What is Plated Through Hole (PTH)?

Plated through hole refers to the process of creating a conductive path between the layers of a multi-layer PCB by drilling holes through the board and plating them with a conductive material, typically copper. These plated holes allow electrical signals to pass through the layers of the PCB, enabling the interconnection of components and traces on different layers.

PTH is essential for several reasons:

1. It provides a reliable electrical connection between layers in a multi-layer PCB.
2. It allows for the mounting of through-hole components, such as connectors, switches, and certain types of capacitors and resistors.
3. It enhances the mechanical strength of the PCB by reinforcing the connection between layers.

The PTH Process in PCB Fabrication

The PTH process involves several steps, each of which contributes to the creation of a reliable and robust electrical connection between the layers of a PCB.

Step 1: Drilling

The first step in the PTH process is drilling holes through the PCB substrate. The holes are drilled using high-speed, computer-controlled drilling machines that ensure precise hole placement and size. The diameter of the holes depends on the specific requirements of the PCB Design, taking into account factors such as component lead size and current-carrying capacity.

Step 2: Desmearing

After drilling, the holes may contain debris and resin smear, which can interfere with the plating process. Desmearing is the process of removing this debris and preparing the hole walls for plating. There are several methods for desmearing, including:

• Plasma desmearing: This method uses a plasma generator to remove the resin smear and etch back the glass fibers, creating a rough surface that promotes adhesion of the plating material.
• Permanganate desmearing: This chemical process uses a permanganate solution to remove the resin smear and etch the glass fibers.
• Mechanical desmearing: This method involves using abrasive materials to scrub the hole walls, removing the resin smear and roughening the surface.

Step 3: Electroless Copper Deposition

Once the holes are desmeared, a thin layer of copper is deposited onto the hole walls using an electroless plating process. This process involves immersing the PCB in a copper plating solution that contains a reducing agent, which causes the copper to adhere to the hole walls without the need for an external electrical current.

The electroless copper layer serves as a conductive base for the subsequent electroplating process and ensures complete coverage of the hole walls.

Step 4: Electrolytic Copper Plating

After the electroless copper layer is deposited, the holes are further plated with copper using an electrolytic plating process. In this process, an electrical current is applied to the PCB while it is immersed in a copper plating solution. The current causes the copper ions in the solution to adhere to the hole walls, building up the thickness of the copper layer.

The thickness of the copper plating is determined by the specific requirements of the PCB, such as the current-carrying capacity and the mechanical strength needed. Typical copper plating thicknesses range from 20 to 40 microns.

Step 5: Resist Stripping and Etching

After the holes are plated, the remaining resist used to define the circuit patterns on the PCB is stripped away, revealing the copper traces on the surface of the board. The exposed copper is then etched using a chemical etching process, leaving only the desired circuit patterns and the plated through holes.

Designing for PTH

When designing a PCB that incorporates PTH, there are several key considerations to ensure optimal performance and manufacturability.

Hole Size and Placement

The size and placement of the plated through holes are critical factors in PCB design. The hole diameter must be large enough to accommodate the leads of through-hole components and to allow for sufficient copper plating thickness. However, larger holes consume more board space and can limit the routing of traces on the PCB.

Designers must also consider the placement of the holes to ensure adequate spacing between components and to avoid interference with other design elements, such as traces and vias.

Aspect Ratio

The aspect ratio of a plated through hole refers to the ratio of the hole depth to its diameter. A higher aspect ratio indicates a deeper hole relative to its diameter, which can pose challenges for the plating process.

As the aspect ratio increases, it becomes more difficult to achieve even copper plating throughout the entire depth of the hole. This can lead to thin or inconsistent plating, which can compromise the electrical and mechanical integrity of the connection.

To mitigate this issue, designers should strive to keep the aspect ratio within manageable limits, typically less than 10:1. This may involve adjusting the PCB Thickness or the hole diameter to achieve the desired aspect ratio.

Copper Thickness

The thickness of the copper plating in the holes is another critical consideration in PTH design. The copper thickness must be sufficient to carry the required electrical current and to provide mechanical strength to the connection.

The copper thickness is typically specified in terms of the minimum acceptable thickness, which is determined by the current-carrying requirements and the mechanical stress that the connection will be subjected to. Designers should work closely with the PCB fabricator to ensure that the specified copper thickness is achievable and meets the necessary performance criteria.

Thermal Management

Plated through holes can also play a role in the thermal management of a PCB. In some cases, designers may choose to use thermal vias, which are plated through holes that are strategically placed to transfer heat away from high-power components and into the PCB substrate or an external heatsink.

Thermal vias can help to dissipate heat more efficiently, preventing components from overheating and extending the lifespan of the PCB. However, the use of thermal vias requires careful design considerations, such as the placement and spacing of the vias, to ensure optimal thermal performance without compromising the electrical integrity of the circuit.

