Printed circuit boards (PCBs) are essential components in almost every modern electronic device. They provide the mechanical support and electrical connections between components that allow devices to function. As electronics have advanced, PCBs have become more complex in terms of component density, operating frequencies, and performance requirements. This has led to continual innovation in PCB design and manufacturing techniques.
One key PCB technology that has stood the test of time is plated through hole (PTH) technology. PTHs provide electrical connectivity between layers in multilayer PCBs and secure component leads to the board. Although declining in usage for some high density designs, PTHs continue to have unique advantages that make them the preferred interconnection method for many applications.
What are Plated Through Holes (PTH)?
A plated through hole is a conductive plated hole that passes completely through from one outer surface layer to the other in a PCB. The key characteristics of PTHs are:
- Hole drilled through the entire PCB, through all layers
- Walls plated with copper to form a conductive tube connecting layers
- Allows component leads or wires to pass through and connect between layers
- Provides mechanical fastening point for leads/wires
PTHs enable connections to be made between layers in a multilayer board easily in a single drilled hole. This makes routing easier versus having to route connections internally between all layers. The plated metal tube also gives connections good conductivity while the hole itself secures component pins from pulling out under mechanical stress.
*Plated through hole diagram (image source: researchgate.net)* </div>Now that we've introduced what PTH technology is, let's look at why PTHs are advantageous to use and where they are best utilized.
Benefits of Using Plated Through Hole Technology
Although declining somewhat in leading edge, high density designs, plated through holes still have unique characteristics that make them beneficial across a wide range of PCB applications:
1. Excellent Electrical Conductivity
A key benefit of plated through holes is that the plated copper barrel forms an excellent conduction path between layers in the PCB. The tube shape gives a large contact area and the thickness of plating applied ensures good conductivity (typical 1 oz/ft2 or greater). This allows PTHs to carry substantial currents when required compared to other interlayer connection methods:
<div style="text-align:center">| Connection Type | Typical Current Capacity |
|---|---|
| Plated Through Hole | 3-6 Amps |
| Buried Via | 0.5 Amps |
| Microvia | 0.2 Amps |
| Signal Layers | 0.2 Amps |
The continuous copper tube connection also avoids issues with alignment accuracy that can affect conductivity with other layered interconnection approaches like microvias. This reliable conductivity makes PTHs well suited for power distribution networks in PCBs requiring high current capacity traces.
2. Strong Mechanical Connection
Another major benefit of plated through holes is the strong mechanical connection they create by allowing component leads and wires to pass fully through the board. The leads can be soldered on both sides securely fastening them in place. This makes PTH mounted components highly resistant to shock/vibration stresses.
PTHs similarly provide solid anchor points for connectors mounted to boards allowing them to withstand repeated insertions and extractions without wearing out the plating. These robust mechanical connections are a key reason PTHs remain very popular for connectors in high reliability applications like industrial controls, aerospace, defense, and medical products.
3. Simplifies Routing on Multilayer Boards
On multilayer PCBs, plated through holes help simplify routing between non-adjacent layers. Rather than having to route connections internally between all layers involved, PTHs allow connections to be made through just a single drilled hole.
This avoids complex internal routing that can add layers solely to enable connectivity between non-adjacent layers. Routing connections through PTHs instead thus helps minimize overall layer counts reducing costs.
4. Well Established, Reliable Technology
Plated through hole technology has been used since the early days of multilayer PCBs in the 1960s. The fabrication processes involved such as drilling, plating, soldermask, etc. are all now highly standardized and well characterized.
This maturity makes PTHs an extremely reliable interconnection method. Manufacturing yields are typically very high using optimized PTH design rules avoiding many defects common with newer interconnect technologies. This excellent reliability makes PTHs a trusted choice for critical applications where quality and dependability are paramount.
5. Lower Cost Than Other High Density Approaches
While not as component dense as microvias or other HDI (high density interconnect) technologies, plated through holes are considerably less expensive to manufacture. PTH PCBs avoid the additional process steps of laser drilling, precise alignment, and complex lamination involved with microvias and other HDI techs that increase costs.
PTH circuit boards can also generally meet target component densities while using fewer layers than would HDI approaches lowering total board costs. The reliability and cost savings of PTHs makes them attractive for many cost sensitive, high volume consumer and industrial applications.
Now that we've covered the major advantages of plated through holes, let's look at some of the key applications where PTH technology excels.
Prime Applications for Plated Through Hole PCBs
With their unique combination of electrical performance, mechanical robustness, and fabrication reliability at moderate cost, plated through holes are the ideal interconnection choice for a wide range of PCB applications including:
Power Electronics
For PCBs distributing and controlling substantial currents and voltages, plated through holes are the perfect fit. Their high conductivity barrels allow even large cross section, high current traces to be routed between layers easily.
PTHs additionally provide solid mechanical anchoring for the screw terminals and large gauge wires used in power electronics equipment. Example applications include:
- Power supplies
- Motor drives
- Battery chargers
- Power converters
High Reliability Controls
In control systems requiring exceptional dependability like flight controls, surgical equipment, and railway signals, PTHs deliver. The field proven longevity and fabrication reliability of PTHs significantly reduce risks of internal shorts and other defects during operation.
Their vibration resistance also suits them well to the harsh operating conditions often seen in industrial control environments. Example high reliability control applications include:
- Avionics systems
- Medical instrumentation
- Railway signal equipment
- Industrial process controllers
Connector Interfaces
The unmatched mechanical integrity of plated through holes makes them ideally suited for PCB connector interfaces. They stand up well to repeated mate/demate cycles without failing or degrading conductivity over long term use.
PTHs also simplify routing of higher current traces to connectors needing them. Some examples include:
- Backplane/daughtercard connectors
- Power input connectors
- High speed data connectors (USB, Ethernet)
- Automotive connectors
Consumer Electronics
While consumer devices are driving towards microvias and HDI for increased compactness, PTH designs still prevail in cost sensitive, medium complexity devices. The dependable manufacturability and cost efficiency of PTH boards keeps them attractive for:
- Smartphones
- Tablets/Laptops
- Smart home devices
- Small appliances
- GPS/Portable navigation devices
Now that we've covered where plated through holes excel, let's look at some best practices to optimize your PTH design and maximize quality.
Design Guidelines for Optimizing Plated Through Holes
Properly designing plated through holes is critical for minimizing fabrication issues and ensuring reliable PCB performance long term. Here are some key PTH design guidelines:
1. Follow Manufacturer Design Rules
PCB fabricators provide detailed design rule documents that specify constraints on PTH placement, sizes, spacing, etc. tailored to their specific capabilities. Tightly following these rules prevents producing an unmanufacturable design and ensures acceptable yields.
Some typical design rules guide:
<div style="text-align:center">| PTH Design Parameter | Typical General Guideline |
|---|---|
| Minimum Hole Size | 0.2-0.25mm (8-10 mil) diameter |
| Minimum Annular Ring | ≥ 0.15mm (6 mil) width |
| Minimum Spacing | ≥ Hole size + 0.15mm |
| Maximum Aspect Ratio | ≤ 8-10:1 (hole depth : diameter) |
Hole to hole spacing and annular ring rules in particular help avoid shorting between holes from misregistration during drilling
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