PTH Technology in PCBs

Rush PCB Inc. is a reputed manufacturer of PCBs or printed circuit boards in the US. The electronic industry uses our printed circuit boards to electrically interconnect electronic components through conductive pathways, typically known as signal traces or tracks etched onto a copper-plated board. We make the board itself from an insulating dielectric substrate. We may have to drill several holes through a PCB, and we drill most of them with coated tungsten carbide drill bits. For our HDI boards, which require very small diameter holes, we use drill bits made of laser beams. Finally, we make the walls of these holes conductive by electroplating them with copper.

PTH Technology in PCBs

Depending on the requirement, we may have to drill different-sized holes in a PCB. Some holes we may leave without any plating, and they are the NPTH or Non-Plated Through Holes. NPTH, having no coating, cannot be used as conductive pathways. For other holes, we may electroplate with copper to create PTH or Plated Through Holes and use them as conductive pathways. Apart from holes, a PCB may have additional copper features such as:

PCB Pads

These are the exposed copper features on the substrate that allow mounting and soldering of the component lead. For a component with a complex footprint, it may be necessary to create multiple pads simultaneously. A pad may or may not have a hole in it depending on whether the pad is for mounting a THT (through hole component), or an SMT (surface mount component).

A PCB pad is considered PTH if the hole has an annular copper ring and a solder stop mask on both sides, and they are linked with a copper barrel extending through the hole. If the PCB is multilayered, there may be additional copper rings in the inner layers, electrically connected to the barrel.

Holes without a copper-plated barrel are NPTH. Such holes may or may not have an annular copper ring around them. Forming NPTH is simpler and quicker than PTH. We typically use NPTH holes for tooling or as support for mounting the PCB in its operational location.

Tracks and Footprints

The most important feature of a PCB is the circuit that interconnects the electronic components mounted on it. Typically, we chemically etch away the excess copper from the substrate to form the necessary circuit, which has pads and footprints matching the leads of the components and these are interconnected with tracks. When mounted and soldered on these pads and footprints, the leads of a component are firmly anchored both electrically and mechanically.

Most PCBs are either double-sided or multi-layered. PTH technology allows component leads to connect not only on the same layer but also with other circuits on various layers.

Comparing PTH and NPTH PCBs

Although PTH technology is a premium option for interlayer connectivity, NPTH presents a cheaper and simpler alternative when applications are less demanding.

It is easy to distinguish between a PTH and an NPTH PCB by checking for the presence of copper plating on the hole wall. An NPTH PCB will have no copper in the hole walls. Additionally, there will be no electrical continuity between the pads on either side of an NPTH.

Cost and Process

The major difference between the PTH and NPTH technology is in the plating process, which the NPTH does not use. This saves production time and cost as compared to the PTH process. However, the absence of plating reduces reliability due to the lack of physical contact with the component lead inserted into the hole.

Consistency and Reliability

NPTH PCBs carry substantial risks for applications subject to shock, vibration, or fluctuating temperatures. Metal movement, expansion, and contraction can disturb the alignment of the component lead inside the hole, progressively leading to increasing resistance or intermittency.

PTH PCBs mitigate the above through consistent bonding of the component lead to the plating along the hole. This bonding reduces the mechanical stresses drastically from environmental conditions. This superior signal integrity and reliability justify the use of PTH PCBs for mission-critical applications.

PTH Functionality in PCBs

PTH technology has two major functions in PCBs. They are mainly for:

Mounting and Soldering TH components

Although the usage of TH or through-hole components has declined substantially to be replaced by surface mount components, some applications still use them. In contrast to surface mount components, through-hole components have longer leads that must pass through holes in the PCB for proper mounting and soldering.

Typically, the PTH for a through hole component requires a larger diameter hole than the pin diameter of the component, to allow the lead to easily pass through.

