Why Use Plated Through Hole Technology in PCB Design?

 Plated through hole (PTH) technology refers to the process of electroplating conductive materials onto the inner walls of holes drilled through a printed circuit board (PCB). This creates a conductive pathway between layers of the board, allowing different signals and power planes to pass through vias and interconnects. PTH has been the backbone of PCB fabrication for decades, with good reason. Here is an in-depth look at the key benefits of using plated through holes in your PCB designs.

Reliability and Durability

One of the biggest advantages of using PTH technology is the reliability and durability it offers. The electroplated metal (usually copper) creates a robust connection through the entire board thickness. The plated walls essentially become a single solid conductor.

This is superior to techniques like through hole via filing where the holes are manually filled with conductive ink or paste. Those methods are more likely to have voids or inconsistencies which can cause intermittent contacts. The solid plating of PTHs avoids that risk.

The tough plated metal also stands up well to repeated load stresses like temperature cycling. That makes PTHs well suited for boards going into demanding environments like automotive or aerospace applications.

Multi-Layer Board Enabling

PTH technology enables complex multi-layer board stacking. Without plated through holes, each board layer would be isolated from the others. By drilling holes through the entire stack and plating their walls, it creates conductive vias between layers.

This is how dense multi-layer boards are constructed today. The layers are aligned with drilled holes, and then electroplated to interconnect signals between layers. A typical high-density board might have 8, 10 or 12 conductive layers stacked together using plated vias. This creates miniature 3D routing networks inside the PCB.

Cross-Section of a Multi-Layer Board Using PTHs

Layer Number	Layer Name
1	Top Layer
2	GND Plane
3	Power Plane
4	Signal Routing
5	Signal Routing
6	Signal Routing
7	Power Plane
8	Bottom Layer

Without PTH technology, this dense routing would be impossible. The layers would remain isolated. So for complex boards with high interconnection needs, plated through holes enable the entire multi-layer approach.

Fine Pitch Capability

As PCB components shrink in size, the pitch (spacing) between pins and vias has become tighter. Plated through holes allow reliable interconnects at very fine pitch. The solid plated holes can safely go to 1mm pitch or even less.

This is due to the precision and consistency of the electroplating process. The desired plating thickness can be dialed in across even tiny via diameters. For comparison, conductive pastes and inks often have more variation at fine pitch. The fluid materials can wick between close vias or create voids.

So PTHs give PCB designers flexibility to route traces and connect pads at tight spacing. This helps maximize board real estate usage, especially with dense ball grid array (BGA) packages.

High Current Rating

The thick plated copper walls of through holes can also carry significant electrical current. The cross-section of metal provides very low resistance and inductance through each via. This makes PTHs well suited for power delivery traces on boards.

High current paths like those feeding modern processors often use an array of parallel PTHs. This provides ample conductive material to avoid voltage drops. Plated through holes reliably handle currents ranging from around 3-5A for a single via, up to 100A or more for an array.

Efficient Manufacturing Process

Another advantage of plated through holes is cost-efficient fabrication. PTH technology has been refined over decades of PCB mass production. The well-established processes are economical at scale.

The basic steps are:

• Drill via holes in stackup
• Electrolessly plate initial copper
• Add resist layer over board
• Strip resist from hole interiors
• Electroplate holes to desired thickness
• Remove resist layer

The automated drilling, plating, and stripping processes enable high volume throughput at low cost. And the horizontal panel designs used today allow even more vias to be plated simultaneously.

So plated through holes offer reliable performance while keeping manufacturing costs down. This combination of technical capabilities and fabrication efficiency has made PTH the standard via approach in the PCB industry.

Challenges of Plated Through Holes

For all their advantages, plated through holes also come with some limitations PCB designers should keep in mind:

• Via stubbing - The remaining laminate "stub" inside plated holes can cause impedance discontinuities, reflections, and crosstalk. Careful modelling is required to mitigate these effects.
• Aspect ratios - There are limits on achievable aspect ratios (height/width) of PTHs due to fabrication constraints. This can restrict routing options, especially on dense boards.
• Thermal relief - Electrical current flowing in plated vias generates heat that is hard to dissipate, since the holes are surrounded by insulating laminate. Thermal management is required to avoid reliability issues.
• Annular ring requirements - PTHs require a minimum annular ring of copper around the drill hole to ensure reliable plating. This consumes extra space on layers.
• Plating control - Getting uniform plating thickness across thousands of vias can be challenging. Poor control leads to reliability concerns.

