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Solder Paste Application in Printing Circuit Board Processes

Application in Printing Circuit Board Processes

In order to mount surface-mount components on a printed circuit board (PCB), the paste must be applied correctly. This delicate process is incredibly precise, and if it’s not performed consistently by experienced professionals, it will result in defective boards. The right amount of solder paste must be applied to the PCB so that each SMD will adhere securely and connect with the pads on the circuit board. Too much solder paste will create an imperfect outline, while too little will prevent the SMDs from attaching to the circuit board.

In the printing circuit boards processes, a metal screen is used to apply the paste onto the PCB. The stencil is cut into the metal using etching, electroforming, or laser cutting methods. The grooves on the stencil, called apertures, must be perfectly aligned and smooth for accurate paste application.

The squeegee used to apply the paste must also be in perfect condition. A squeegee with a scratch, tear, or residue can cause the paste to fail to adhere to the PCB or to miss the correct position. In addition, the speed and pressure with which a squeegee is operated must be controlled carefully.

Solder paste is made from mixing a tin-alloy lead-based powder with a flux material to make it stick. It is important to use the right mixture for a specific job, as different types of solder have different melting points. For example, tin-lead paste can be used to solder electronics that require a low melting point, while tin-free solder paste is required for parts with higher temperatures.

Solder Paste Application in Printing Circuit Board Processes

An expert PCB manufacturer knows how to program the screen printer properly, as well as how to apply the correct amount of solder paste and at what temperature. The physical characteristics of the paste, such as its viscosity and flux levels, must be checked regularly by performing in-house tests.

Once the paste is applied to the stencil, a device called a pick and place machine will place each SMD on top of it. The SMDs must be placed in preprogrammed locations, and the proper amount of paste must be applied so that each one will stick properly. If the wrong amount of paste is used, the circuit board will be short-circuited. If too much is applied, it will be difficult to remove the SMDs.

Once the surface-mount components are in place, the circuit board is put into a special oven called a reflow oven. This heats up the surface of the paste, which melts and bonds with the pads on the circuit board. When the temperature is just right, the solder can then be reformed to a solid state. The SMDs will then be able to function normally. The reflow oven temperature is usually set to a level recommended by the paste’s manufacturer. This temperature needs to be gentle enough not to explode the paste, but it must be high enough to melt the metal and activate the flux. A quick cool-down is then needed to ensure that the paste doesn’t re-oxidize.

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