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JingHongYi PCB (HK) Co., Limited

Through Hole Technology Assembly
Through Hole technology is a method for constructing electronic circuits in which the pin-through hole (PTH) components are inserted through holes drilled into printed circuit boards (PCBs). The ends, or leads, are then affixed to pads on the opposite side with molten metal solder using wave soldering or reflow soldering equipment. This process is also called through hole assembly, THT Assembly for short.

Through Hole technology replaced early electronics assembly techniques such as point-to-point construction. From the second generation of computers in the 1950s until surface-mount technology became popular in the late 1980s, every component on a typical PCB was a through hole component.

Automated and Manual Through-Hole Assembly

We use automated through-hole placement techniques for radial and axial components, preferably for small-volume production. The layout of the auto-placement area is intended to reduce material handling and labor. The manual placement technique is used by our engineers to deal with compound through-hole assembly challenges. Wave-soldering is used for both Surface mount and Through-hole components, but we prefer to use it for simple through-hole components. In mixed technology boards, the first board is wave-soldered and then it is reflowed.

Through-hole mounting of components provides strong mechanical bonds when compared to the surface-mount technique, and gives extra space for the designer to route the tracks. However, surface mount components can allow for denser circuit configurations and utilize both sides of the board. Through-hole mounting is necessary for larger and heavier components as they require stronger bonds for a robust connection. Our dedicated team procures our components through authorized North American distributors to ensure authentic high-quality parts. Easy availability of all requisite components enables us to provide fast manual, automated and combination through-hole assembly.

Through-Hole Mounting (THM):

Through-hole mounting is the process by which component leads are placed into drilled holes on a bare PCB. The process was standard practice until the rise of surface mount technology (SMT) in the 1980s, at which time it was expected to completely phase out through-hole. Yet, despite a severe drop in popularity over the years, through-hole technology has proven resilient in the age of SMT, offering a number of advantages and niche applications: namely, reliability.

Through-hole components are best used for high-reliability products that require stronger connections between layers. Whereas SMT components are secured only by solder on the surface of the board, through-hole component leads run through the board, allowing the components to withstand more environmental stress. This is why through-hole technology is commonly used in military and aerospace products that may experience extreme accelerations, collisions, or high temperatures. Through-hole technology is also useful in test and prototyping applications that sometimes require manual adjustments and replacements.

Overall, through-hole`s complete disappearance from PCB assembly is a wide misconception. Barring the above uses for through-hole technology, one should always keep in mind the factors of availability and cost. Not all components are available as SMD packages, and some through-hole components are less expensive.

However, that doesn`t negate that fact that, in a modern assembly facility, through-hole is considered a secondary operation.

Axial vs. Radial Lead Components

There are two types of through-hole components: axial and radial lead components. Axial leads run through a component in a straight line ("axially"), with each end of the lead wire exiting the component on either end. Both ends are then placed through two separate holes in the board, allowing the component to fit closer, flatter fit. Radial lead components, on the other hand, protrude from the board, as its leads are located on one side of the component.

Both through-hole component types are "twin" lead components, and both have their distinct advantages. While axial lead components are used for their snugness to the board, radial leads occupy less surface area, making them better for high density boards. Generally, axial lead configuration may come in the form of carbon resistors, electrolytic capacitors, fuses, and light-emitting diodes (LEDs). Radial lead components are available as ceramic disk capacitors.

Advantages: THM provides stronger mechanical bonds than SMT, making through-hole ideal for components that might undergo mechanical stress, such as connectors or transformers. Good for test and prototyping.

Disadvantages: On the bare PCB side, THM requires the drilling holes, which is expensive and time consuming. THM also limits the available routing area on any multilayer boards, because the drilled holes must pass through all the PCB`s layers. On the assembly side, component placement rates for THM are a fraction of surface mount placement rates, making THM prohibitively expensive. Further, THM requires the use of wave, selective, or hand-soldering techniques, which are much less reliable and repeatable than reflow ovens used for surface mount. Most of all, through-hole technology requires soldering on both sides of the board, as opposed to surface-mounts, which only -- for the most part -- require attention to one side of the board.

Surface Mount Technology (SMT):

SMT the process by which components are mounted directly onto the surface of the PCB. Known originally as [planar mounting," the method was developed in the 1960s and has grown increasingly popular since the 1980s. Nowadays, virtually all electronic hardware is manufactured using SMT. It has become essential to PCB design and manufacturing, having improved the quality and performance of PCBs overall, and has reduced the costs of processing and handling greatly.

The key differences between SMT and through-hole mounting are (a) SMT does not require holes to be drilled through a PCB, (b) SMT components are much smaller, and (c) SMT components can be mounted on both side of the board. The ability to fit a high number of small components on a PCB has allowed for much denser, higher performing, and smaller PCBs.

Through-hole component leads, which run through the board and connect a board`s layers, have been replaced by "vias" -- small components which allow a conductive connection between the different layers of a PCB, and which essentially act as through-hole leads. Some surface mount components like BGAs are higher performing components with shorter leads and more interconnection pins that allow for higher speeds.


There are perhaps too many terms that describe different aspects of surface mount technology. Here`s what they mean:

  • SMA (surface-mount assembly) – a build or module assembled using SMT.
  • SMC (surface-mount components) – components for SMT.
  • SMD (surface-mount devices) – active, passive, and electromechanical components.
  • SME (surface-mount equipment) – machines used for SMT.
  • SMP (surface mount packages) – SMD case forms.
  • SMT (surface-technology) – the act and method of assembling and mounting electronic technology.
  • Common of Surface Mount Devices (SMDs)

The taxonomy of surface mount devices (SMDs) is so expansive and ever-changing that covering it in full would be impossible. But here are several types that are very common and very important to know.

