Almost all electronic equipment we use today contain various active and passive components mounted on one or more printed circuit boards (PCBs). This combination of components and PCBs is known as a PCB assembly or PCBA. Apart from electronic components, the PCBA may also contain other parts and accessories such as wires, connectors, shields, and heat sinks. With the advancement of technology and popular demand from users, manufacturers are increasingly making electronic equipment that are more compact.
To facilitate making thinner and smaller equipment, manufacturers of electronic components are also making miniature components and high definition interconnect PCBs. As manual assembly of such tiny components is not commercially viable, the technology for mounting them is also changing. As a result, there are various types of PCB assemblies depending on the application and their design:
- Through-Hole Components Assembly
- Surface Mount Components Assembly
- Mixed Assembly
- Single-Sided PCB Assembly
- Double-Sided PCB Assembly
Through Hole Components Assembly
Through hole components (THC) are those with wire leads for connecting. Mounting THC requires PCBs with holes in the pads, through which the assembler pushes the leads until the component is flush with the board. The assembled boards go over a wave soldering machine that solders all the leads to pads on the underside of the board. The assembler then cuts off the excess leads using a rotating blade cutter.
Fig 1: THT
For mounting components at high speed, assemblers often use programmable auto-inserting machines, which have the necessary components lined up in reels. After the machine inserts the components, it cuts off the extra leads and clinches them inwards to prevent the component from falling off. Passing the board over a wave soldering machine completes the soldering.
Surface Mount Components Assembly
With the increasing density of components on the PCB, manufacturers shifted towards surface mount component (SMC) technology, which allowed several advantages. The size of the SMCs being much smaller than the through hole types meant the assembly could be of much higher density. Additionally, these being lead-less components meant PCBs did not need holes drilled in them, reducing their cost substantially
Fig 2: SMT
For assembling PCBs with SMCs, however, a host of new technologies was necessary—starting from handling these miniature components, to placing them in position, and then soldering them.
These boards have both SMCs and through-hole components on them.
Single-Sided PCB Assembly
These boards have either SMCs, through-hole components, or both, mounted on only one side of a single- or multi-layered PCB. The assembly process involves the assembler mounting the throughhole components first, and soldering them using the wave soldering machine. The assembler then mounts the SMCs and subsequently solders them using the reflow soldering machine.
If there are only through-hole components present on the PCB, the operator needs to use only the wave soldering machine to solder them. Likewise, if there are only SMCs present, the assembler uses only the reflow soldering machine.
Double-Sided PCB Assembly
Unlike the single-sided PCB assembly, the double-sided PCB assembly has components on both its sides. However, mounting all through-hole components on one side of the board is advisable from the point of view of manufacturing ease. On the other hand, it is possible to mount SMCs on both sides.
PCB Assembly Technology
For increasing the efficiency of the assembly process, several technologies have evolved, replacing many of the manual methods that assemblers used earlier. These are:
- Wave Soldering
- Solder-Paste Deposition
- Reflow Soldering
Auto-Insertion : As explained earlier, this technology is primarily for mounting through-hole components. The assembler must program the auto-inserter machine with the position, orientation, and span of each component with respect to a specific mounting hole on the PCB. Based on the information fed in by the operator, the machine arranges the components in the form of a continuous reel.
While mounting, the auto-insertion machine uses components from its reel, cuts off the leads according to the span information, and inserts them into the respective position in the PCB, and clinches the ends of the leads. This allows transporting the boards without the danger of components falling off.
Wave Soldering : This technology is also primarily suitable for through-hole components. The wave soldering machine has a bath of solder kept molten with heaters. Impellers within the tub push the molten solder through a long open spout, and closely controls the height of the overflowing solder.
A conveyor carries the PCB assembly over the bath, such that the bottom of the PCB just grazes the top of the overflowing solder. The rest of the machine has a pre-heater to bring the components and the board closer to the temperature of the molten solder., and a cooling zone to bring the temperature of the assembly down after soldering is over.
The machine operator closely controls the pre-heat temperature, the speed of the conveyor, and the soldering temperature to achieve good-quality soldering. Once they achieve a proper combination of the above parameters, the operator stores it as the profile for the specific PCB assembly.
Glue-Dosing : Some designs have SMCs on both sides of the board, while others have a mixed assembly with through-hole components on the top and SMCs at the bottom. SMCs on the underside of the board can fall off when the assembler mounts the through-hole components.
To prevent them from falling off, the assembler uses a glue-dosing machine to deposit a tiny drop of glue at each point on the underside of the PCB where he/she will later place an SMC. After placing all the SMCs, the board must undergo curing to dry and fix the glue. The SMCs along with the through-hole components then undergo wave soldering.
Solder-Paste Deposition : This technology is meant exclusively for SMCs, as they have relatively short terminals rather than long leads, and do not require holes in the PCB to mount.
The operator places a tiny amount of solder paste on each SMD pad on the PCB. Rather than doing this manually, the operator uses a stencil with the necessary openings. The operator places the stencil on the PCB, registering it accurately. He/she then uses a rubber squeegee to push the solder paste through the openings of the stencil, and deposit it on the PCB. As the stencil is very thin, the amount of solder paste it deposits is very small.
As SMCs are very small, it is not practical to mount them manually. Rather, assemblers use pick-nplace machines for the placement. Manufacturers offer SMCs arranged in cassettes, reels, tubes, and trays. The operator must program the machine to pick the right component and place it at a specific position on a PCB on which the operator has already deposited either glue or solder paste.
This is a technology for soldering SMCs. The reflow soldering machine has a chamber with a conveyor and an infra-red heater arrangement. Operators feed PCBs with solder paste and SMCs mounted on them via the conveyor, and adjust the temperature inside the chamber to allow the boards to pre-heat before the solder paste can melt. The speed of the conveyor, the pre-heat temperature, and the temperature at which the soldering takes place, are interrelated and unique for a specific board and its components.
PCB Assembly Process
Fig 3: PCB Assembly Process