Flexible PCB Through Holes: The Ultimate Guide

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Oakley Mae

What are flexible PCB through holes, and what roles do they play in circuit boards?

At least every technology enthusiast or potential future PCB designer should answer these questions.

But even if you can’t, we got you covered. 

We can all agree that circuit boards are covered with limitless holes.

These holes offer different functions, with the larger one providing a surface for mounting components.

However, small holes offer inter-layer signal connectivity.

These holes are called through holes; without them, the circuit board is as good as dead. 

Read more about flexible PCB through holes, including design guidelines and considerations.

Let’s get started. 

Table of Contents

Defining Through-Hole Technology

Electric green PCB through-hole

(Electric green PCB through-hole )

Through-hole technology involves constructing circuits that insert pin-through hole components into holes you drill on the board.

You can then affix pads to the ends or leads using molten metal solder with either reflow or wave soldering equipment. 

You’ll hear people referring to it as a through-hole assembly background, and it replaced traditional techniques like point-to-point construction. 

Advantages of Through-Hole

  • Through-hole technology creates a stronger bond between the circuit and board than surface mount technology (SMT). Therefore, through-holes are perfect for designing devices that you expect will undergo environmental stress. 
  • Also, you’ll find through-hole components easy to swap out, thus perfect for testing and prototypes. 

Plated Through-Hole in Flexible PCB Process

A component being inserted into PCB holes

(A component being inserted into PCB holes)

Plating through holes involves adding conductive copper in the holes to enable the connection between layers.

You can only initiate the plating process after you drill the holes and composite the PCB together using either pressure or heat.

Below is the step-by-step process: 

  1. First, clean the through holes to remove residue. The common residues include contaminants like burrs and residue resins. Therefore, employ abrasive processes and strong chemical agents for this task. 
  2. Second, you chemically coat a thin copper layer via the through-holes and on the board surface. Surprisingly, this electroless deposition will offer a strong base for sticking the plating. 
  3. Thirdly, you can micro-etch the inside of the through-holes. This will further improve the base for the plating to anchor. 
  4. Fourthly, you create circuitry on the outer layers by exposing an image on the photoresist covering. Repeat the same process for the interior layers, only that you employ a reverse image this time. 
  5. At this point, you remove the soft and unexposed material using chemicals. This will expose the copper circuitry. Don’t worry about the circuit board as it gets protected by the hardened resist. 
  6. Finally, you use copper for plating the exposed circuitry and the drilled holes. The plating process is complete now, and you can use the board to design electronics

Flexible PCB Through Hole Design Guidelines

Holes featuring on a blue circuit board

(Holes featuring on a blue circuit board)

You should not expect this to be the last time you hear about circuit boards through holes. If anything, they’re around to stay for a long time.

Therefore, the best you can do is to understand their design guidelines, and we have highlighted them below: 

Soldering

In most cases, you assemble through holes in the circuit using wave soldering.

You then pass the board through a molten solder wave and force it into the through holes, and the inserted component leads to offer a reliable solder joint. 

However, if you plan to apply surface mount parts on the PCB simultaneously, you’ll have to block them off from the wave.

Alternatively, you can opt for manual soldering.

Generally, surface mount parts don’t go through the above process of through-holes.

Therefore, always communicate with the manufacturer to avoid creating such bottlenecks. 

Spacing

To handle automated soldering systems effectively, you have no option but to space the components well.

Always note that creating large holes sometimes overshadows smaller SMT parts and prevents effective soldering.

Also, allowing enough space between parts and components gives enough room for manual reworks. 

Footprints

At all times, stick to the footprint dimensions the manufacturer recommends.

This way, the circuit board can be assembled without problems.

However, you can use some parts, such as through-hole resistors, in different-spaced holes. 

Hole Size

There is the recommended through-hole size, which you’re supposed to stick to.

If you ignore this and design large through-holes, they won’t capture molten solder and might translate into poor solder joints.

Also, smaller through-holes will not allow you to insert the component leads. 

Through-Hole Components

Larger through-hole technology resistors

(Larger through-hole technology resistors)

We have the radial and axial lead through-hole components, which are ‘twin’ lead components but with contrasting benefits.

Let’s consider the below: 

Axial Lead Components

They run through a component along the axis in a straight line, with the lead wire end exiting the component on both ends.

You then place the end into two different holes on the circuit board to give the component a closer fit. 

We recommend axial lead components if you’re looking for a tight and compact fit.

Generally, they are available as light-emitting diodes, carbon resistors, fuses, and electrolytic capacitors. 

Radial Lead Components

These will protrude from the circuit board and have leads on a single component side.

Surprisingly, they occupy less surface area, thus ideal for high-density flex board designs. In most cases, you’ll find them as ceramic disk capacitors. 

Why Are Through-Hole Components Necessary?

Although SMT parts dominate the industry over through-hole components, the latter is still necessary for the following reasons: 

Strength

Flexible PCBs are designed to offer high flexibility but maintain great strength.

Surprisingly, through-holes give the PCB more strength, thus making it possible to connect and interface the different parts and components.

This gives through-holes an edge over SMT which is not strong enough to hold connectors in place when you apply a strong force. 

Power

Generally, through-holes are designed for high currents.

Moreover, they have high thermal and mechanical stability.

Therefore, you can comfortably use them in power applications and still solder them easily without worrying about heating the metal. 

Heat

Plated through-hole components are better conductors of heat than SMT options.

Therefore, you can employ them in high-temperature parts and applications.

Surprisingly, you can bolt them on your circuit to offer extra thermal dissipation via the ground plane. 

FAQ

What are the disadvantages of through holes?

Generally, through-holes have the following drawbacks:

  • First, you must pre-drill holes which is a time-consuming and expensive task. Also, pre-drilling limits the routing area and the components on a single side of the PCB. 
  • One of the main reasons manufacturers prefer SMT to through-holes is that the latter is very expensive.
  • The through-hole soldering exercise makes the solder joints unreliable. 

Is plated through-hole expensive to design?

As we mentioned, plating through holes involves adding conductive copper into the holes.

This is an extra expense that makes the entire process expensive.

However, they take up less space than the non-plated options. 

Is SMT more expensive than through-hole?

The two fundamental technologies in flexible circuit boards are surface mount and through-hole.

However, the surface mount is more common than through-hole thanks to its low cost and high reliability. 

Generally, placing SMT components on the circuit board is faster than through-hole components.

Therefore, they achieve more manufacturing capacity.

But we consider through-hole to offer better performance and can withstand environmental stress. 

Conclusion

As we highlighted, plated through-hole technology involves constructing circuit boards by inserting pin-through hole components in holes you drill on the board.

Surprisingly, this technology isn’t as popular as surface mount technology but offers better performance.

For example, it withstands environmental stress more than SMT. 

However, you’ll find through-hole technology more expensive.

Before you decide to use the technology, discuss it with your manufacturer.