Manufacturing Considerations for PTH

In addition to design considerations, there are several manufacturing factors that can impact the quality and reliability of plated through holes in PCB fabrication.

Material Selection

The choice of PCB substrate material can have a significant impact on the PTH process. Different materials have varying thermal expansion coefficients, which can cause stress on the plated holes during the fabrication process and in the finished product.

For example, PCBs made from high-temperature materials, such as polyimide, may require special processing to ensure the stability of the plated holes during the high-temperature assembly processes, such as Reflow Soldering.

Designers and manufacturers must carefully consider the material properties and select a substrate that is compatible with the PTH process and the intended application of the PCB.

Plating Chemistry

The chemistry of the plating solutions used in the PTH process can also have a significant impact on the quality and reliability of the plated holes. The composition of the plating solutions, including the concentration of copper ions and the additives used, must be carefully controlled to ensure consistent and uniform plating.

Manufacturers must also monitor and maintain the plating solutions to ensure that they remain within the specified parameters throughout the production process. Contamination or depletion of the plating solutions can lead to poor plating quality and reduced reliability of the plated holes.

Quality Control

Rigorous quality control measures are essential to ensuring the consistency and reliability of plated through holes in PCB fabrication. Manufacturers employ a range of inspection and testing techniques to verify the quality of the plated holes, including:

• Cross-sectional analysis: This involves cutting a sample PCB and examining the cross-section of the plated holes under a microscope to verify the plating thickness and uniformity.
• Electrical testing: Resistance and continuity tests are performed to ensure that the plated holes provide the necessary electrical conductivity and connectivity between layers.
• Mechanical testing: Plated holes may be subjected to mechanical stress tests, such as pull tests or thermal cycling, to evaluate their mechanical strength and durability.

By implementing comprehensive quality control measures, manufacturers can identify and address any issues with the PTH process, ensuring that the finished PCBs meet the required performance and reliability standards.

FAQs

1. What is the difference between plated through holes (PTH) and non-plated through holes (NPTH)?

2. PTH refers to holes that are plated with a conductive material, typically copper, to create an electrical connection between the layers of a multi-layer PCB. NPTH, on the other hand, are holes that are not plated and do not provide an electrical connection. NPTH are often used for mounting purposes or as tooling holes.

3. Can plated through holes be used for surface mount components?

4. While plated through holes are primarily used for through-hole components, they can also be used in conjunction with surface mount components. In some cases, designers may use a combination of PTH and Surface Mount Technology (SMT) on the same PCB to accommodate different component types and to optimize board space utilization.

5. What is the minimum hole size that can be achieved with PTH?

6. The minimum hole size achievable with PTH depends on several factors, including the PCB thickness, the aspect ratio, and the capabilities of the PCB fabricator. In general, most PCB manufacturers can produce plated through holes with diameters as small as 0.2 mm (8 mils). However, smaller holes may be possible with advanced manufacturing techniques and specialized equipment.

7. How does the aspect ratio affect the PTH process?

8. The aspect ratio of a plated through hole, which is the ratio of the hole depth to its diameter, can have a significant impact on the PTH process. Higher aspect ratios (deeper holes relative to their diameter) can pose challenges for achieving even and consistent copper plating throughout the entire depth of the hole. To mitigate this issue, designers should strive to keep the aspect ratio within manageable limits, typically less than 10:1.

9. What are the advantages of using thermal vias in PCB design?

10. Thermal vias, which are plated through holes strategically placed to transfer heat away from high-power components, offer several advantages in PCB design. They can help to dissipate heat more efficiently, preventing components from overheating and extending the lifespan of the PCB. Thermal vias can also improve the overall thermal management of the PCB, ensuring more stable and reliable operation, especially in high-power applications.

Conclusion

Plated through hole (PTH) technology is a fundamental aspect of PCB fabrication, enabling reliable electrical connections between the layers of multi-layer PCBs and facilitating the mounting of through-hole components. The PTH process involves a series of steps, including drilling, desmearing, electroless copper deposition, electrolytic copper plating, and resist stripping and etching.

Designers must consider various factors when incorporating PTH into their PCB designs, such as hole size and placement, aspect ratio, copper thickness, and thermal management. Additionally, manufacturers must carefully control the material selection, plating chemistry, and implement rigorous quality control measures to ensure the consistency and reliability of the plated holes.

By understanding the intricacies of the PTH process and considering the design and manufacturing factors involved, engineers can create high-quality, reliable PCBs that meet the demands of today’s complex electronic systems.

	Plated Through Hole (PTH)	Non-Plated Through Hole (NPTH)
Hole Function	Electrical connection	Mounting or tooling
Plating	Copper plated	No plating
Components	Through-hole components	Not applicable
Thermal Management	Can be used for thermal vias	Not applicable