- The board has only a few components
- The board has mainly through-hole components
- Only a few boards need assembly
Contract manufacturers populate and solder printed circuit boards in the US with components to make them functional. Essentially, they must solder a specific component at its designated place for the board to perform as intended. The component density and the complexity of the board defines the manner of PCB manufacturing and PCB assembly in the US—whether the manufacturer does it manually or by using machines.
Manual Assembly
Manual assembly of printed circuit boards is an optimum process when:
However, with manual assembly, there is a greater possibility of several types of errors such as mounting wrong components, mounting components at wrong positions, mounting components with wrong polarity, and missing components.
Moreover, manual assembly cannot produce high throughput of assembled boards. Therefore, when high throughput is necessary, along with high accuracy, high repeatability, high quality, and lower costs, manufacturers prefer using machines for PCB manufacturing and assembly in the US.
Machine Assembly
Several steps are necessary when machine assembling a printed circuit board in the USA. Although manufacturers also use manual operations for some steps, for best results, it is advisable to use automation for all the steps. Moreover, the efficiency of machine automated assembly is at its maximum when it uses surface mount technology or SMT. However, the PCB circuit design in the USA must support SMT.
Automated machine assembly steps are the same whether the printed circuit board undergoing assembly is a rigid board, flexible, or High Definition Interconnect type. These are:
Automated Solder Paste Deposition
Solder paste deposition on a bare printed circuit board typically involves the use of a stencil and solder paste. Manufacturers make stencils from thin stainless-steel sheets, and they have precision cutouts matching the pads on the PCB. After properly registering the stencil on the board, the operator pushes solder paste using a squeegee across the stencil, allowing the solder paste to drop through the openings onto the pads.
Assemblers typically use an automated inspection stage following the solder paste deposition stage. This ensures the deposition is clean and without smear. The inspection stage verifies that the height of solder paste on the pads is adequate to solder the component. High levels of solder paste means the volume of solder deposited is more, and this may lead to solder bridging among neighboring pads/components. On the other hand, lower height of solder paste may lead to inadequate or dry soldering.
Advantages of Automated Solder Deposition
The major advantage of using automation for solder paste deposition is that once the operator has adjusted the machine for optimum deposition height, the results are highly predictable and repetitive, and the machine provides high throughput. At this point, the automated inspection stage may change over to sampling inspections instead of continuous checking.
Automated Component Mounting
The next stage involves an automated pick-and-place machine. It selects components from feeders and places them on individual locations on the PCB. Operators must program pick-and-place machines to allow the machine to operate with precision and repeatability. Larger machines may have several heads that pick components from various cassettes and reels, positioning the components on pre-programmed locations on the PCB.
Initial stages of production may require the operator to use an automated inspection stage to determine a satisfactory outcome from the pick-and-place machine. Once this is established, the inspection stage may resort to sampling inspections.
Advantages of Automated Components Mounting
Substantial advantages result from using a pick-and-place machine rather than a manual placement process. The major advantage is high throughput with substantially low errors in placement. The automated process surmounts errors from the manual placement process. Errors that the automated process can overcome are wrong component placement, wrong polarity, missing components, and for close-pitch components, loss of accuracy.
Automated Reflow Soldering
After automated mounting of components, conveyor belts carry the printed circuit boards through a machine for soldering components. The reflow soldering machine typically has many sections with the temperature increasing in each section in a gradient. The soldering section has a temperature adequately high enough to melt the solder and bond the component to its pad on the printed circuit board. Once the soldering is over, the PCB and its soldered components pass through a cooling section.
For proper reflow soldering, the operator must initially calibrate the speed of the conveyor and temperature profile for a uniform assembly. The two parameters are specific to each board and its components. It is important the printed circuit board and components follow a specific temperature gradient up to the point of actual soldering.
Advantages of Automated Reflow Soldering
Machine soldering offers a much higher throughput of assembling an entire board with several components. For manual soldering, an operator typically uses a soldering iron or a hot-air gun for melting the solder. This can possibly dislodge a component from its intended position, as most SMCs are very small. Moreover, the finite size of the soldering iron bit is totally inadequate for the pitch of high-density components, leading to solder shorts.
Machine soldering tremendously increases the throughput of PCB manufacturing and assembly in the US. Use of machine soldering tends to reduce the possibility of several errors. These errors are inevitable during a manual soldering process and can result in solder shorts, missed solder joints, dry soldiers, burnt components, and many more. The manual process typically causes uneven soldering, while automated soldering offers an even and uniform soldering for all the components in one go.
Automated Testing
There are two ways of automatically testing a printed circuit board in the US—using a fixture or flying probes. For both, the PCB circuit designer must have provided test pads at strategic points on the board during its design.
Advantages of Automated Testing
A dedicated fixture with multiple probes can substantially reduce a board’s testing time. The multiple probes of the fixture usually test multiple parameters at a time with high accuracy. This would not be possible in manual testing. However, a specific fixture for an individual board assembly involves an expensive fabricating process. That is why manufacturers prefer using fixtures only when the production run of the printed board is of a substantially large quantity.
Another method of automated testing during PCB manufacturing and assembly in the US uses a pair or more of flying probes. These testing machines are programmable, and its test probes connect to various test pads serially to check and measure parameters. As the testing is serial, the speed of testing with flying probes is lower than that using a fixture. However, using flying probes has the advantage of lower cost, repeated use, and rapid reconfigurability when producing various types of boards in quick succession.
Conclusion
Overcoming human error is the major advantage of machine assembly of printed circuit boards in the US. Human error is natural and inevitable. Humans are prone to variable judgment and operational fatigue. Judgment for humans is highly subjective, and dependent on the individual. This often leads to different conclusions by various operators. Human operators typically suffer from operational fatigue after a few hours of intense concentration, leading to operational errors. Machines do not suffer from these shortcomings, as their judgment has its basis on a few rules that the operator has programmed into their memory. Machines also do not experience fatigue. However, machines do need regular preventive maintenance. Using machine assembly also has the benefit of reducing overall manufacturing costs.
