Introduction
When you’re designing a multi-layer circuit board, you require a reliable method of connecting the various layers. Plated Through Hole (PTH) is the traditional solution and the foundation of through-hole technology. This guide takes a look at the PTH process and how this is done in your plated through hole pcb. You will get to know its advantages, design parameters and ways of providing long-term reliability.
What is a Plated Through Hole (PTH)?
A Plated Through Hole (PTH) is one of the fundamental components of your printed circuit board. You will begin by drilling a hole that goes through the substrate on board. Then you plate the inside of this hole with a conducting metal, such as copper. This plating forms an electric bridge between the circular pads and between the traces on the top and bottom sides of your board.
This simple plated through hole performs two important jobs in your circuit. First, it enables the passage of an electrical current between the layers and this is a key component of through-hole technology. Second, it makes a strong mounting point in case you want to mount through-hole components and you have great mechanical strength and PTH reliability after soldering.
Benefits of Using PTHs
Implementing a Plated Through Hole (PTH) in your plated through hole pcb design has great benefits. PTHs give higher mechanical strength to parts and very good electrical contact between layers. Compared with other types of connections, this technology increases general PTH reliability and can support higher currents, thus a more powerful and versatile option.
Inter-Layer Connectivity
The main duty of a Plated Through Hole is to connect various layers of your circuit board. You can use it to connect traces and the vias on different layers together using only a vertical connection. This allows you to do more complex and tight circuit designs with much less hassle. This form of connectivity is a fundamental advantage of through-hole technology, simplifying your routing challenges.
Component Anchoring
Plated Through Holes give your through-hole components an anchor that is incredibly strong. You just place the component leads in the holes and do through-hole soldering. This forms a strong mechanical bond and one that resists vibration and stress. This leads to a better physical connection, which is one difference when comparing PTH to SMT mounting.
High Current Capacity
Another important advantage of a PTH is that it can carry high electrical currents. The cross-sectional area of the plated copper barrel is significantly larger than that of a normal trace or via. With this, it can safely conduct or deliver amperes of current without getting hot. When designing a power application, the main thing is to adhere to PTH design recommendations in order to ensure that your board can handle the current.
Mixed-Connection Options
With a PTH you have so much design flexibility as there are both sides of the board with pads. This will enable you to mount through-hole components on either side. Connections to one or both of the sides of traces may also be made as needed in your layout. The flexibility will enable you to maximize your board space.
Improved Reliability
Plated Through Hole directly increases the reliability of PTH through its robust structure. Their continuous plated barrel is much more stronger than the more delicate interconnects like blind or buried vias. This reduces the chances of your connections failing under stress. A well-designed PTH annular ring also helps in reinforcing the solder joint so that there is a long-term performance.
Ease of Assembly
The PTH process will make your manual and automated assembly much easier. In manual applications, it is easy to insert components and solder with through-hole methods. Through-holes are easy to manage as they are larger in size and in the proper position in automated systems. This simple assembly has the ability to minimize the manufacture flaws and build faster.
Plated Through Hole Process
The PTH process is a very specific series of manufacturing steps that produce your Plated Through Hole. The process converts a hole drilled in the PCB into a strong electrical connection to mechanically mount through-hole components. It is essential to follow every step properly to see a high PTH reliability on your plated through hole pcb.
- Drilling: The first step is drilling all the required holes in the laminated PCB substrate. These holes will be the basis of your Plated Through Hole.
- Cleaning and Deburring: You clean the holes that have been drilled so that they are completely clean of debris or resin that may still be inside or on them. This important step makes sure that the plating will stick correctly, which is fundamental in the reliability of the PTH.
- Electroless Copper Deposition: Then, you will add a thin, conductive coating of electroless copper to the top of the whole board. This coats the non-conductive walls inside the holes, to prepare for the next step.
- Imaging: Next, you place a photoresist film over the surface of the board. This film serves as a mask, covering off the areas that should not be plated while leaving your circuit pattern exposed.
- Electrolytic Plating: At last, you get the board into an electrolytic bath. Copper electroplates onto the exposed pattern and inside of the holes to eventually develop the Plated Through Hole to its final and strong thickness. Your plated through-hole PCB can now be assembled with the components.
