PCBA Assembly

1. DFA: Verifies Gerber/ODB++ and BOM

Design for assembly is the first stage of PCBA. During this stage, DFA engineers verify the data present in the Gerber/ODB++ and circuit board BOM files.

As a designer, you should always follow the DFA guidelines to avoid board respins. The prirmary objective of this step is to avoid potential assembly errors and reduce the overall cost of your board. DFA engineers ensure:

Correct implementation of parts listed in the BOM
Component footprint dimensions are accurate
Adequate spacing between the components
Drill file specifications are followed
Incorporation of adequate circuit board thermal relief techniques
Board edge clearance rules are in compliance

2. SMT assembly process using pick and place machine

SMT assembly involves placing and soldering electronic components to the board using an automated system called the pick and place machine.

Before mounting the SMDs, the assembly house checks if there are any non-washable components. If they are present, we will assemble them at a later stage (after washing the circuit board). Surface mount assernbly involves the following steps:

a. Solder paste screening/inspection

In this step, we will apply solder paste uniformly using stencils. Solder paste is a combination of powdered metal solder like tin, silver, and copper and a flux medium.

Our assembly experts apply the solder paste using an SMD stencil. An SMD stencil is a thin sheet of material (usually stainless steel) with a series of apertures that resemble the component pads on the board. The stencil accurately deposits the right amount of paste on the surface mount pads.

If you would like your CM to use any specific solder paste, you can call out the same in your PCB assembly notes.

After applying the solder paste, we inspect the paste with the help of solder paste inspection (SPI) machines. There are two types of SPI devices: 2D and 3D.

2D equipment can measure the height and width of solder paste deposits. 3D machines can calculate the length, width, and volume of the applied solder peste. These devices can also detect assembly errors such as missing paste and solder bridges.

b. SMT component placement

After applying the solder paste, a pick and place machine mounts components such as BGAs, ICs, resistors, and capacitors. This device picks a component from a tape, rotates it to the desired orientation, and places the part on the board.

Components per hour. This enables us to assemble your prototypes with a 24-hour turnaround time.

c. Reflow soldering

Refiow soldering is conducted only for SMT assembly (wave soldering for through-hole components will be discussed later). The circuit board, along with the mounted components, passes through the reflow oven. In this stage, the solder paste melts and establishes a solid connection between the components and their pads, upon cooling. The components are rigidly fixed to the board.

The temperature range of reflow soldering depends on the type of solder paste used. 180- 220°C for lead solder paste and 210-250"C for lead-free solder paste.

d. Optical Inspection

Worker inspect the solder joints and components on the PCB for defects. They identify flaws such as missing components, solder difficulties, wrong component orientation, lifted and misplaced components, open circuits, solder shorts and insufficient or excess solder. This process aids in quality assurance.

3. Cleaning of assembled boardse

After mounting the components, the team cleans the board using deionized water or a kaizen solution to remove flux residues and contaminants. They treat the circuit board with deionized water jets at high temperatures and pressures

We maintain the water temperature at 144 F while applying a pressure of 45 pounds per square inch. After washing, they dry the circuit board in the air using powered air jets.

4. Soldering of non-washable components

If there are any non-washable components in the design, they are soldered to the board at this stage. No-clean flux is generally used to solder these parts. This type of flux doesn't need to be cleaned up after the soldering procedure.

This flux has resins and activators that are used to make the solder flow easier and less likely to oxidize.

5. Final inspection and testing

After completing the PCB assembly, the team sends the board for the final inspection. Next we perform a quality inspection to check for physical and electrical flaws, such as missing components, solder defects, open circuits, short circuits, and incorrect component values.