Plated through hole knowledge collection

Electronic devices have been designed with a board known as PCBA which holds all the electronic components together to complete the conductivity circuit. Manufacturing the circuit board primarily is made up of an etching process where a copper sheet is plated into a non-conductive substrate material and holes drilled onto the end product board. The purpose of the drilled hole is to ensure conduction between the board’s copper layers and the components that are fixed in the board. Remember that there exist different types of circuit boards with different layers, but the simplest form of the board that utilizes the through hole assembly technology only has a top and a bottom layer. The drilled holes will never be conducive until an advanced process known as electroplating that triggers the conductivity is utilized. This advanced process involves the application of a thin copper layer on the walls of the drilled holes. The electroplating of the drilled holes is what leads us to what we refer to as plated through holes (PTH). This article will focus on the PTH PCBA and how we can compare it to the NPTH PCBA. Let us get started!

The difference between PTH and NPTH

In PCBA, the definitions of the two are different, PTH stands for plated through hole while NPTH stands for non-plated through hole.

PTH is where the drilled hole has a solder stop mask and copper pad designed to be larger than the diameter of the hole which is about 6 mil than the designed minimum width.

For an NPTH, the plated copper pads appear tinier than the drilled hole and in other cases, the copper is absent while its copper overlay on the pads exceeds the drilled hole size with a buffer zone of size 6 mil separating the slot and copper

NPTH & PTH PCB Drilling Rules

The existence of holes is to ensure the smooth progress of PCB assembly projects. For this reason, we need to consider the design and manufacturing rules of PTH and NPTH, which will directly affect the assembly quality. The following are the drilling rules of the two listed by FS Technology:

Non-Plated Through Hole Drilling Rules:

Minimum complete size of the hole should be 6 mil;
The minimum hole edge-to-edge clearance should be 5 mils from the total placed surface elements.

Plated Through Hole Drilling Rules:

The minimum size of the annular ring should be 4 mil
The minimum size of the complete hole should be 6 mil
The minimum hole edge-to-edge clearance should be 9 mils from the total placed surface elements.

In general, you should consider the size of the drilled hole, the type of vias, the spacing, and the aspect ratio during the process of drill hole DFM regulations and guideline development. For example, if your intention to set the DFM rules is to save space, then you will have to make use of the via-in-pads.

The difference between PHT and NPHT in the manufacturing process

Apart from the difference noted in the definition of the two, we can differentiate the two through-hole processes based on the PCB manufacturing process. The PTH hole drilling occurs before the process of electroless copper while for the NPTH, the hole drilling process occurs after the board has undergone the process.

Defining the two is a very straightforward process, but implementing them in a PCB assembly process is what proves to be a bit difficult. For example, if a PCBA manufacturer receives a file that has a copper overlay pad wider than the drilled hole but with the absence of the solder stop, the mask appears confusing. The manufacturer is tied between plating or not plating it. Therefore, it is a requirement that every PCB designer pays adequate attention to designed Gerber files before submitting them to the fabrication house.

From the feedback from most PCBA manufacturers, if all the drilled holes have solder-stop masks overlay and are larger than the drilled hole then:

Those that are connected to the copper traces and have broader copper pads than the drilled holes qualify to be labeled as plated through hole;
Those without copper are labeled as Non-plated through hole.

PTH/ NPTH Typical Applications

These two through-hole technologies are widely used in PCB assembly. PTH metal holes have two uses depending on their size. The larger aperture is used to solder DIP components, and the smaller aperture is called “via hole”, which is used to connect copper foil lines between PCB layers. The NPTH PCB may be used as a screw hole because there is no copper in the hole. The following are typical applications of PTH/NPTH:

Voltage Isolation Air gaps for milling slots; This is because they can be easily replaced when electrical arcs that burn out components occur.
Rectangular components connection; such components include DC jacks which require a reliable connection joint.

Plated Through Hole Assembly Process

FS Technology inserts the leads of DIP components into the corresponding PCB plated through holes through automated equipment:

Diodes & Resistors Axial Insertion Process
Electrolytic Capacitors radial insertion process
Odd form insertion process for connectors, transformers, etc.
Manual Process used for many other electronic components

After completing the insertion of components process, the soldering of the placed through-hole components is done by use of the following soldering processes and techniques:

Manual Soldering;
Intrusive reflow soldering or wave soldering;
Selective Soldering.

Although SMT assembly is more in line with the needs of miniaturized electronic devices, THT assembly soldering can provide the advantage of stronger joints. This is simply because the leads of the electronic components are placed through the PCB.

Plated Through Hole PCB Assembly Failure

Through PCBA failure analysis, we know that assembly is a complex and subtle process, and any mistakes in this process will lead to assembly failure. The following are the failures that may be encountered during the PHT assembly process of the PCBA factory:

Barrel cracking
Interconnection defects
Thermal excursion stress
Foil cracks

From the list above the biggest failure problem that is experienced in the plated through hole assembly is the interconnection defects (ICD). This occurs dues to mechanical stress that originates from the difference in the thermal expansion coefficient and the system of the PTH. The glass reinforcement of the manufacturing system limits the material expansion along the normal X and Y-axis hence allowing the material to expand faster along the remaining unrestricted Z-axis. The expansion movement creates adhesive stress between the interconnections and plated copper which can lead to separation. This same stress is what leads to other issues that are experienced in the PTH PCB assembly.

To solve or prevent such failures, first, you need to adopt the best copper plating processes and practices and secondly, you have to try adapting the solutions listed below:

Ensure that the electroless copper thickness is of equal size on the interconnected plated copper.
Ensure you have a sturdy deposit of copper structure grain.
The choice of the resin material should factor in the ability of the material to withstand thermal strains without losing its weight.
Ensure that you carry out the high aspect-ratio test by subjecting the fabricated boards to repeated shocks in the process of prototype PCB assembly to have the knowledge of the range of temperature at which the board failure occurs.

PTH Production Advantages

Prototyping process especially for the breadboards
Ideal for small batch PCB assembly projects
Used when a sturdy or stronger attachment is needed such as transformer attachments.
Sturdy and strong mechanical bonds as compared to SMT assembly.
Easy to repair and replace faulty electronic components using hand instead of whole circuit replacement.
The process is very useful when it comes to the use of bulky components in the circuit.

Summarize

Plated through hole involves drilling and electroplating holes on the PCB before the connection of the components;
PTH PCB has broader copper pads while NPTH has no copper pads;
The DIP components are placed on the board using either the axial insertion process, radial insertion process, or manual process;
PTH and NPHT components are soldered into the board by use of either hand soldering, wave soldering, or selective soldering;
Non-plated through hole has a hole of minimum size 6 mil while the edge-to-edge clearance should be 5 mils.
Plated through hole has an annular ring size of 6 mils, edge-to-edge clearance of 9 mils, and minimum complete hole size of 6 mils.
Failures can occur during the PTH PCBA process due to barrel cracking, interconnection defects, thermal excursion, and foil cracks.
PTH and NPTH are suitable for applications that require sturdy joints, prototyping, and small production run.