So while PTHs are extremely capable overall, designers still need to apply them thoughtfully and account for their limitations.

PTH Design Guidelines

To leverage the full benefits of plated through holes, while avoiding any issues, PCB designers should follow these guidelines:

• Annular ring size - Allow sufficient annular ring space for plating process tolerances. Recommended minimum is 4-6 mil for standard vias.
• Capture pad assignment - Assign capture pads on at least 2 layers to ensure plating reliability. More vias = more opportunity for defects.
• Thermal relief - Use thermal relief cutouts or spoke patterns on large copper pours connected to PTHs to reduce heat buildup. Leave at least 55 mil clearance between planes.
• Aspect ratios - Limit aspect ratios to between 3:1 and 10:1 for most reliable results. Higher AR means lower yield and more plating defects.
• Barrel fill - Request barrel fill plating for high aspect ratio PTHs. This plates the entire hole rather than just walls, improving conductivity.
• Breakout routing - Allow sufficient space for breakout routing from BGA/chip vias to avoid dense stitching. Plan ahead when positioning PTH arrays.
• Via in pad - Use via in pad construction where possible to gain routing area rather than sacrificing pad space for annular rings.
• Modelling - Perform modelling of PTHs with stub lengths to assess signal performance. Tune stub length by adjusting stop-off plating depth.

Following these guidelines in your PCB design will ensure you maximize the benefits of plated through holes while minimizing any potential drawbacks.

Evolution to Other Via Technologies

While plated through holes have been the standard in PCBs for many years, alternative via technologies have also emerged to address some of the limitations of PTHs. These include:

• Blind vias - These are plated holes that connect layers sequentially rather than passing fully through the board. Blind vias help manage aspect ratios and stubbing effects.
• Buried vias - These fully plated vias are embedded internally in the layer stackup rather than drilling through from external layers. They simplify routing between inner layers.
• Microvias - Smaller laser drilled and plated vias with diameters below 8 mils. They allow ultra-fine pitch connections.
• Stacked vias - Vertically stacked vias and pad structures. Allows routing vias in same area without high aspect ratios.

So while plated through holes continue to be the backbone of most boards, PCB designers now have other via options to help mitigate limitations of traditional PTHs. The demands of high density designs are driving adoption of these newer via types to supplement PTH technology.

Frequently Asked Questions

Why are plated through holes better than through hole vias filled with ink or paste?

Plated through holes form a solid, uniform conductor along the entire via length. Filling holes manually with conductive ink or paste often leads to inconsistencies like voids or gaps that reduce reliability. Electroplating ensures continuous conductive walls without defects.

What is the minimum and maximum aspect ratio that is possible with standard plated through holes?

A good rule of thumb is to keep plated through hole aspect ratios between 3:1 and 10:1 for reliable fabrication yields. The minimum diameter is driven by the drill bit size, typically 8 mils. Maximum height depends on plating thickness capabilities, around 100 mils.

Do plated through holes introduce impedance discontinuities on high speed signal layers?

Yes, plated through holes create impedance disturbances due to the remaining laminate stub inside the barrel of the hole. Careful modelling of the stub length is required to assess these effects. Adjusting hole depth by varying stop-off plating can help manage impedance and reflections.

Why do plated through holes need to be captured on multiple pads when routing?

At least two capture pads on different layers should be assigned to each PTH to ensure the hole gets reliably plated through. If a PTH only has one capture pad, it may end up only partially plated, creating an open circuit. More vias means more chances for plating defects.

What construction techniques help reduce the negative effects of plated through holes on routing space?

Using blind and buried vias instead of PTHs where possible saves space by eliminating stub lengths. Also, a via in pad design where the breakout pad overlaps the via can regain some of the space lost to annular ring clearance requirements.