Through Hole PCBs
Printed Circuit Boards electrically connect electronic components through conductive pathways, tracks or signal traces that are etched onto a copper-plated sheet. The PCB board itself is made of a non-conductive substrate and has several holes drilled through it. These holes are drilled with coated, solid, tungsten carbide drill bits and are prominent in diameter. The holes are made conductive by electroplating with copper.
By using PCBs “through-hole” or “thru-holes” technology, electronic components are mounted onto the substrate using the components’ legs (or leads). The leads are placed into the holes and soldered onto pads on the opposite side through manual assembly (placing by hand) or using automated insertion mount machines.
Printed circuit boards may have different sized holes, which are differentiated as PTH (Plating-Through-Hole), which has been described above, and NPTH (Non-Plating-Through-Hole), which have no coating as they are not used for conducting pathways.
PTH Vs. NPTH
PCB pads are the exposed metals on the substrate where the component lead is soldered. Multiple pads are used synchronously to create a land pattern (component footprint) on the PCB. Pads can be through-hole ones or surface mount pads. A hole is considered PTH if its pad has copper along with the solder stop mask. This should be larger than the hole/slot with a minimum of 6 mils in width and be overlaid.
Holes in which the pad’s copper size is smaller than the hole or if there is no copper at all are NPTH. Also, even if the pad’s copper is overlaid or larger than the hole, there will be a six mil space of clearance between the copper and the hole.
The manufacturing process of NPTH is simpler, hence quicker. They are most used (but not always) as Tooling/Mounting holes to fix the PCB to its operational location.
Land Patterns and Component Footprints
The most important part of printed circuit boards is their circuits that interconnect electronic components. Most of the time, components are soldered to PCBs, but soldering can only be done under conducive conditions in order to be effective. The leads of a component must have a consistent electrical and mechanical connection to the etched copper traces.
Also, each component must have a footprint or land pattern. This footprint is how the pads are arranged in through-hole technology or surface-mount technology and are used to attach and electrically connect components to the PCB physically. The component’s land pattern is the etched copper features that matches the leads of the component. These land patterns are normally made slightly larger than component leads to allow soldering space.
How can you tell the difference between PTH and NPTH?
The easiest way to distinguish between Plating-Through-Hole and Non-Plating-Through-Hole PCBs is by visually checking for traces of plating on the borehole wall in the PCB. Non-Plating-Through-Hole PCBs will not have any traces of copper in the borehole wall.
Plated-Through-Holes PCBs are more expensive than non-plated-through-hole PCB. Also, PTH printed circuit boards are often smaller than NPTH printed circuit boards.
The Use of Plated-Through-Holes in PCBs
A plated-through-hole PCB works best for the ceramic diamond copper plating, copper plating, and resin copper plating, amongst others. These PTH holes have two purposes:
- Component holes for welding DIP components.
In such cases, the hole diameter must be larger than the pin of the component so that the component can be inserted in the PTH.
Vias can be between outer and inner layers, or inner layers only, or from surface-to-surface. They connect and conduct wiring between different layers in PCB. The size of these PTH—Plating-Through-Holes will be smaller than component holes. PCB vias should ideally provide conductive paths, passing electrical signals from one layer to another to integrate each PCB layer they pass.
The Use of Non-Plated-Through-Holes in PCBs
NPTHs are simpler PCBs. Therefore the manufacturing process for these is faster. They are frequently used as Tooling/Mounting holes, to fix the PCB to its operational location. They are also used to mount components in some single-sided PCBs.
PTH Components Handling Processes
PTH or Plated-Through-Hole PCBs are ones in which the leads (or legs) of electronics components are inserted into designated ‘holes’ in the printed circuit board. These are soldered into place using a number of soldering techniques such as using a hand solder, selective soldering, wave solder, or intrusive reflow process. This entire process is called conventional assembly. Soldering forms very strong joints as the component are soldered completely through the PCB and not just onto the surface of the PCB like Surface Mount Technology (SMT) components.