Electroplating
The crucial part of PTH process is electroplating where you add copper thickness. You connect your plated through hole pcb to a negative charge and immersed in a chemical bath. The surfaces on the board are plated with the dissolved copper, including the inside of the holes. This very precise process gives a uniform plating, forming the strong barrel of the Plated Through Hole which is critical to the reliability of PTH.
Challenges of Plated Through Holes
Although Plated Through Hole technology is reliable, you may encounter difficulties through manufacturing. These will affect the performance and the reliability of PTH on your board. Knowledge about each of these possible difficulties allows you to have successful PTH process. Your best protection against defects is proper design and process control.
Lack of Copper in PTH
The main issue is an inconsistent plating in Plated Through Hole. By using too little copper, you leave voids that block the flow of current and damage reliability. On the other hand, excess copper may block the hole, and thus insertion of components is impossible. You have to manage this variable with care in the process of PTH. A good board requires consistent thickness of copper.
Plughole
A PTH may be unusable due to debris blocking it and thus unable to mount through-hole components. It occurs when particles of drilling or plating are not completely removed from the hole. A plugged hole does not allow the insertion of components and cannot be soldered reliably through the hole. This is why there are such strict cleaning and filtration around the production process. All Plated Through Holes should be absolutely clear.
Lamination and Blistering on a Board
On your plated through hole pcb, blistering or delamination may take place. This flaw is usually caused by contamination of the surface, such as oil or adhesive residue. Chemical incompatibility between your solder mask and the other board materials also can be a possible cause. This is avoidable by making sure that there is perfect cleanliness during the PTH process. The choice of materials is also important in achieving long-term reliability of PTH.
Risks of Hole Plating
Although a Plated Through Hole is a trustworthy interconnect, you need to think over the risks coming with the thermal stress.
As soon as you power your device, the plated through-hole PCB will get hot, and as you turn it off, it will cool. This process adversely affects the copper in your Plated Through Hole. With time, such stress may result in fatigue and cracking of the copper, resulting in an open circuit and failure of the product. The most important element in this prevention is the thickness of your copper plating. To control this risk, the IPC standard defines three classes according to the thickness of the plating:
- IPC Class 1: This applies to products that have a shorter lifespan such as the disposable consumer electronics. It must have an average of at least 20 microns of copper thickness.
- IPC Class 2: This is the standard of most commercial electronics, including laptops and servers. This Plated Through Hole category must also have an average thickness of 20-microns although this category has tighter inspection requirements in continuous use.
- IPC Class 3: You specify this class when you want a high reliability or mission-critical product, such as aerospace applications or medical equipment applications. It requires a thicker and stronger plating, averaging at least 25 microns, to guarantee optimum thickness.
Plated Through Hole Preparation & Plating Preprocessing
Alkaline Cleaning
The steps start with alkaline cleaning so that everything in the holes and all the contaminants on the surface of the board are eliminated. This procedure removes oils, fingerprints, and any other contaminants which will contaminate plating. Then after cleaning, you correct the surface charge in the hole. This preparation will make the wall of the hole uniformly attract the catalyst later, which is the core to good PTH reliability.
Micro Etching
The next is carrying out a micro-etch to prepare the copper faces to be bonded. This can eliminate any surface oxidation, and lightly roughens the copper foil and the exposed base copper in the hole. This rough surface allows a very high mechanical bond to the new copper plating. It will make sure that the plating on your Plated Through Hole will not separate or peel away overtime.
Prepreg
The adhesive used to assemble your multilayer PCB is called prepreg. It is a resin impregnated fiberglass cloth. In the lamination process, you apply heat and pressure to bond together the individual layers and copper cores into one solid board. This leaves a solid base on which your Plated Through Hole will later be drilled. The whole process of PTH is dependent on a void-free lamination.
Activation
Activation is where you use the catalyst, which enables electroless copper plating. You place the board in a solution full of colloidal palladium particles. These particles attach themselves to the treated surfaces of the hole, forming a seed layer. It is this palladium nucleus that draws the copper ions in the plating bath and sets the process of creating the conductive Plate-Through Hole into motion.
Decontamination
The last part of the chemical preparation is decontamination, which is also known as acceleration. You have to remove the tin ions which protect the palladium particles. To release the active palladium nucleus you dissolve the tin in a decontamination bath, usually with an acid. This “accelerates” the plating reaction, which serves to have the electroless copper deposit evenly and quickly throughout the internal surface of the hole.
