Tag Archives: Printed Circuit Boards

How useful are PCB Vias?

Designers use a plated through via as a conduit for transferring signals and power from one layer to another in a multi-layer printed circuit board (PCB). For the PCB fabricator, the plated through via are a cost-effective process for producing PCBs. Therefore, vias are one of the key drivers of the PCB manufacturing industry.

Use of Vias

Apart from simply connecting two or more copper layers, vias are useful for creating very dense boards for special IC packages, especially the fine-pitch components such as BGAs. BGAs with pitch lower than 0.5 mm usually do not leave much space for routing traces between neighboring pads. Designers resort to via-in-pads for breaking out such closely spaced BGA pins.

To prevent solder wicking into the via hole while soldering and leaving the joint bereft of solder, the fabricator has to fill or plug the via. Filling a via is usually with a mixture of epoxy and a conductive material, mostly copper, but the fabricator may also use other metals such as silver, gold, aluminum, tin, or a combination of them. Filling has an additional advantage of increasing the thermal conductivity of the via, useful when multiple filled vias have to remove heat from one layer to another. However, the process of filling a via is expensive.

Plugging a via is a less expensive way, especially when an increase in thermal conductivity does not serve additional value. The fabricator fills the via with solder mask of low-viscosity or a resin type material similar to the laminate. As this plugging protects the copper in the via, no other surface finish is necessary. For both, filled and plugged vias, it is important to use material with CTE matching the board material.

Depending on the application, fabricators may simply tent a via, covering it with solder mask, without filling it. They may have to leave a small hole at the top to allow the via to breathe, as air trapped inside will try to escape during soldering.

Trouble with Vias

The most common defect with vias is plating voids. The electro-deposition process for plating the via wall with a layer of copper can result in voids, gaps, or holes in the plating. The imperfection in the via may limit the amount of current it can transfer, and in worst case, may not transfer at all, if the plating is non-continuous. Usually, an electrical test by the fabricator is necessary to establish all vias are properly functioning.

Another defect is the mismatch of CTE between the copper and the dielectric material. As temperatures rise, the dielectric material may expand faster than the copper tube can, thereby parting the tube and breaking its electrical continuity. Therefore, it is very important for the fabricator to select a dielectric material with a CTE as close as possible to copper.

Vias placed in the flexing area of a flex PCB can separate from the prepreg causing a pad lift and an electrical discontinuity. It is important designers take care to not place any vias in the area where they plan the PCB will flex.

What are Wearable PCBs Made of?

The Internet of Things market is growing at a tremendous speed. Among them, wearables represent a sizeable portion. However, there are no standards governing the small size PCBs or Printed Circuit Boards for these wearables. The unique challenges emerging in these areas require newer board level development and manufacturing experiences. Of these, three areas demand specific attention – surface material of the boards, RF or microwave design and RF transmission lines.

Surface material of the boards

PCB materials are typically composed of laminates. These can be made of FR4, which is actually fiber-reinforced epoxy, of polyamide, Rogers’s materials of laminates, with pre-preg as the insulation between different layers.

It is usual for wearables to demand a high degree of reliability. Although FR4 is the most cost-effective material for fabricating PCBs, reliability is one issue the PCB designer must confront when going for a more expensive or advanced material.

For example, with applications requiring high-speed and high frequency operation, FR4 may not be the best answer. While FR4 has a Dk or dielectric constant of 4.5, the more advanced Rogers series materials can have a Dk of 3.55-3.66. The designer may opt for a stack of multilayer board with FR4 material making up the inner cores and Rogers material on the outer periphery.

You can think of the Dk of a laminate as the capacitance between a pair of conductors on the laminate, as against the same pair of conductors in a vacuum. Since there must be very little loss at high frequencies, the lower Dk of 3.66 for a Rogers’s material is more desirable for high frequency circuits, when compared to FR4, which has a Dk of 4.5.

Typical wearable devices have a layer count between four and eight. With eight layer PCBs, the layer structuring offers enough ground and power planes to sandwich the routing layers. That reduces the ripple effect in crosstalk to a minimum, while significantly lowering the EMI or electromagnetic interference. For RF subsystems, the solid ground plane is necessarily placed right next to the power distribution layer. This arrangement reduces crosstalk and system noise generation to a minimum.

Issues related to fabrication

Tighter impedance control is an important factor for wearable PCBs. This results in cleaner signal propagation. With today’s high frequency, high-speed circuitry, the older standard of +/-10% tolerance no longer holds good and signal-carrying traces are now built to tolerances of +/-7%, +/-5% or even lower. This influences the fabrication of wearable PCBs negatively, as only a limited number of fabrication shops can build such PCBs.

High-frequency material such as Rogers require to have a +/-2% of Dk tolerance and +/-1% is also a common figure. In contrast, for FR4 laminates it is customary to have Dk tolerances of +/-10%. Therefore, Rogers’s material presents far lower insertion losses when compared to FR4 laminates.
In most cases, low cost is an essential factor. Although Rogers’s material offers low-losses with high-frequency performance at reasonable costs, commercial applications commonly use hybrid PCBs with FR4 layers sandwiched between Rogers’s material. For RF/microwave circuits, designers tend to favor the Rogers’s material over FR4 laminates, because of their better high-frequency performance.