB design, via refers to a pad with a plated hole that connects copper tracks from one layer of the board to other layer(s). High-density multi-layer PCBs may have blind vias, which are visible only on one surface, or buried vias, which are visible on neither, normally referred to as micro vias. The advent and extensive use of finer pitch devices or products and requirements for smaller size PCBs creates new challenges. An exciting solution to these challenges uses a recent, but common PCB manufacturing technology with self-descriptive name, “VIA IN PAD”.

Via in pad helps to reduce inductance, increase density and employ finer pitch array packages. The via in pad approach places a via directly under the device’s contact pads. This allows higher component density and improved routing. Consequently, via in pad provides the designer significant PCB space savings. For example, traditional fan-out places four components, whereas with via in pad, eight components can be placed within the same board outline.

Filled via in pad is a way to achieve intermediate density with an intermediate cost compared to using blind or buried vias. Some of the key advantages associated with using the via in pad technology are:

 

• Fan out fine pitch (less than 3mil) BGAs
• Meets closely packed placement requirements
• Better thermal management
• Overcomes high speed design (HSD) issues and constraints i.e. low inductance
• No via plugging is required at component locations
• Provides a flat, coplanar surface for component attachment

 

However, there are some disadvantages associated with this technology. The most prominent and worrisome is the cost impact associated with adopting a new technology. PCB manufacturers and suppliers identified two primary cost drivers associated with specifying via in pad technology: Additional HDI PCB manufacturing process complexity and the underlying material cost for the conductive fill.

Specifically, via in pad technology adds 8 to 10 steps to the PCB manufacturing process while via filling cost is a function of the via size and actual number of via instances on any given design. However, the reduction in layer count realized by using via in pad technology compensates for the added cost associated with this process.

 

Not Filled or Filled Via in Pads?

 

If vias in pads do not be filled, your assembled board will likely be missing connection between some of the BGA balls and the board. If you placing the via in the BGA pad, your only option is to have the vias filled and plated over at the PCB manufacturing house. If you placing the vias between the pads, you have two options. You can put soldermask dams on the short trace between the pad and the open via. This will prevent solder paste from migrating. The other, and the better, option is to cap the vias with solder mask. This gives a bit of extra protection in case of any of the solder mask dams are too thin or chip off. Just make sure you cap these things on the solder side. If you cap them on the back, solder paste can still spread on the trace and partway into the via. That still puts the electrical and mechanical connection at risk.

 

Solder Mask Filled or Epoxy Filled?

 

Solder mask cannot be used for filling the vias (microvias) in pad when forming Via in Pads since removing solder mask clearance from the SMT pad would make it un-solderable. In the solder mask filling, there was concern that nearby vias might wick away (straw and suck) some solder paste from a pad, so of course this is an even greater concern when the via is within the pad itself. In this case, MADPCB’s PCB production team will selectively fill the vias with an epoxy material but not solder mask before the surface finish is applied, which requires some additional cost and lead time compared to standard vias.

 

Conductive or Non-Conductive Epoxy Filled?

 

The epoxy is most often a non-conductive substance, since conductive substances expand much more quickly with heat, which can be troublesome during the reflow soldering process. Non-conductive filling does not impede the via’s ability to conduct electrical current, since the walls are still plated with copper just like any other via. In cases where the via must carry out a lot of current or heat through the PCB board, conductive epoxies are available as well. When the via in pad is plated through and then filled with a conductive or non-conductive epoxy plugging ink, the epoxy ink is then planarized to create a flat surface and plated over to get a fully filled and capped vias in pads.

When you send inquiry with PCB design files to MADPCB for quote, it would be helpful to note the presence of any vias within component pads. These will be taken into account for the pricing and lead time you see on your quote, and the CAM engineer can help you to determine if specific vias on your board will require filling of any kind.

 

Via-in-Pad Fabrication Process

 

The Via in Pad technology proves to be a good technique to save real estate while making the layout of the PCB thus allowing denser PCBs. Not all PCB manufacturers can conduct via in pad manufacturing process as excellent as MADPCB. Here we list the main Via in Pad fabrication process for thru-hole PCB boards as below.

 

 

Maybe you find this is via in pad process is for thru-hole boards. Yes, but similar. In Microvia in Pad Process, the required selective microvias need to be fully filled plugged (/filled) with epoxy ink, then planarized, plated-over to get invisible capped microvia surface.

 

Manufacturable Via in Pad Technical Parameters