PCB Assembly Process in 14 Photos – Plus How Much?

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PCB Assembly Process in 14 Photos – Plus How Much?

Dec 27, 2021

PCBs bring electronics to life, so it’s no surprise you want to understand how the PCB assembly process works; well, I’m about to make the PCB assembly process easy to understand -How do you make them?

 

Have a look. Swipe left

1.PCB Assembly – PCB Board and Laser Marking

 

Laser marking on printed circuit boards is a must; it’s a regulatory requirement by IPC standards.

 

This is extremely important for printed circuit boards and their PCB manufacturers as it shows the quality of the PCB and its manufacturing process.

 

PCB Assembly laser marking

 

Think of it as labeling a PCB so anyone can trace where it comes from, how the manufacturer made it, and to check whether the PCB meets regulation standards.

 

You can label a PCB using different methods

 

  • Ink marking
  • Stamping
  • Stickers

 

Laser marking is the best option because 

 

  1. Low cost 
  2. Better label clarity 
  3. Fast
  4. Machines for laser marking are super easy to use 
  5. Laser marking lasts longer 
  6. Environmentally friendly 

 

pcb assembly service

 

The laser marking process is straightforward; the PCBs go into the laser-marking machine; a conveyor belt moves them through; PCBs go under a low-power laser beam that marks them.

 

With laser-marking machines is easy to select the number or QR code you want to print on the PCBs; there’s of of course a software to upload the necessary files.

 

2. PCB Assembly – Applying Solder Paste

 

Now comes the part of applying solder paste on the bare board; a solder paste printer will only apply the solder paste on the areas where the components will be mounted as per the design in the Gerber files.

 

At this stage, a solder paste printer and stainless steel stencil are placed on top of the bare board (PCB) and held together thanks to a mechanical fixture.

 

Then solder paste is applied on top of the stencil, filling all the openings.

 

PCB Assembly solder mask

 

This has to be extremely precise, you can not add a bit more or a bit less, so at this stage of the PCB assembly process, we can expect a relatively high rate of defects.

 

The stencil is removed after the solder paste application, and the solder paste stays in place.

 

In case you’re wondering, this solder paste is 

 

  • 96.5% tin
  • 3% silver
  • 0.5% copper

 

It is ultimately lead-free.

 

Quality is something to look out for here; this part of the PCB assembly is susceptible to environmental conditions such as

 

  • low/ high temperature
  • Humidity

 

This is the reason for the many inspection stages that follow.

 

3.PCB Assembly – Solder Paste Inspection (SPI)

 

This part of the process can be automated thanks to the use of machines with

 

  • x-ray
  • High definition cameras
  • Light sources

 

It’s known as automatic optical inspection.

 

solder paste inspection assembly

 

The purpose is to inspect and evaluate the solder paste deposit quality on each PCB.

 

This is highly important because, in many cases, PCB’s improper functioning is due to bad solder paste printing.

 

By identifying solder paste defects early in the production cycle, you’ll avoid rising costs and time delays; imagine going into PCB mass production with faulty PCBs.

 

4. PCB Assembly – Pick and Place Machine

 

An exciting part of the printed circuit board assembly process is the automated placement of components.

 

More specifically, mounting SMT (surface mount technology) components or SMT assembly.

 

These are small components; some of them are too tiny to pick up with your bare hands; that’s why machines do this soldering process.

 

Learn more about SMT and SMD components

These machines are known as pick and place robots; because that’s exactly what they do; they pick up the small electronic components and place them on the corresponding solder joint.

 

If you’re wondering how the machine can know where all the components are supposed to go; well, this is thanks to design files fed into the machine; the file has the x, y coordinates of all the electronic components for PCB assembly.

 

Before having pick-and-place machines, technicians used tweezers to pick up the components and then solder by hand.

 

mixed technology pick and place machine for pcb assembly

 

You can imagine how time-consuming and expensive the PCB assembly services were.

 

A manufacturing plant offering PCB assembly services usually has three pick and place machines.

 

  • A fast Pick and place machine.
  • When accuracy is paramount, a machine with a camera for small and subtle components.
  • A very robust machine with high pressure for boards that require force when placing components.

 

rigid pcb flexible pcbs smt

 

We have PCB assemblies that move fast with high accuracy and flexibility thanks to these different pick and place machines.

 

5. PCB Assembly – Reflow Soldering

 

So, now we have the PCBs with the SMT components mounted on them; the next step in PCB assembly is reflow soldering.

 

Operators place the PCBs with their components on a conveyor belt which moves them into a big reflow soldering oven at a temperature of 250c.

 

This temperature melts the solder and causes the components to stay fixated, creating the joints.

 

Reflow soldering pcb assembly

 

After enduring high temperatures, the printed circuit boards go into a cooler that solidifies the solder joints.

 

This is what creates that strong and permanent joint between PCB and component.

 

What happens with double-sided PCBs? 

 

Well, the PCB side with fewer and smaller components will be treated first (steps 2,3,4,5); then the other side follows.

 

6. PCB Assembly – Visual inspection

 

PCB assembly second inspection time.

 

There’s a chance that during the reflow soldering, there was an erroneous movement in the PCB holding tray, causing a misalignment that will most likely end up in open connections or a short circuit.

 

visual inspection for quality assurance on PCB

 

You obviously don’t want that. That’s why we have another round of inspections.

 

PCB Assembly – Manual Inspection

 

You can do it this way, but it is not very accurate. This is because the SMT ( surface mount technology) components are too small for the human eyes to verify whether they’re installed correctly.