MELF (Metal Electrode Face Bonded): Consisting of two terminals bonded to a cylindrical body, these SMD components are less expensive than flat chips but require special handing during assembly. Furthermore, one of their biggest disadvantages is their tendency to roll off solder pads during assembly. Generally speaking, they come in the form of diodes, resistors, and capacitors.

SOT Transistors and Diodes: These are usually rectangular and easy to place, though they're a bit outdated. The most common SOTs are SOT 23, SOT 89, SOT 143, and SOT 223. Its most common packaging is tape & reel.

Integrated Circuits (ICs):

Small outline Integrated Circuit (SOIC) – These are good SMT alternatives to the duel in-line package (DIP), due to their dramatically reduced size. In general, they take up 30 – 50% less space and 70% less thickness than an average DIP.

Thin Small Outline Package (TSOP) – TSOPs are low profile packages with fine-pitch leads. TSOPs are typically meant to accommodate large silicon chips in high density packages (RAM or flash memory ICs), largely because of their low volume/high pin count.

Quad Flat Pack (QFN) – QFNs are high lead count packages (44 – 304). Its leads are typically gull wing. There are many kinds of QFNs, and they are one of the most common surface-mount ICs.

Plastic Leaded Chip Carrier (PLCC) - Connections are made on all four edges of a square package with a relatively high pin count. PLCCs can have roughly 18 – 100 leads (usually J-leads). Many of them can fit into IC sockets and can be easily replaced in the field. PLCCs have long been a popular option.

Lead-less Chip Carrier (LCC) – Not to be confused with PLCC, LCCs have no leads. Rather, LCCs are soldered directly onto PCBs by their (castellation) solder pads. These are usually designed for Mil Spec because, with no leads to damage, they're quite "rugged." LCCs are great for high temperature and aerospace applications.

Pin Grid Array (PGA) – PGAs are typically square or rectangular, with pins arranged underneath the package. They're design was highly influential on the now ubiquitous BGA.

Flip Chip – Flip chips are bare die packages, with small bottom-side solder bumps that act as leads. They are soldered directly onto the PCB.

Ball Grid Array (BGA) – BGAs are perhaps one of the best performing SMT packages in use today, due to their high densities. The BGA is a descendent of the PGA, yet instead of pins, it has solder balls that can be placed directly onto the PCB. Because of their high density, BGAs are typically used to house microprocessors.

Advantages: SMT allows for smaller PCB size, higher component density, and more real estate to work with. Because fewer drilling holes are required, SMT allows for lower cost and faster production time. During assembly, SMT components can be placed at rates of thousands-even tens of thousands-of placements per hour, versus less than a thousand for THM. Solder joint formation is much more reliable and repeatable using programmed reflow ovens versus through techniques. SMT has proven to be more stable and better performing in shake and vibration conditions.

Disadvantages: SMT can be unreliable when used as the sole attachment method for components subject to mechanical stress (i.e. external devices that are frequently attached or detached).

Overall, surface mounting will almost always prove more efficient and cost-effective than through-hole mounting. It is used in more than 90 percent of PCBAs today. However, special mechanical, electrical, and thermal considerations will continue to require THM, keeping it relevant well into the future.

The benefits of through-hole PCB assembly

The benefits of through-hole PCB assembly include:

  • Stronger physical connections - THT component leads can withstand more environmental stress because they run through the board rather than simply being secured to the board`s surface, as with SMT components.

  • Easier prototyping - In addition to being more reliable, through-hole components can easily be swapped out.

  • Higher heat tolerance - Combined with their durability in extreme accelerations and collisions, high heat tolerance makes THT the preferred process for military and aerospace products.

  • Better power handling capability - Through-hole soldering creates a stronger bond between components and the board, making it perfect for larger components that will undergo high power, high voltage, and mechanical stress, including transformers, semi-conductors, connectors and electrolytic capacitors.

Through-hole technology can be seen everyday in the LED lights on billboards and in stadiums. Through-hole LEDs are extremely durable and bright, making them perfectly suited to handle the elements. Industrial machinery and equipment are often home to THT boards. The harsh conditions these machines are exposed to require the physical endurance and strength that the through-hole PCB assembly process offers.

Drawbacks of the Through-Hole Assembly Process

THT has higher production costs (due to the board drilling that is required) and the more involved assembly process leads to longer production times. Leads being fed through the board means only one side of the PCB is useable and THT boards thus tend to have slower operating speeds. However, the strength and reliability that THT offers can mean the difference between a successful product and a failed one.

JHYPCB: Through-Hole PCB Assembly Experts

For over 8 years, JHYPCB has been offering, surface mount, mixed, and through-hole PCB assembly services. With a reputation for quality and satisfaction, Telan is your #1 choice for PCB assembly.

Our diligent staff ensures there is no compromise on quality since they are trained to follow all the inspection steps for low volume and prototype through-hole PCB assembly. We have a quality assurance team to monitor, inspect the processes and support continual process improvement. We currently employ numerous advanced test equipment including, AOI Testing, Visual inspection, In-Circuit test, and optional functional tests to produce high-quality and cost-effective assembly.

PCB Through Hole Assembly Video

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