SMD Soldering Guide: Technology Sharing

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SMD (Surface Mount Device) is a packaging method for electronic components that uses surface mount technology to solder electronic components to the surface of a circuit board. This type of packaging is characterized by small size, light weight, material saving, high reliability and powerful performance.SMD soldering involves placing electronic components in specific locations on the circuit board and then soldering them by melting the solder to firmly connect the components to the board.

SMD Soldering Tools

Surface mount device soldering requires some specialized tools in order to handle tiny components and make precision solder joints. Here are some of the essential items you’ll need:

Soldering Iron – A fine-tipped soldering iron in the 15-30W power range is ideal for SMD work. Tips as small as 0.5mm can be used. Temperature control features help avoid overheating.

Solder paste– Solder paste consists of a mixture of powdered solder alloy and flux cream. It allows solder to be precisely applied to SMD pads before components are placed.

Microscope – A stereo microscope or magnifying glasses are indispensable for inspecting small solder joints and component placement. A microscope with 20x to 40x magnification is typical.

Tweezers – Fine-tip tweezers allow precise handling and placement of SMD components as small as 0201 or 01005 sizes (0.25mm x 0.125mm). Anti-static tweezers are preferred.

Soldering Helping Hands – Helping hands tools with magnifying lenses allow hands-free positioning of PCBs under a microscope during soldering.

Stencil–PCB stencils are thin metal sheets laser-cut with a pattern of openings matching the PCB’s solder pad layout. To apply solder paste, the stencil is aligned to the PCB and the paste is screened onto the pads through the stencil’s openings. Using a stencil allows precise and efficient solder paste application prior to SMD component placement.

Jigs – Jigs help position boards at an angle which improves visibility and access to solder joints underneath components during hand soldering.

Solder Sucker/Desoldering Tools – Specialized vacuum tools are used to remove or rework solder joints and desolder components for repair work.

Surface Mounting Steps

▶Mounting the substrate: Fix the substrate on the countertop.

▶ Point paste or glue: according to the size of the components, the SMD adhesive coated in the pre-determined position, if the assembly process using reflow soldering, it is necessary to apply the paste in the substrate pads, the current commonly used in the medium-high temperature level Sn-Ag solder paste.

▶ Mounting of SMD: Generally, automated professional mounter is used, which mainly includes: suction and loading head for picking and placing SMD, X-Y working table, program control system and feeding part.

▶ Thermal curing: carried out after the dispensing, SMD, under a certain temperature, time control through the curing oven to make the adhesive curing. The curing oven is controlled by a certain temperature and time to improve the adhesive strength of the SMD, and to avoid the components being shifted by vibration and shock during storage and transportation.

▶ SMD soldering: Wave soldering with SMD adhesive bonding and reflow soldering with solder paste bonding are used.

▶Cleaning: Remove residual adhesive to prevent corrosion of the substrate.

▶Inspection and testing: Solderability is inspected according to standards and test requirements.

SMD soldering needs to pay attention to the following points:

1. Keep the soldering iron tip clean to avoid oxidizing or staining its surface with impurities, which may hinder the heat conduction between the tip and the soldered parts.

2. Before soldering, solder paste should be evenly applied to the pads of the PCB, and make sure that the amount of solder paste applied is appropriate.

3. Components should be accurately placed on the PCB to avoid misalignment or tilting.

4. The temperature of the reflow oven should be strictly controlled to ensure that the solder paste melts and solidifies at the correct time and position.

5. The soldering time should be properly adjusted to ensure the quality of soldering.

6. Soldering pressure should be properly adjusted to ensure the density and strength of the solder.

7. Reflow soldering process parameters should be strictly controlled, including the temperature, time and pressure of the preheating zone, uniform heat zone, reflow zone and cooling zone.

8. The welding environment should be kept clean to avoid external factors interfere with the welding quality.

9. Process inspection should be carried out to ensure that the welding quality meets the requirements.

Common SMD package size

Common SMD mounting method is divided into SO mounting, QFP mounting, LCCC mounting and PLCC mounting four.

(1) SO mounting is divided into SOP mounting and SOL mounting, the use of wing-shaped electrode pin shape, pin spacing 1.27mm, 1.0 m m, 0.8mm, 0.65mm and 0.5mm.

(2) PQFP mounting rectangle on all sides of the wing-shaped electrode pins, the thickness of 1.0mm or 0.5mm. QFP packaged chips are generally large-scale integrated circuits, the number of electrode pins for the 20 to 400, the minimum pin pitch is 0.4mm, the largest is 1.27mm.

The minimum pin spacing is 0.4mm and the maximum is 1.27mm.

(3) LCCC mounting is not a pin mounting, the chip is mounted on a ceramic carrier, no lead electrode soldering ends are arranged on the bottom of the four sides of the mounting surface, the number of electrode pins 18 ~ 156, the spacing of 1.27mm.

(4) PLCC mounting is a rectangular mounting of integrated circuits, its pins hooked back to the inside, the number of electrode pins 16 ~ 84, the pitch is 1.27mm.

SMD soldering is a very delicate work, which is nowadays done by fully automated production lines. Of course, for beginners to understand and learn manual welding is also very necessary. Because this way we can more quickly familiar with the entire welding process, and better able to find problems, solve problems.