SMT PCB Assembly
The demand for high-tech products is rapidly growing; from small electronics like our handheld Smartphones to enormous, complex systems such as the 787 Dreamliner airplanes.
SMT PCB assembly is a critical service to build these high-tech products.
Surface Mount PCB assembly is the backbone to integrated electronic systems. PCB assemblies come in a variety of shapes and sizes and are the powerhouses of electronic systems whether large or small.
The miniaturization of applications requires printed circuit boards to fit in small spaces, be able to handle extreme temperatures, withstand excessive vibration and yet perform over and over again with precision.
Surface Mount Technology (SMT) Process Overview
SMT circuit boards come single-sided, double-sided and multi-layered.
The stenciling process is the foundation of PCB assembly; if the application of solder paste is off, you will most likely have defects down the road.
A stencil is required for most surface-mount assemblies. Where SMT components are on the top side, a solder paste stencil is required. Where parts are glued to the bottom or solder side of the board for wave soldering, a glue stencil is usually required. Bottom side reflow also requires a paste stencil. Stencils have to meet the manufacturing requirements for frame size, stencil thickness, step-downs or other techniques for fine-pitch, aperture sizes on all components and stencil adhesive compatibility.
Component placement is done automatically with pick-and-place machines. For time-sensitive orders, your assembly shop should ensure the latest technology is utilized, including multi-headed, multi-gantry machines that can place components even faster than ever – well over 135,000 per hour. This technology utilizes cameras to line up the placement in just a split second.
Components are soldered by one of several solder methods: wave, reflow, or selective.
Any through-hole components are wave-soldered to solder the leads to the board. The SMT components use a reflow oven that melts the solder paste that was stenciled onto the SMT pads.
A selective solder machine is used for components that cannot withstand the heat of the reflow oven. Selective solder is a very precise process, so damage is not incurred to the surrounding components. Selective solder is also used for mixed technology – SMT and through-hole components and devices on the same board.
There are inline cleaners that remove any dust particles from the boards before the soldering process. Cleaning before soldering has proven to aid in reliability. After the soldering process, the board goes through cleaning to remove any flux residue that is left on the board. The flux residue needs to be removed so the board is not damaged or found non-operational. There are several means available including cleaning agents, heat, and agitation or vibration. There are also assemblies that are designed as “no-clean” parts.
Some of the most common SMT inspection tools are the Automated Optical Inspection (AOI) and X-ray testing. These types of equipment test the quality of the solder joints. The boards are also visually inspected to see if anything is out of alignment, missing or has other issues.
Depending on the type of product, the boards may be inspected by sampling. If the product is for a more critical application such as manned aerospace or implantable medical devices, the inspection process includes as high as 100% of the final lot.
Automated Optical Inspection has many benefits:
- Improved quality – by detecting catastrophic failures such as missing components or quality defects, the AOI process will help save time and improve the output. Once issues are found, adjustments can be made. The defective board can be removed from the line and reworked.
- Breadth of inspection – this is a non-contact test that will detect a wide variety of defects in just one inspection point on the line. It will save much time in the long run.
Testing includes in-circuit, functional, and environmental tests dependent on the requirements. The testing process validates that the boards work as they are intended to. The environmental screening puts it through the rigors of temperature and vibration cycles to ensure the board can withstand ruggedized situations.
There are many issues that can cause a board to fail a test. This is why it is critical to cover all the basics:
- In-circuit – this type of test is checking for shorts, opens, capacitance, and resistance among other things.
- Functional – this type of test is looking for how well the board software performs against the design specification.
- Environmental – this type of test verifies the board can withstand certain harsh environments such as vibration and shock.