Interconnection Between Layers

In double-sided and multilayered PCBs, we typically use the PTH technology for electrically interconnecting inner and outer layers. In this function, the PTH is also termed as a via. There may be three types of vias:

1. Through Hole Vias

These PTH typically pass through the entire thickness of the board, interconnecting circuits on the outermost layers, and, if necessary, to some layers inside.

2. Blind Vias

These PTH begin on one of the outermost layers and terminate on an inner layer, interconnecting other inner layers in between, if necessary. These PTH are visible only on one side of the board.

3. Buried Vias

These are PTH not visible from the outer sides of the board. They begin on an internal layer and terminate on another internal layer, interconnecting other layers in between, if necessary.

4. Via-in-pad

This is a special case that we typically use in our HDI boards to increase routing density. Rather than creating a via with its pads, we place the via inside the component pad of a surface mount component. This allows us to save the space that the via pad would have taken up and use the space for routing. This technique is very useful when routing high-density boards such as for BGAs.

However, via-in-pad has a downside. During reflow soldering, molten solder wicks down into the via hole, leaving the joint dry. We counter this by filling up the via hole with a non-conducting epoxy and closing the hole by plating it over.

Rules for PTH and NPTH PCBs

We follow some general rules when making PTH and NPTH PCBs. Following these rules significantly impacts the turnaround times:

Rule	NPTH	PTH
Minimum edge-to-edge clearance	5 mil	9 mil
Minimum finished hole size (Mechanically drilled)	6 mil	6 mil
Minimum annular ring	12 mil	4 mil

 

For DFM or Design for Manufacturing, we consider the above rules and guidelines for drill holes, additionally considering drill hole sizes, aspect ratio, and spacing for the specific via type. We use via-in-pads to save space, but it can result in increasing the PCB manufacturing process by extending it by two days.

For PTH designs, annular ring dimensions are important. The annular ring is the copper pad area extending past the drill hole on the PCB. If the annular ring is too small, it may easily detach from the PCB when subject to mechanical stress due to heat or vibrations.

Large Annular Rings

The minimum annular ring size for PTH is the distance from the border of the hole to the extent of the pad. Reducing the minimum annular ring from the standard requirements tends to increase PCB costs.

Lower Hole density per Unit Area

Whether we use mechanical drilling techniques or laser, each machine has its own efficiency. Drilling a higher hole density per unit area takes longer to complete the drilling process, increasing the throughput time. We recommend decreasing the hole density per unit area to lower costs.

Larger Diameter Holes

Drilling small-diameter holes requires high-precision equipment, increasing the cost of the PCB. We upcharge for holes with diameters between 0.15-0.3 mm. We recommend using larger diameter holes and annular rings to keep costs down.

Hole Shape

We make holes either round or oblong, depending on customer requirements. Round holes have the advantage of uniform annular rings, while for oblong holes, the dimensions may be non-uniform. As larger annular rings cost less, we recommend using larger diameter holes, whether round or oblong.

Advantages of PTH Technology

PTH technology forms much stronger bonds between the connecting components and the circuit board, as compared to those from surface mount methods. Manual repair of components is easier for PTH boards. However, PTH boards can be expensive.

PTH technology is useful for heavy components that need adequate mounting strength to withstand high mechanical stresses, such as in power supplies, electronic modules, and LED applications.

We have evolved a mixed technology with investors and customers demanding lower costs, less equipment usage, fewer manufacturing processes, and higher profit yields. We use both surface mount technology and PTH in a two-phase process.

Conclusion

At Rush PCB Inc., we use PTH technology in our printed circuit boards to offer durable connectivity of pads, traces, and component pads across all layers. The conductive plating of the PTH technology allows unimpeded signal travel across the width and thickness of our multilayered boards regardless of environmental conditions.

Although NPTH boards are cheaper, PTH boards offer improved robustness and reliability that justify the higher expense. With extensive expertise in producing boards with PTH technology, our engineers at Rush PCB Inc. can guide you in harnessing the advantages of this technology for your projects. Talk to our experts today and let us design your next board.