Can Manual Soldering be Used in SMT Assembly Manufacturing Process?

As electronic technology rapidly develops, electronic products tend to become miniaturized with weight and cost dramatically shrunk. As far as SMT (Surface Mount Technology) assembly is concerned, SMCs (Surface Mount Components) are mostly soldered onto PCB through reflow soldering that is carried out in reflow soldering oven, an automatic device. In spite of the high automation SMT assembly adheres to, manual soldering is still of quite necessity in its manufacturing process. Thus, this article will introduce the significance of manual soldering to SMT assembly and some tips for it.


Advantages of SMT Assembly

a. High Assembly Density


Compared with traditional through-hole components, chip components call for smaller real estate of board surface. Additionally, the application of SMT assembly leads electronic products to shrink by 60% in terms of volume and 75% in terms of weight.


b. High Reliability


Chip components feature high reliability and shock resistance due to their small size and low weight. Automated production is used so that soldering and placement feature high reliability. Therefore, nearly 90% of electronic products are produced by SMT assembly.


c. High Frequency


Since chip components cover no leads, both parasitic inductance and capacitance are reduced with frequency improved.


d. Cost Reduction


Due to rapid progress and wide applications of chip components, the cost of chip components also goes down in such a high speed that a chip resistor features the same price with through-hole resistor. SMT assembly simplifies the overall manufacturing procedure and reduces manufacturing cost. As far as SMCs are concerned, their leads don’t need to be reorganized, bent or cut so that the whole production procedure is shortened and manufacturing efficiency is improved. As soon as SMT assembly is applied, the overall manufacturing cost can be decreased by 30% to 50%.


Comparison between SMT Assembly and THT Assembly

The characters of SMT assembly can be fully indicated through the comparison between SMT assembly and THT (through-hole technology) assembly. Based on the mounting technology, the essential difference between SMT assembly and THT assembly lies in the difference of placement and through hole. Besides, both parties differentiate from each other in terms of a couple of aspects including base board, components, devices, soldering joint and assembly technology, which can be summarized in the following table.

The difference between SMT and THT actually derives from the difference between component types, including component structure and lead types. Since no-lead or short-lead components are applied in SMT assembly manufacturing, the essential difference between SMT and THT is that the figures of components and PCB are not totally the same and components are fixed onto PCB in different ways.


Fundamental Requirement of Manual Soldering for SMT Components

Requirement#1: Soldering Material


Finer tin wires should be used and active tin wires with diameter being in the range from 0.5mm to 0.6mm are better. Solder paste can be also used but it should feature no-clean flux with low corrosion and no residues.


Requirement#2: Tools and Equipment


Constant-temperature soldering iron and specialized tweezers should be used. The power of constant-temperature soldering iron should be lower than 20W.


Requirement#3: Operators


Operators are required to master sufficient techniques on SMT inspection and soldering. Certain working experience should be accumulated.


Requirement#4: Operation Regulations


Rigorous operation regulations have to be implemented during the process of SMT assembly.


Commonly-used Tools and Equipment for SMC Manual Soldering

• Tweezers


Tweezers are a type of soldering tool specially used for SMCs. Component soldering can be easily finished as two terminals of SMCs are caught by tweezers.


• Constant-temperature Soldering Iron


Constant-temperature soldering iron features a soldering head whose temperature can be controlled. Constant-temperature soldering iron is applied because it adheres to constant heating, saving electric energy by a half and it leads to a rapid temperature rise.


• Special heating head of soldering iron


After special heating heads with different dimensions are equipped with soldering iron, a lot of SMCs can be soldered onto PCB with different lead count, including QFP, diodes, transistors and ICs.


• Vacuum Tin-absorbing Gun


Vacuum tin-absorbing gun mainly consists of tin-absorbing gun and vacuum pump. The front end of tin-absorbing gun is a hollow soldering head, capable of heating.


• Hot-air Soldering Table


As a type of semi-automated equipment using hot air as heating source, hot-air soldering table is capable of easily soldering SMCs, which is more convenient than soldering iron. Additionally, hot-air soldering table is capable of soldering numerous types of components.


SMC Manual Soldering Tips

• Resistor, capacitor and diodes soldering tip


First, tin is melted on pad and soldering iron should not be far away from pad to keep tin melting. Second, place a component onto pad with a tweezer. Third, one terminal is soldered and then comes the other terminal.


• QFP soldering tip


First, IC should be placed onto corresponding position and a little solder paste is used to fix the three leads on IC so that chips can be accurately fixed. Second, flux is smoothly coated onto leads with each lead soldered. In the process of soldering, if bridging occurs between leads, a little flux should be coated onto the bridging position.


• Hot-air soldering table application tip


Hot-air soldering table can be used much more conveniently than soldering iron and numerous types of components can be dealt with. ICs can be soldered by hot-air soldering table but solder paste should be used as solder instead of tin wires. Solder paste can be coated on pad in a manual method first. After SMCs are placed, hot-air nozzle is used to quickly move along chip so that all pads are smoothly heated with soldering completed.


As a conventional soldering method, manual soldering still plays a key role in electronic manufacturing no matter how technology develops. SMT assembly has become a leading assembly method owing to its high assembly density, high manufacturing efficiency, low cost, high reliability and wide range of applications. The combination of automated soldering and manual soldering will definitely bring forward active effects to electronic manufacturing.


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