Test Methods to Validate PTH Reliability
Once you have manufactured the boards, you need to have the board quality checked. PTH reliability can be verified with the help of a few industry-standard test techniques. These are designed to create intense operating conditions to test any possible weakness in the Plated through-hole structure. The correct method of choice guarantees that your plated through hole PCB expresses excellence and performance that lives up to the task, as well as ensuring long-lasting use.
- Modeling and Simulation: Prior to manufacturing, you can model and interpret how your thermal and mechanical stress would behave within your PTH using software. It gives an indication of where things may fail, so you can refine your PTH design recommendations at the earliest stages.
- Cross-Sectioning: A destructive test that involves the cutting of a Plated Through Hole to view its internal structure when viewed under a microscope. Then you may solder and shock test the sample to inspect it for barrel cracks or any other forms of defects.
- Thermal Shock Testing: You expose the board to very fast and drastic temperature changes. The test will be used to accelerate the aging process of determining how the PTH will work over time in response to thermal pressure, and this aspect is extremely important in terms of long-term PTH reliability.
- Interconnect Stress Testing (IST): An IST is a test where you apply current to the board, heating the interconnects internally very fast. Changes in resistance in such cycles can be monitored to give an immediate indication of fatigue failure in the Plated Through Hole.
- PCQR2 Testing: This is a standard IPC program that you employ to benchmark the process capability of a manufacturer. It offers a standard test coupon and regimen, providing you with a dependable method to compare the PTH reliability and quality of the PTH process among various suppliers.
- Highly Accelerated Thermal Shock (HATS): HATS is a more accelerated and more aggressive test of thermal shock. You use it to perform quick process monitoring so that you can tell quickly of manufacturing faults that may compromise the integrity of your Plated Through Hole.
FAQs
Why Do Plated Through Holes Fail?
A Plated Through Hole often fails by cracking of the copper barrel, a condition called barrel fatigue. This is because the PCB material expands and contracts as the heat changes compared to the copper plating which expands and contracts to a much lower degree. When this stress occurs after numerous thermal cycles it causes the copper to crack and electrical connection to break thus affecting PTH reliability. A PTH is quite a lot more vulnerable to such a failure when the copper plating thickness simply does not suffice.
What Are the Four Methods of Plating?
The four main techniques are:
- Barrel Plating
- Rack Electroplating
- Selective Plating
- Vibratory Barrel Plating
How Are PTH Holes Aligned Between Layers During PCB Fabrication?
In the PTH process, you ensure that there is uniform alignment of layers with the use of registration marks printed on individual inner layers of the board. These are marks which are positioned very accurately with the help of an optical system so that the drill will hit the same point at every layer. This exact position is important when forming a functional Plated Through Hole that will properly connect all the various layers of your PCB.
What Is Tenting and When Is It Used?
Tenting is where a via is covered by a solder mask. You apply this method in protecting the conductive barrel against exposure and in ensuring that solder does not wick into the hole in the assembly process. You normally test via those that are not used as test points. Such a practice is often observed in PTH design guidelines in order to enhance the reliability and manufacturability of your board.
How Do PTHs Affect Characteristic Impedance for Signals?
The impedance of high-speed signal path can be interrupted by a Plated Through Hole. The electrical environment is different between barrel and pad as compared to a uniform trace, and induces parasitic capacitance and inductance. Such a discontinuity may lead to signal reflection, and reduced performance. This effect should always be simulated and you should not route critical high-speed signals through a PTH whenever possible.
Conclusion
The Plated Through Hole is not merely a hole–it is the basic element of modern electronics. It is critical to understand PTH process in order to provide high PTH reliability and successful designs. Through-hole technology can be used to make quality and well-designed circuit boards to serve your any application.
Whenever you require a manufacturing partner with profound experience in plated through-hole PCB manufacturing, think of MV Flex Circuit. With years of experience in precision PTH plating, we excel at handling various complex circuit boards. From microvias to heavy-copper through-holes, we ensure uniform plating layers that meet strict IPC standards. Every PTH circuit board undergoes rigorous AOI and electrical testing to meet our strict product quality requirements. Send us your Gerber files today, and get your free DFM check and quote.
