Surface mount technology (SMT) components are easily assembled by automated machines, making assembly quicker, faster, and cost-effective.
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Pick-and-place equipment is generally used to assemble SMT devices. Such assembly begins with board and component inspection.
Surface mount technology assembly is a technique in which electrical and electronic components are mounted directly on the surface of a circuit board. Electrical and electronic components that follow this mounting technique are known as surface mount devices (SMD). This technology minimizes fabrication costs and increases efficiency.
An SMD is a leadless electronic component that is mounted on a PCB directly without drilling holes. And SMT refers to the process (technology) used to mount SMD components on a circuit board.
To learn learn more, check different SMD component package sizes.
A few benefits of using SMT for board design include:
For more details, see the advantages and disadvantages of SMT.
The storage temperature for BGA devices that are temperature-sensitive is between 20 – 25°C with a relative humidity (RH)of 10%. Once BGA packages are opened, they must be used within 8 hours under the operating environment conditions of ≤30°C/60% RH. To prepare BGA devices for soldering, they are baked at 125°C to remove any moisture content.
The circuit board drying temperature for dehumidification is <60°C (low-temperature heat), and the water vapor content in the atmosphere should be <0.05g/m3.
The following steps are implemented for board preparation:
Like circuit boards, components are also prepared for assembly by classifying them into kits. This process is as follows:
After loading the right components in the cartridges, programming the pick-and-place machine with the placement data, and solder paste application on the boards, the SMT assembly kicks off.
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This process is repeated for the secondary side in case the circuit board has components on both sides.
While a majority of components can be placed using pick-and-place machines, some of them can only be assembled manually. The reasons for this are stated below:
The Printed Circuit Board assembly process involves the placement and soldering of the electronic components on the bare board (Circuit Board minus the components) which can be accomplished either by manual assembly or Surface Mount Technique (SMT) line assembly methods. Surface mount technique involved soldering of components on the bare board, while other method involved drilling holes in the bare board, and that’s called through-hole assembly. Both through-hole and surface mount components can be soldered manually on a PCB. With the shrinking chip sizes and dense circuitry, SMT line assembly is the widely used PCB assembly technique today.
Initially, through-hole components were exhaustively used in the PCBs until the demand for compact circuit boards increased in the electronics industry. The small board size couldn’t accommodate large through-hole parts and hence the Surface Mount components became popular. SMT parts are smaller in size and have leads that can be directly soldered on the PCB surface. They are cheaper than through-hole parts and offer better electrical performance. SMT passive parts have majorly replaced their through-hole counterparts already.
The different methods used for soldering the electronic parts on the PCB are manual soldering, wave soldering, and reflow soldering. Manual (Hand) soldering is the simplest method used in PCB assembly. The soldering iron tip is positioned above the joint and pressed against the component placed on the pad along with the solder wire. After removing the soldering iron, the solder joint hardens and establishes the connection. This method is also known as soft soldering. It offers a strong connection that can also be reworked later. Manual soldering is the best option to solder components that are difficult to access by huge machines.
Usually, through-hole parts are wave soldered and SMT parts are reflow soldered during PCB assembly. In the wave soldering method, the through-hole components are inserted in the drilled holes and a molten solder wave is applied to the PCB that connects the leads to the PCB holes. In the case of reflow soldering, the solder paste is applied to the pads and SMT parts are accurately placed using automated pick and place machines. Next, the board is run through a pre-heated oven which reflows the solder paste to make a solder joint between the leads and pads of the SMT parts. This process is faster and cheaper compared to the wave soldering method used for through-hole components.
Manual assembly depends on skilled technicians for the physical placement of components and uses the hand-soldering method. For low-quantity orders, the machine assembly will not be cost-effective. Manual assembly can reduce project delays as the assembly staff will visually inspect the PCBs for any incorrect footprints, misalignments, etc. A machine inspection may miss out on a few faults or even halt the assembly process for failures. But the manual process can quickly resume, as the technician can inspect and correct those mistakes on the spot.
Based on the requirement of a pick and place machine for complete automation or manual loading of the components, the SMT lines are classified as automatic and semi-automatic types. Several machines are used in SMT lines to enable complete automation of the assembly like Solder paste mixer, SMT loader/ unloader, Solder paste printing machines, and AOI machines.
The SMT line assembly has become a standard assembly method but has few exceptions. It is not suitable in applications like prototypes built for minimum quantities or boards with special components (like heat-sensitive, unique size or shape, etc) Through-hole components on small build volume (lesser quantity) PCBs are ideal contenders for the manual assembly process. Boards having press-fit components or custom parts may also require manual assembly. Components having longer lead times that have to be assembled separately can be handled by manual assembly methods.
A manual process can also be used to verify existing PCBs that are assembled using the SMT line method. Any rework or touch-up requirements for SMT parts can be done by manual soldering. During the automated assembly process, tiny SMT parts may get displaced or soldering defects like bridges (bridge is the connection of two or more pads’ unwanted connection, usually happened while soldering) might occur. The manual soldering method can attend to such SMT assembly issues for clean-ups.
The increased complexity of circuits has made the PCBs very compact. Placing the tiny components closely is not feasible in the manual assembly process and hence SMT line assembly is used with automated pick and place machines. The SMT line offers high-speed assembly which can bring out the final product swiftly. This is a huge competitive advantage that has made the SMT automated assembly process a universally accepted method. Low cost, reduced faults, consistent quality, and miniaturization are other added advantages of the SMT line assembly.
Knowing these benefits and drawbacks, it is easy for you to choose the right assembly method for your PCB product. An experienced Contract Manufacturer can offer you a high-quality assembly service by using a combination of automated equipment and skilled technicians together.
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