PCB Assembly Fabrication Methods
PCB or Printed Circuit Board assembly at Rush PCB UK Ltd involves combining the bare PCB, electronic components, and other accessories effectively to allow the assembly to function as the designer intended. Broadly, the fabrication methods involve a number of major steps:
- Component placement
However, depending on the nature of the PCB undergoing the assembly, above steps may involve further activities.
Methods of placing components on the PCB depend on several factors such as:
- Single or double side component placement
- SMD components only
- Through-hole (TH) components only
- Mix of SMD and through-hole components
A PCB may have components present only on one of its sides or on both, although assembling a PCB with through-hole components on both sides is a rare occurrence. Single side component placement with either all SMDs or all through-hole components is more common, while single sided PCBs with a mix of SMDs and through-hole components is also to be found.
For assemblies requiring SMDs on both sides of the PCB or a mix of SMD and through-hole components on the top and only SMDs on the bottom, the PCB assembly process must be broken up into several intermediate steps, and the assembler needs to take special precautions for each of them.
Single-Side Component Placement
This type of PCB may have only SMDs, only TH components, or a mix of both. If it is only SMDs, the necessary steps for assembly are:
- Solder paste printing
- SMD placement
- Reflow soldering
- Manual Inspection
- Electrical and Functional Testing
When the PCB has only TH components, solder paste printing is not necessary, in place of SMD placement, there is either manual placement or machine placement of the TH components, and wave soldering replaces reflow soldering. For a mix of both components, the assembler places TH components after anchoring the SMDs to the underside using adhesive and follows it up by wave soldering. This is because SMD components at the bottom of the board require to be held in place, while component leads extending on the underside of the PCB are undergoing wave soldering.
Double-Side Component Placement
Similar to the single sided PCBs, double sided PCBs may have SMDs alone, TH components alone, or a mix of SMD and TH components. Although it is usual to mount SMD components on both sides, mounting TH components on both sides is not feasible. However, it is possible to have a mix of SMD and TH components on top, and only SMDs on the bottom side of the PCB.
With both sides of the PCB holding components, multiple steps are necessary for the placement and soldering of the individual sides. SMDs on the underside need anchoring to the bottom side of the PCB to prevent them from falling away. Assemblers use a glue dispenser or a glue stencil to deposit a small drop of glue at the coordinate where the pick-n-place machine will deposit the SMD on the bottom side of the PCB. Baking is necessary to allow the SMD to permanently stick to the board before the soldering process.
The top side of the board may have only SMDs, only TH components, or a mix of the two. If the board has only TH components on its top side, the assembler inserts them and sends the board for wave soldering. However, if there are only SMDs on the top or a mix of SMDs and TH components, the assembler mounts the SMDs first, solders them with reflow, mounts the TH components and then wave solders the bottom side of the PCB.
Soldering is the process of joining two dissimilar metals using a molten filler metal alloy. The molten metal alloy enters the joint, and as it cools and solidifies, bonds with the adjoining metals. Primarily, there are three common ways of soldering:
- Manual Soldering
- Wave Soldering
- Reflow Soldering
Manual soldering requires the use of a soldering iron whose tip is heated either by gas or electricity. The pcb assembler applies the hot tip to the component lead and pad to heat them up. Solder held on the heated junction melts and solidifies to form a joint. The assembler may apply flux to the surfaces to help the process of soldering.
Wave soldering is a bulk soldering method. The PCBs to be soldered pass over a bath of molten solder. The bath has a pump to create a wall of solder that washes the underside of the PCB as it passes over the wall, effectively soldering the components to the underside of the PCB.
Temperature of the molten solder in the bath and the time the PCB is exposed to the solder are the two important parameters governing the quality of the solder joints formed. It is also necessary to preheat the board mounted with components to allow proper wetting. Although wave soldering is primarily effective for TH components, it can solder SMD components anchored with adhesive on the underside of the PCB.
Assemblers use reflow as the most common method for soldering SMD components to the top side of the PCB. However, before they can solder the components, they must deposit solder paste and mount the components in place. The sequence they follow are:
- Solder Paste Printing
- Mounting SMD Components in place
- Passing the assembly through a Reflow Soldering Oven
The assembler first deposits solder paste using a prefabricated stencil onto the pads of the PCB. Instead of using solid solder, SMD assemblers use a paste of solder and flux, which they draw with a squeegee over a stencil placed on the PCB. Appropriate cutouts in the stencil allow the solder paste to deposit onto the pads. On removing the stencil, the pads that will hold the components only retain the solder paste.
A pick-n-place machine then places respective SMDs onto their locations on the PCB. A pre-assigned program on the machine allows it to pick a specific SMD component from reels or cassettes and position it on a predefined position on the solder paste.
This assembly of PCB, solder paste, and components then passes through an oven on a conveyor belt, where heat melts the solder paste, effectively soldering the SMDs to the board. The speed of the board through the oven and the oven temperature are very important settings for achieving a good quality of solder joints.
After soldering is over, some flux residue may remain on the printed circuit board. Over time, this can turn acidic, and corrosively damage solder joints. The flux residue can also attract fingerprints and make the PCB look unclean.
For cleaning and removing flux residue, Rush PCB UK Ltd prefers washing the PCB assembly with high-pressure deionized water. A quick drying cycle with compressed air removes all traces of water, and the PCBs are ready for inspection and testing.
Inspection is necessary at various stages of assembly. For instance, one stage of inspection is necessary when the operator has stuffed TH components into the board, and again once the assembly has undergone wave soldering.
Likewise, for reflow soldering, inspection is necessary once after solder paste printing is over, then again after mounting the SMD components, and once again after the boards have exited the reflow oven.
Inspectors in both cases above look for missing components, wrong components, wrong polarity, solder bridges, excess solder, inadequate solder, dry joints, and many more defects. For minor faults, a touch up station is enough to rectify them, but for more severe and persistent faults, a change in process parameters is the solution.
AOI & X-Ray Inspection
For large batches of PCB assembly, the manual inspection process may be too slow. A faster process is the Automatic Optical Inspection or AOI process. AOI machines have video cameras to capture images of the PCB, the components, and the solder quality, and they can compare the images with standard images in the machine’s memory. The AOI machines work at high speed processing a large number of PCBs within a relatively short time.
Fine pitch components such as BGA conceal the solder joints under the component body, preventing manual or automatic inspection. The only way to assess the quality of solder joints hidden under such components is by passing the board through an X-Ray machine, capturing the images on camera, and examining the results.
Electrical & Functional Testing
Electrical testing may be necessary after the assembly has passed through the above stages, to test whether the assembly functions as intended by the designer. This may involve programming the board, or calibrating certain components before the actual testing can begin.
Electrical and functional testing involves applying specific voltage/voltages to the circuit on the PCB and looking for normal/abnormal behavior at predefined outputs. Some tests may require a voltmeter and ammeter, while others may need more sophisticated instruments such as an oscilloscope or a waveform analyzer to complete the testing.
PCB assembly is a complicated process involving several technical processes with important setup parameters. Eminent assemblers such as Rush PCB UK Ltd are always careful with these setup parameters to allow the final product achieve the desired quality