Surface Mount Technology and Assembly Implications
Surface mount technology (SMT) is a design and fabrication process that replaces the through-hole mounting of PCB components with direct placement on the printed circuit board surface.
Fortunately, SMT components and packages are largely standardized in size such that uniformity in components can be realized. These standards are specified by the Joint Electron Device Engineering Council (JEDEC) to simplify the development processes for PCB designers, component manufacturers, and companies that manufacture and utilize SMT fabrication equipment.
SMT Assembly Benefits
SMT is now utilized in the vast majority of printed circuit boards being designed and manufactured for modern electronics, including computer boards, smartphones, televisions, and even home appliances such as refrigerators and laundry machines.
Creating mass-produced PCBs built with SMT construction requires highly-technical design tools such as CAD software and specialized PCB development programs. Design engineers now have software products at their disposal that interact with fabrication tools, such that consideration for manufacturing capabilities is integrated into the initial design. This closely-coupled process from design to fabrication is often referred to as design for manufacturing (DFM).
Once the design is completed for a PCB, an initial prototype is created to prove the intended functionality of the end product. SMT is sometimes not considered for this phase of development, due to the flexibility provided by less automated fabrication techniques to more readily rework the PCB. This accommodates any changes required in the board design prior to full-scale manufacturing.
Approved designs are most often released for manufacturing incorporating SMT assembly processes. SMT provides many advantages for manufacturing, as well as for the company designing the board for a product:
Reduced cost – through surface mounting of components, manufacturers eliminate (or significantly reduce) operations for hole drilling to affix components and devices to board materials.
Weight/Size – lighter boards are produced through the attributes of smaller devices. Capability of installing SMT components on both sides of the board results in fewer or smaller boards to take less space in the end product. This is a very important factor in today’s shrinking electronic devices.
Automated fabrication processes – Manufacturers can leverage efficient fabrication equipment to provide high-quality PCBs that utilize surface-mount components consistently and reliably.
SMT Assembly Techniques
SMT assembly can be done very efficiently with high-speed specialized equipment that place SMT components on boards quickly and accurately.
Solder pads on the board surface have a solder paste or similar substance applied to the points where SMDs are to be placed. This is typically done through utilizing a stencil process similar to silk-screening, in order to control application of the solder mixture.
Mechanized equipment then places the designated components onto the corresponding solder pads at high speed, eliminating manual processes of placing component leads through holes that have been drilled in the board material.
Soldering ovens then warm the board to the proper temperature to melt the solder paste, forming a permanent bond to the solder pads on the board surface. Several techniques are in practice for performing this operation, partly due to environmental concerns raised by lead content, generating more strict controls over lead-based soldering. Additional methods for accomplishing the solder connections include infrared reflow, convection reflow, and vapor phase reflow. Each of these methods has its own advantages and disadvantages, and application for use.
When a PCB includes dual-sided application of SMDs, this process will be repeated for the subsequent side. If there are heat-sensitive components included, they may be installed manually after these heat-generating processes are complete.
Residue is typically removed from the PCB surface through a washing and drying process.
Quality inspection is performed to examine boards for missing components or those that may be misaligned during the manufacturing process. This is followed by functional testing to ensure the PCB circuit works properly.
Rework is one of the biggest issues for SMT boards. SMDs that are defective, incorrectly positioned, or missing must be installed manually using very precise operations for installation of the faulty component. This can be tedious work that requires both considerable time and significant skill.
SMT Fabrication Considerations
SMT design and manufacturing has taken over the market for PCB providers, due to the call for miniaturization, speed of manufacturing, and cost reductions.