The components inserted in a PTH PCB are done through these automated processes:
- radial insertion (electrolytic capacitors),
- axial insertion (resistors and diodes),
- odd form insertion (connectors, transformers, etc.) or
- manually (by hand.)
Drilling Rules for PTH and NPTH PCBs
Some general rules should be applied to the NPTHs and PTHs that you are designing and manufacturing. These rules can significantly impact the turnaround time for your board.
Non-Plated-Through-Hole (NPTH) Drilling Hole Rules
- Minimum edge to edge clearance (from any other surface element) = 0.005″
- Minimum finished hole size = 0.006″
Plated-Through-Hole (PTH) Drilling Hole Rules
- Minimum edge to edge clearance (from any other surface element) = 0.009″
- Minimum finished hole size = 0.006″
- Minimum annular ring size = 0.004”
When considering simple PCB Design For Manufacturing (DFM) rules and guidelines for drill holes, consider drill hole sizes, spacing and aspect ratios, and via types. If your DFM purpose is space-saving, use via-in-pads. Alternatively if DFM requires faster turnaround time, explore alternatives via options. You are employing via-in-pad increases the PCB manufacturing process by two days.
Annular ring dimensions are important for through-hole designs. The annular ring is the copper area extending past drilled holes on PCBs. If the PCB annular ring is too small, the copper around the hole may easily detach when heated during repair or when subjected to mechanical stress. The recommended annular ring value for any NPTH hole is 0.30mm (12mil)
The Reason NPTH Use Declined
Non-Plating-Through-Hole PCBs have simple features that are either drilled or milled through your circuit board. Since components do not have copper-plating of conductive copper, the hole has no electrical properties. NPTHs were popular when PCBs had copper traces printed on only one side, but waned as multilayer PCBs became common. Now they are mostly used as mounting holes to allow screws or other fasteners to pass through PCBs.
Why Surface Mount Technology—SMT is now Popular
Now SMT Surface Mount Technologies have almost completely replaced through-holes because they reduce the production cost, increases automation, and improve quality.
The mounted electrical component of an SMT is called a surface-mount device (SMD). They are popular as SMT capacitors and SMT resistors. Since SMDs can be made very small, SMT can hold many more components on a given area of the substrate.
Advantages of PTH Production
The mechanical bonds connecting components to the circuit board are much stronger in PTH assembly than those produced by surface-mount methods. Faulty connections can be repaired individually by hand rather than by replacing an entire circuit board. However, PTH PCBs are more expensive.
PTH assembly is used for bulky components, such as large package semiconductors that need high mounting strength to withstand high stresses or in electrolytic capacitors, such as in electronic modules, power supplies, and the assembly of LED applications.
Since investors demand less equipment, lower costs, fewer manufacturing processes, and higher profit yields, process and design engineers have evolved a mixed technology that uses through-hole and surface mount methods together in a two-phase process.
Cost-Effective Ways to Lower Manufacturing Costs for Plated-Through-Hole PCBs
Annular rings, hole density, and hole design/size all affect your PCB cost. Here are tips to decrease costs.
Larger Annular Rings:
Minimum annular ring size is defined as the border of the hole’s and the pad’s minimum distance for hole pads and vias. PCB costs increase if the minimum annular ring is less than standard requirements.
Lower Hole Density Per Square Meter:
The machines for mechanical and laser drilling each have their own efficiency. Higher hole density per square meter, take longer to complete the drilling process during PCB mass production runs; it is recommended to decrease hole density per square meter if your PCB design supports it, as it lowers costs.
Drilling small holes requires high-precision machines, which cost more. PCB fabricators upcharge for holes between 0.15mm and 0.3mm in diameter, so it is better to use larger diameter holes and annular rings.
Holes can be either round or oblong. Round through-hole pads or vias annular rings are uniform around the hole, while dimensions of oblong-shaped through-hole are non-uniform. Though the shape doesn’t matter, for both, larger annular rings cost less and increase PCB quality and efficiency.
If you need more information on through-hole technology and PTH assembly, or to determine PTH and NPTH manufacturing costs, contact us to learn more. We are happy to answer your queries.