 

It will inevitably cause technicians to tire and let faulty PCBs go through.

 

 

However, this inspection method might work for a PCB with through-hole components and a low density of components.

 

PCB Assembly – Automated Optical Inspection

 

There’s a machine for this. At this station, we ensure that each component was placed correctly at the right angle and that the soldering is good.

 

This is the ideal inspection method for large batches of PCBs as the machine is fast it dramatically speeds up PCB assembly.

 

pcb inspection and pcb requirements

 

Once we make sure the SMT components are correctly mounted, we can move to THT soldering.

 

7. PCB Assembly – Through Hole and Manual Assembly

 

A PCB usually has more than SMD components; it also has THT or through-hole components.

 

They differ from SMD because they have leads that go through the PCB and are soldered on the other side. THT components can not use the soldering method used for SMT components for THT ones.

 

 

pcb assembly services tht

 

There are two ways to do this soldering.

 

Manual Soldering

 

This is pretty much a bunch of technicians manually working on placing THT components on the PCB and soldering by hand.

 

As you can imagine, the process is tedious and time-consuming; that’s why you should avoid THT components as much as possible when you’re designing your PCB.

 

 

However, sometimes you need them in your design to secure essential components to the PCB; THT soldering is stranger than SMT, so if your device is in a situation of constant stress, you better consider using THT.

 

Wave Soldering

 

Manual soldering is not the only option; you can also go for wave soldering, a faster soldering method.

 

The PCB is placed on a conveyor belt, and a molten solder wave is splashed on the PCB bottom layer where the leads are; this solder all pins at once.

 

wave soldering

 

The problem with this is that it can not be performed on double-sided PCBs.

 

Those PCBs with BGA (Ball Grid Array) components on their design would go into BGA assembly now.

 

8. PCB Assembly – Electrical Testing

 

At this stage, and after doing two inspections, the PCB should be working as per design, but we better test.

 

An electrical test is done to check the interconnectivity of the PCB functions. Some machines can do fast electrical testing and make PCB assembly safe and secured.

 

overall cost SMT assembly electrical testing

 

These are the two methods used to perform electrical tests on the PCB

 

A bed of nails; this is done for high volume PCB assembly; it allows for shorter testing time but requires expensive development of fixtures.

 

The second method is a flying probe, great for high component density and small volume; however, it takes longer to program the flying probe and run the test.

 

9.PCB Assembly – X Ray

 

This is not done on every PCB, only those with pretty advanced components, such as BGA.

 

For large BGA components, we need to check that the pins reach the correct layer; these pins are usually behind the component and therefore can not be tested with Automated Optical Inspection machines.

 

x ray inspection

 

In this case, we need to use an X-ray to take an x-ray image of the PCB and make sure that the connectivity and the soldering are right.

 

10.PCB Assembly – Functional Test

 

Functional test (FCT) is the final PCB assembly step done by PCBA manufacturers. It provides a pass/fail result on finished PCBs before shipping.

 

It’s great that we can test the functionality of the completed product, but remember that the preference is always to find defects through the different processes before this last stage.

 

pcb manufacturing

 

Functional testers typically interface to the PCB under test via its edge connector or a test-probe point. This testing simulates the final electrical environment in which the PCB will be used.

 

The PCB assembly process has been completed.

 

PCB Assembly Service

 

When looking for a PCB assembly service, even for a prototype PCB, this is the info you’ll need to provide.

 

The quantity you want to order

 

PCB Type

 

  • FR4
  • Flexible PCB
  • Flex-Rigid PCB
  • Aluminum
  • Rogers

 

Layers

 

  • 1 Layer
  • 2 Layer
  • 4 Layer
  • 6 Layer
  • 8 Layer
  • 10 Layer
  • 12 Layer
  • 14 Layer
  • 16 Layer
  • 18 Layer
  • 20 Layer

 

Material Details

 

  • FR4 Standard TG 140C
  • FR4-Tg 150C
  • FR4-High Tg 170C
  • FR4-High Tg 180C
  • FR4-Halogen-Free
  • FR4-Halogen-Free & High T

 

Board Size

 

  • 3mm minimum
  • 1200 maximum 

 

Thickness for finished board

 

  • 1.6 mm
  • 0.5 mm
  • 0.6 mm
  • 0.8 mm
  • 1.0 mm
  • 1.2 mm
  • 2.0 mm
  • 2.2 mm 
  • 3.2 mm

 

Surface Finish

 

  • HASL – Hot Air Solder Leveling
  • Lead-Free HASL RoHS
  • ENIG Electroless Nickle/Immersion Gold RoHS
  • Immersion Silver RoHS 
  • Immersion Tin RoHS 
  • OSP – Organic Solderability Preservatives – RoHS 

 

Copper Weight 

 

  • 35 um
  • 70 um
  • 105 um
  • 140 um
  • 175 um
  • 210 um
  • 420 um
  • 525 um

 

Solder Mask

 

  • Both Sides
  • One Side

 

Solder Mask Color

 

  • Green 
  • Blue
  • Red
  • Black
  • Yellow

 

Silkscreen Legend

 

  • 1 Side
  • 2 Sides

 

Smallest Holes Size 

 

  • 0.40 mm
  • 0.30 mm
  • 0.20 mm
  • 0.15 mm
  • 0.10 mm

 

Min Tracing/Spacing

 

  • 0.20 mm
  • 0.15 mm
  • 0.10 mm

 

How much will it cost to manufacture your PCBs? If you share your PCB bill of materials with us, we can quote you a price. 

You can send an email to [email protected] 

Bob Moore
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