PCBA--Printed Circuit Boards Assembly Process

Step 1: Solder Paste Stenciling

The first step of PCB assembly is applying a solder paste to the board. This process is like screen-printing a shirt, except instead of a mask, a thin, stainless-steel stencil is placed over the PCB. This allows assemblers to apply solder paste only to certain parts of the would-be PCB. These parts are where components will sit in the finished PCB.

The solder paste itself is a greyish substance consisting of tiny balls of metal, also known as solder. The composition of these tiny metal balls is 96.5% tin, 3% silver and 0.5% copper. The solder paste mixes solder with a flux, which is a chemical designed help the solder melt and bond to a surface. Solder paste appears as a grey paste and must be applied to the board at exactly the right places and in precisely the right amounts.

In a professional PCBA line, a mechanical fixture holds the PCB and solder stencil in place. An applicator then places solder paste on the intended areas in precise amounts. The machine then spreads the paste across the stencil, applying it evenly to every open area. After removing the stencil, the solder paste remains in the intended locations.

Step 2: Pick and Place

After applying the solder paste to the PCB board, the PCBA process moves on to the pick and place machine, a robotic device places surface mount components, or SMDs, on a prepared PCB. SMDs account for most non-connector components on PCBs today. These SMDs are then soldered on to the surface of the board in the next step of Printed Circuit Boards Assembly Process.

Step 3: Reflow Soldering

After the pick and place process concludes, the PCB board is transferred to a conveyor belt. This conveyor belt moves through a large reflow oven, which is somewhat like a commercial pizza oven. This oven consists of a series of heaters which gradually heat the board to temperatures around 250 degrees Celsius, or 480 degrees Fahrenheit. This is hot enough to melt the solder in the solder paste.

Once the solder melts, the PCB continues to move through the oven. It passes through a series of cooler heaters, which allows the melted solder to cool and solidify in a controlled manner. This creates a permanent solder joint to connect the SMDs to the PCB.

Step 4: Inspection and Quality Control

Once the surface mount components are soldered in place after the reflow process, which doesn't stand for completion of PCBA and the assembled board needs to be tested for functionality. Often, movement during the reflow process will result in poor connection quality or a complete lack of a connection. Shorts are also a common side effect of this movement, as misplaced components can sometimes connect portions of the circuit that should not connect.

Step 5: Through-Hole Component Insertion

Depending on the type of board under PCBA, the board may include a variety of components beyond the usual SMDs. These include plated through-hole components, or PTH components.

A plated through-hole is a hole in the PCB that's plated all the way through the board. PCB components use these holes to pass a signal from one side of the board to the other. In this case, soldering paste won't do any good, as the paste will run straight through the hole without a chance to adhere.

After this soldering process is finished, the PCB can move on to the final inspection, or it can run through the previous steps if the PCB needs additional parts added or another side assembled.

Step 6: Final Inspection and Functional Test

After the soldering step of the PCBA process is finished, a final inspection will test the PCB for its functionality. This inspection is known as a "functional test". The test puts the PCB through its paces, simulating the normal circumstances in which the PCB will operate. Power and simulated signals run through the PCB in this test while testers monitor the PCB's electrical characteristics.

If any of these characteristics, including voltage, current or signal output, show unacceptable fluctuation or hit peaks outside of a predetermined range, the PCB fails the test. The failed PCB can then be recycled or scrapped, depending on the company's standards.

Testing is the final and most important step in PCB assembly process, as it determines the success or failure of the process. This testing is also the reason why regular testing and inspection throughout the assembly process is so important.

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