SMT คืออะไร และกระบวนการผลิต SMT

What is SMT?

SMT, short for surface mount technology, is a method that attaches electronic components onto the surface of PCB.

Basically, it solders SMC (surface mount components) onto the boards through reflow soldering.

Process of SMT manufacturing

Flow chart of SMT process

1. Material preparation and examination Prepare the SMC and PCB and exam if there are any flaws. The PCB normally has flat, usually tin-lead, silver, or gold plated copper pads without holes, called solder pads.

2. Stencil preparation Stencil is used to provide a fixed position for solder paste printing. It’s produced according to the designed positions of solder pads on PCB.

3. Solder paste printing Solder paste, usually a mixture of flux and tin, is used to connect the SMC and solder pads on PCB. It’s applied to PCB with the stencil using a squeegee on an angle range from 45°-60°.

4. SMC placement The printed PCB then proceeds to the pick-and-place machines, where they are carried on a conveyor belt and the electronic components are placed on them.

5. Reflow soldering Soldering oven: after SMC was placed, the boards are conveyed into the reflow soldering oven. Pre-heat zone: the first zone in the oven is a pre-heat zone, where the temperature of the board and all the components is raised simultaneously and gradually. The temperature ramp-up rate in this section is 1.0℃-2.0℃ per second until it reaches 140℃-160℃. Soak zone: the boards will be kept in this zone at a temperature from 140℃-160℃ for 60-90 seconds. Reflow zone: the boards then enter a zone where the temperature ramp-up at 1.0℃-2.0℃ per second to the peak of 210℃-230℃ to melt the tin in the solder paste, bonding the component leads to the pads on the PCB. The surface tension of the molten solder helps keep the components in place. Cooling zone: a section to ensure solder freezes at the exit of heating zone to avoid joint defect. If the circuit board is double-sided then this printing, placement, reflow process may be repeated using either solder paste or glue to hold the components in place.

6. Clean and inspection Clean the boards after soldering and check if there are any flaws. Rework or repair the defects and store the products. Common equipment related to SMT includes magnifying lens, AOI (Automated Optical Inspection), flying probe tester, X-ray machine, etc.