Tag Archives: Thermal Conductivity

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.

Heat Pipes for Electronic Applications

Electronic applications such as mobile, embedded computing, and servers often use intermediate level heat pipes to cool systems dissipating 15-150 Watts. Usually, such heat pipes use copper tubes and sintered copper wicks with water as the working fluid.

System designers incorporate heat pipes when the thermal design has limitations of space and/or weight and other materials such as solid aluminum or copper heat sinks cannot achieve the desired cooling. However, heat pipes for electronic applications require several considerations.

Manufacturers often publish thermal conductivity of heat pipes as ranging from 10,000 to 100,000 W/mK, which is nearly 500 times that of solid aluminum, and 250 times that of solid copper. However, unlike solid metal, the effective thermal conductivity of copper heat pipes varies extensively with their length and other factors affect this figure as well, although to a lesser extent.

For instance, a sample heat pipe could reach its published thermal conductivity of 100,000 W/mK when its length was 100 mm, when transporting heat from a 75 W power source. However, on increasing the length of the same heat pipe to 200 mm, its thermal conductivity fell to less than one-third the published figure.

Moreover, customization for a specific electronic application can severely limit the operational and performance characteristics of a heat pipe. This is much like the type of transformation a vehicle requires when fitting it out for a specific activity such as track racing or off-roading.

An engineer often has to customize the heat pipe when navigating the crowded route from the heat source to the evaporator or condenser. They do this by flattening or bending the heat pipe. For a lower thermal resistance, the designer may have to machine a heat pipe assembly to create a flat surface capable of direct contact with the heat source. This may require elimination of the solid metal base plate and the extra TIM layer. To keep the entire fitment small enough, the heat pipe may have to use a diameter that is different at one end from the other.

Drastic changes to customize the heat pipe to the electronic application often results in the heat pipe unable to meet the power handling requirements. To make it suit the application, the engineer may have to make changes to the internal structure of the heat pipe design.

This often requires changing the porosity and thickness of the wick, allowing the engineer to tune the heat pipe to meet specific operating parameters and performance characteristics. For instance, the heat pipe may have to operate at higher power loads or against gravity, requiring a larger pore radius or an increase in the capillary pressure of the wick.

However, a larger pore radius works against gravity, and therefore, in place of increasing the pore radius, increasing the wick thickness may make it more effective. Alternately, the engineer may increase both the wick thickness and porosity along the length of the tube. Some suppliers specialize in custom heat pipes and use unique mandrels and/or custom formulated copper powder to fabricate them. The outward physical characteristics also affect the performance of a heat pipe.