Types of PCB Assembly Processes

PCB Assembly Process


The manufacturing process of an assembled PCB is known as PCB assembly process or PCBA. In this process, the required electronics components are mounted on the PCB.

The copper lines on bare board, known as traces, electrically link connectors and components to each other.

These traces run signals between these features, allowing the circuit board to function in a specifically designed way.

The PCB assembly process consists of several automated and manual steps. With each step of the process, a board manufacturer has both manual and automated options from which to choose.

In this article, we are going to discuss the three basic technology of PCB assembly process. Such as :

  • Thru-Hole Technology (THT) Assembly Process
  • Surface Mount Technology (SMT) Assembly Process
  • Mixed Technology

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Through-Hole Technology (THT) Assembly Process

Through Hole Technology (THT) 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.

through-hole Assembly

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.

The three key steps of Through Hole Technology (THT) assembly process are as below:

Step 1: Components Placement

This step is achieved manually by professional engineering staff. Engineers need to quickly, yet precisely place components on corresponding positions based on client’s PCB design files.

Component placement must conform to regulations and operation standards of through-hole mounting process to guarantee high-quality end products.

Step 2: Inspection & Rectification

Once component placement is completed, the board is then placed in a matching transport frame where board with components plugged in will be automatically inspected so as to determine whether components are accurately placed.

If issues concerning component placement are observed, it’s easy to get them rectified immediately as well. After all, this takes place prior to soldering in PCBA process.

Step 3: Wave Soldering

Now the THT components should be accurately soldered onto circuit board. In the wave soldering system, the board moves slowly over a wave of liquid solder at high temperature, approximately 500°F.

Surface Mount Technology (SMT) Assembly Process

Surface-mount technology (SMT) is a method for constructing electronic circuits in which the components are mounted directly onto the surface of printed circuit boards (PCBs) with solder paste.

Electronic devices made this way are called surface-mount devices (SMDs).

surface mount technology

An SMT component is usually smaller than its through-hole counterpart because it has either smaller leads or no leads at all.

The three key steps in surface-mount technology are paste, place, and reflow.

Step 1: Solder Paste Printing

Solder paste is applied on the board through a solder paste printer.

A template or stencil or solder screen ensures that solder paste can be accurately left on correct places where components will be mounted.

Step 2: Components Mounting

After coming out of solder paste printer, PCB will be auto-sent to pick-and-place machine where components or ICs will be mounted on corresponding pads due to the tension of solder paste.

Components are mounted on PCB board through component reels in the machine. Component reels carrying components rotate to provide parts to the machine, which will quickly stick parts to the board.

Step 3: Reflow Soldering

After every component is placed, the board passes through a 23-foot-long furnace (reflow oven) with a temperature of 500°F .

The solder paste must be heated until it melts and forms strong and reliable joints between the components and the surface of the board. This is accomplished through the use of a reflow oven which heats the solder to the proper temperature and then cools it down to a solid again.

This way the SMD components are bound firmly to the board.

surface-mounted devices are usually small and lightweight.

As compared to through-hole mounting process, Surface mounting process has a high degree of automation, reducing labor costs and increasing production rates. SMDs can be one-quarter to one-tenth the size and weight, and one-half to one-quarter the cost of equivalent through-hole parts.

Industries that utilize SMDs include telecommunications, aviation, consumer electronics, wireless, automotive and instrumentation.

While through-hole mounting provides stronger mechanical bonds than surface-mount technology techniques, the additional drilling required makes the boards more expensive to produce. It also limits the available routing area for signal traces on multilayer boards since the holes must pass through all layers to the opposite side. For these reasons, through-hole mounting is normally reserved for bulkier components such as electrolytic capacitors or semiconductors in large packages that require additional mounting strength to endure physical stress.

Mixed Technology

With the recent developments, electronic products are becoming increasingly complex. These complex electronic products need more complicated, integrated and smaller size PCB boards.

It is almost impossible for PCBAs containing only one type of component to perform such complex tasks.

Hence, for this purpose, a mixed technology board with both SMD and through-hole components will be required.

Such type of boards carry Thru-hole components and SMD components, which requires collaboration of thru-hole technology and surface mount technology.

Nevertheless, soldering is a complicated process that tends to be affected by too many elements. Thus, it becomes extraordinarily significant to better arrange the sequence of thru-hole tech and surface mount technology.

PCBA with application of mixed technologies should be carried out in the following situations:

Single Side Mixed Assembly:

Single side mixed assembly process has the following manufacturing procedure:

Fig.1: Single Side Mixed Assembly Process

Hand soldering can be applied instead of wave soldering only when a small quantity of THT components is required for this type of assembly.

One Side SMT & One Side THT:

One side SMT and One side THT assembly process has the following manufacturing procedure:

one side smt and one side tht assembly

Fig.2 : One side SMT and One side THT Assembly Process

This type of PCB assembly procedure is not recommended since adhesives will burden the total cost of PCBA and possibly lead to some soldering issues.

Double Side Mixed Assembly:

In case of double side mixed assembly methods, there are two alternatives: PCBA with application of adhesives and PCBA without adhesive.

The application of adhesives increases the overall cost of PCB assembly. Moreover, during this PCBA process, heating has to be carried out for three times, which tends to lead to low efficiency.

double side mixed assembly

Fig.3: Double Side Mixed Assembly (1)

double side mixed assembly2

Fig.4: Double Side Mixed Assembly (2)