6 Points We need to Pay Attention to in Printed Circuit Assembly

Printed Circuit Boards (PCBs)

A majority of us would easily recognize printed circuit boards when we bump on them. They are small green chips that are covered by copper parts and traces. The traces are like super-highways that lead traffic to some direction.

But are you aware of a printed circuit assembly process? Unfortunately, a lot of suppliers, manufacturers, and PCB enthusiasts don't know much about this. This article points out six notes in a PCB assembly process.

(Printed circuit assembly, workers, are manually assembling PCB)

1.Solder Paste Stenciling

The first step into a printed circuit assembly process is applying some solder paste to the board. Solder paste stenciling closely resembles screen-printing. The only difference is that instead of using a mask, a thin and stainless-steel stencil gets placed on top of the PCB.

The above process is vital for a good reason. It allows printed circuit board assemblers to effectively apply paste only on the board's selected parts meant to form the final panel. These are the parts that will finally sit in the finished printed circuit board.

Solder paste consists of 96.5% of Tin, 3% of Silver, and 0.5% Copper. In terms of appearance, it is somewhat greyish. If you examine it even more closely, you will realize that this greyish paste consists of tiny metal balls. These metal balls are the solder.

The solder paste does mix the solder with some flux, a chemical substance designed to assist the solder to melt effectively.

Additionally, it helps the solder to bond to a surface. Solder paste is grey, and you have to apply effectively at the exact places of the board. Assemblers must also ensure that they use the adhesive precisely according to the right amounts.

In a professional and mass PCB assembly, a particular mechanical fixture does the entire holding and solder work stenciling the PCB.

(printed circuit assembly A worker undertaking PCB solder paste stenciling)

2.Printed Circuit Assembly Pick-and-Place

After the first step is complete, the printed circuit assembly process now moves on the pick-and-pace equipment. A pick-and-place machine is a robotic machine that precisely mounts surface mount components on a PCB. You have to solder these on the PCB then.

Not so long ago, this was a straightforward process that was achievable using a pair of tweezers. Here, printed circuit assemblers were only picking and placing components using their hands.

However, with technology evolving, this process is now automated by the use of specialized machines. The shift was fast, mainly because the equipment tends to be highly precise and consistent than humans.

Even though humans can work faster, eyestrain and fatigue soon set, especially when spending hours with small components. Unlike human beings, machines can work for many hours without fatigue. The only thing that they may require is some maintenance.

The pick-and-place machine begins the process by effectively picking a printed circuit board. With a vacuum grip, it then moves and places it on a place station. After that, it begins the process of orienting the printed circuit board by placing SMTs to the PCB's surface. The machine sets the components on top of the paste, though in some programmed locations.

(a pick-and-place machine at work, printed circuit assembly )

3.Printed Circuit Assembly Reflow Soldering

The moment the solder paste, in addition to the surface mount components, is in place, they have to stay there. In other words, the solder paste has to solidify to adhere to the parts on the board. Printed circuit assembly does accomplish this through a process that's known as "reflow."

After the completion of pick-and-place, the incomplete PCB board now gets moved to the conveyor belt. The conveyor belt does a lot of work, chief among them moving through a large reflow oven. The reflow oven can be likened to a large commercial pizza oven that has got some heaters. These heaters slowly heat the boards to temperatures of around 250 degrees.

Temperatures of 250 degrees are hot enough to melt the solder material in the solder paste effectively. The moment the solder has properly melted, the printed circuit board continues moving through the oven. Here, it passes through some series of cooling heaters. These cooling heaters allow the solder to cool and subsequently solidify.

Cooling is a process that sees the creation of permanent solder joints connecting the surface mount devices to the PCBs. Plenty of PCB assemblers must pay special attention during reflow. Special attention is needed even more in two-sided printed circuit boards. These two-sided boards require stenciling and reflowing on each of their sides separately.

(PCB reflow soldering, printed circuit assembly)

4.Inspection and Quality Control

At this point, you may think that PCB assembly is almost over. However, this is not the case, as you may have to subject your boards to several tests. Tests are vital for assessing the functionality of boards before being put up for sale. During the reflow process, the lack of proper connections may occur. Also, short may occur, with misplaced components sometimes connecting parts of the circuit that should not join.

Checking for misalignments and errors involve several inspection methods. However, some of the most common ways include:

• X-ray inspection

Even though this is not highly popular, it's still a reliable test method. X-ray inspection is ideal for layered or complex PCBs. X-ray inspection allows assemblers to effectively see through the layers to identify if there are potential hidden problems.

• Manual checks

Despite the rise of automated test methods, manual checks are still reliable. Manual checks are ideal, especially for small batches. However, manual checks are getting increasingly impractical and inaccurate. Such is authentic, especially if the number of boards that require inspection increases. Examining smaller components for several hours can easily lead to instances of visual fatigue. With fatigue, there will be fewer accurate inspections.

• Automatic Optical Inspection

This is one of the best inspection methods, suitable for massive PCBAs batches. Here, printed circuit board assemblers use an optical inspection machine to conduct a series of tests. Automatic Optical Inspection Machines consist of high-powered cameras. These cameras are at different angles aimed at viewing solder connections.

The fate of faulty boards depends on the printed circuit assembly is company standards. If the boards are not functioning correctly, then the company will have to incur extra repair costs. But all is not lost. With inspection and testing, there's a possibility of avoiding such costly errors. Checks ensure the detection and subsequent fixing of errors.

(PCB inspection, printed circuit assembly)

5.Through-Hole Component Insertion

A plated through-hole in a printed circuit board is a hole that is drilled from top to bottom. The components of printed circuit boards use the holes to pass signals from one side of the board –say the top side-, to the other side of the board, such as the bottom side. Here, soldering paste isn't as ideal as such. Solder paste can run straight through the board without adherence.

Instead of the soldering paste, plated-through-hole components need specialized soldering methods later in the PCBA process. Some of the most basic ones include:

• Manual Soldering

This is a straightforward process. Here, an individual stationed at one place inserts and solders one component only. After that, the board gets taken to the next station where another assembler manually inserts and solders another. The cycle continues until the insertion of all the parts is complete.

• Wave Soldering

Unlike manual soldering, wave soldering uses specialized machines to insert and solder PCB components. It is an updated version of manual soldering.

(Through-hole component insertion)

6.Final Inspection and Testing

Upon the completion of either manual or wave soldering in PCBA, a final inspection step follows. Inspections and tests are necessary to determine the functionality of a printed circuit board. The test effectively puts PCBs along its paces and simulates the actual circumstances upon which a PCB will operate.

Simulated and power signals do run through a PCB during this test. Here testers monitor the board is electrical characteristics. In case matters such as signal output, current or voltage indicate some unacceptable fluctuation; the PCB is deemed to have failed the test.

Testing is essential and the final step in the printed circuit assembly process. This is because testing doe determines either the success or failure of the entire printed circuit assembly process.

(PCB inspection and testing)

Summary

There you have it, six notes in the printed circuit assembly process. Do you have any printed circuit assembly needs? Or perhaps, you can not handle PCB assembly as it deserves? If so, feel free to contact us at OurPCB for immediate solutions.

We specialize in high-quality PCBA using a team of highly trained and qualified PCB assemblers. At OurPCB, our vision has always been to assemble PCBs within the shortest time possible. If you have any questions regarding our PCB assembly services or products or would like a free quote, please feel free to contact us.

You can trust us to get the job done from low volume to low complexity assembly to high-volume assembly. Call us today for assistance.