Is PCB Assembly Technology Different for Different PCBs?

As a PCBA engineer with more than 10 years of experience, I first throw my answer to the above question: YES

PCBs or printed circuit boards can be classified into many many types. Different PCBs have different features, especially in thermal dissipation and the maximum temperatures they can withstand. Besides, electronic components on PCBs may be sensitive to oxygen, moisture, and high temperatures. For these reasons, in the PCB assembly process, every minor detail should be considered, and the PCBA technology should be specialized for the PCBs.

First, let’s check out the regular PCB assembly process!

A regular SMT assembly line includes feeders, a solder paste printing machine, a solder paste inspection (SPI) machine, a high-speed SMT machine, a functional SMT machine, a reflow soldering oven, and an automatic optical inspection (AOI) machine. All these pieces of equipment are connected by transfer conveyors, and specialized technicians on the SMT line monitor the process and do visual inspections. For most PCBs, the PCBA manufacturing process is:

1. Solder paste printing. The PCBs are transferred from the feeders into the solder paste printing machine, where a scraper presses the solder paste to go through holes of an SMT stencil to print on copper pads of the PCB.
2. SPI inspection. The SPI machine scans the PCBs panel by panel to make sure the size, shape, thickness, and position of solder paste on PCB pads are qualified for PCB assembly.
3. Placing electronic components on PCB pads. In the high-speed functional SMT machine, small-footprint SMDs like resistors, capacitors, and inductors are automatically picked and placed on PCBs. And the functional SMT machine, large-footprint SMDs like ICs, connectors, BGAs, and so on are picked and placed on PCB pads.
4. Reflow soldering. In a reflow soldering oven, there are 10 temperature zones, where PCB panels are preheated, reflow soldered, and cooled down. The tin/silver/copper alloy powder in the solder paste melted and wet the copper pads, and flux in the solder paste activate the reaction between the solder and copper pads to form metallic compounds and then evaporates away. The reflow soldering temperatures and peak time duration are different for different PCBs.
5. AOI inspection. In an AOI machine, an infrared camera scans the PCBA, and a computer compares the scanned images with reference images to make sure no significant defects happen, such as missing components, tombstones, offsets, bridges, and voids.

After the SMT assembly, it is the THT assembly process. A regular THT line includes a manual THT group and wave soldering machines, which include a preheater and a wave soldering oven. The regular THT assembly process is:

1. Inserting THT component pins through holes. The THT components can be fusses, type-C connectors, etc. If inserting the component pins through holes manually is cheaper and quicker than using a specialized machine, a group of workers does it. Vice versa, we invent specialized machines to do that.
2. Wave soldering. A PCBA is placed in a jig that only leaves the places that need wave soldering exposed, and then the whole PCBA is preheated. In a wave soldering oven, the places that need wave soldering are soldered at once in tin flows.

Later, the long pins are trimmed to be short, and workers visually inspect and add tin to the PCBA if necessary. In the post-assembly from the EMS manufacturer PCBONLINE, functional testing, conformal coating, IC programming, box build assembly, and even enclosure and manual design are available.

Now, what is different in PCB assembly for different PCBs?

Different PCBs have different requirements for PCB assembly.

First, lead-free PCB assembly is different from tin-lead PCB assembly. Lead-free PCB assembly requires lead-free solder paste, and the whole PCB lines should be lead-free – that means, the lines are used only for lead-free PCB assembly. And the reflow soldering temperatures for lead-free PCB assembly are about 240°C to 270°C. In contrast, lead-containing PCB assembly is at a relatively low level, and the reflow temperatures are about 210°C to 245°C. For lead-free PCB assembly.

Second, heavy-copper PCB assembly is different from PCB assembly of copper thickness smaller than 3oz. Heavy copper means much copper in the circuit layers, and thermal is quick to dissipate from PCB pads. For this reason, the reflow soldering temperatures for heavy-copper PCB assembly are 290°C to 300°C. If you have heavy-copper PCB fabrication and assembly demands.

Third, flexible PCB assembly is different from rigid PCB assembly. As you know, flexible PCBs are very thin, light, and easy to move during the assembly process. So flexible PCBs must be fixed by a jig. The jig can be a non-magnetic board or a magnetic board. You can check two kinds of jigs below. In the whole PCB assembly process, flexible PCBs (including rigid-flex PCBs) are fixed on a jig. Wanna flexible PCB fabrication and assembly?

Four, BGA PCB assembly requires additional X-ray inspection. BGAs have solder balls beneath. When the functional SMT machine places BGAs on PCB pads, we can’t see solder balls and don’t know whether they are qualified for reflow soldering. For this reason, we use an on-line X-ray machine to make sure the solder balls are in good shape and right positions before the reflow soldering step.

Five, reflow soldering for PCBA with components easy to oxidize should be oxygen-free. Nitrogen reflow soldering prevents oxidation of the PCBA and boosts solder wetting. From PCBONLINE, both regular and nitrogen reflow soldering are available.

Six, it’s better to replace wave soldering with selective soldering for PCBA with THT components having different temperatures and soldering duration for soldering. With selective soldering, thermal shocks and THT soldering defects can be prevented. If you need selective soldering for your PCBA project.

Besides, double-sided PCBs should experience twice of SMT assembly processes, as each component-mounting side requires one time.

Take care – minor neglect can cause PCBA failure

In the whole PCB assembly process, every detail should be taken into consideration. Minor neglect, such as an opening of the SMT stencil not allowing enough solder paste to print on the PCB pad, an extra solder ball on the PCBA, drop-down of the polyester film on the hole during the reflow process, and so on can cause failures of the PCBA and end products.

If you plan to finish PCB assembly on your own, consider all steps and details in the DFM stage.

Hoping my content about PCB assembly can help you. If you have questions, you can leave your comments.