In electronics manufacturing, a printed circuit board (PCB) is a composite structure that connects and supports active and passive electronic components. Printed Circuit Boards are categorized based on a.) substrate material type–Rigid or Flexible and b.) number of layers–Single/Double/Multi-Layered mainly. In the following section, we are going to shed light on to double-sided PCB assembly process. More particularly, we are going to discuss below processes involved in double sided PCB assembly:
- Solder Printing
- Solder Printing Inspection (SPI)
- Component Placement
- Automated Optical Inspection (AOI)
- X-ray Inspection
- Changeover Process to Second Side of PCB
As the term directly denotes, a double-sided PCB is a type of PCB with circuitry on both sides of the board. This means that copper traces are being etched on both sides of the board and connected through a drilling process. Double sided assembly process starts with an incoming inspection to check any faults during PCB fabrication. Below are the key manufacturing steps during double sided PCB assembly, more technically known as Surface Mount Technology or SMT.
Solder Printing is the process of applying solder paste material into the PCB. Solder paste is a type of adhesive which joins and holds the components in place into the solder pads. It is typically composed of a metal filler, resin, and flux. The flux helps prevent oxidation during subsequent heating processes. Viscosity is a critical property of solder paste which is defined as a material’s resistance to flow. A solder paste is a type of a “thixotropic” material which is a type of material that decreases in viscosity is when stress is applied. There is various solder paste application methods like automatic dispensing and solder screen printing. Solder printing is very important in PCB assembly process, since any offset may cause serious problems.
Solder Print Inspection (SPI)
Solder print inspection is a step that checks the solder printed PCB. The solder print pattern is compared against the criteria to check if the size and position of the solder paste is good. This is done by automatic machines since manual inspection is impractical to tediously scan all pads for any visual abnormality. SPI machines, just like other inspection and measuring equipment, should undergo Gauge Repeatability and Reproducibility (GR&R) analysis to ensure quality of inspection. SPI should be used in high volume PCB assembly process.
High speed automatic Pick-and-Place machines are used to place the components on the PCB with solder paste in PCB assembly process. The placement head consists of cartridge that holds the reels with components. The boards typically contain fiducials that are used to position the board optically and efficiently in the component placement machines.
To achieve a metallurgic bond between the component and solder pad, the PCB is subjected to reflow process. The reflow process, otherwise known as curing process, follows a recommended thermal profile based on the solder paste’s technical data sheet. First, the temperature of the reflow oven gradually ramps up to evaporate the volatile gasses on the solder paste. The gradual ramp up of temperature during pre-heating helps prevent thermal stresses and unwanted movement of components due to abrupt change in temperature. Then, there is a dwell time at the peak temperature to completely melt the solder paste. Lastly, the temperature gradually decreases to complete the curing or the cross-linking of the epoxy material of the solder paste. The reflow profile should be adjusted specially for every type of circuit board in PCB assembly process, according to their component density, size, PCB thickness., etc. Reflow should be done with an N2 (Nitrogen) environment to blanket the PCB from oxidation. Nitrogen is an inert gas that is non-reactive to other materials and gasses, protecting the parts when subjected to high temperature.
Automated Optical Inspection (AOI)
With the advancement of optical inspection systems, PCB manufacturers implement an in-line Automated Optical Inspection (AOI) to scan the PCB for any quality issue. It is a non-contact method of inspection, primarily situated after the reflow machine to inspect defects like placement issues, solder bridging, missing components, etc. The PCB is transported into the inspection platform and fixed in position by vacuum or mechanical clamping. The vision system of the AOI captures high-resolution images of the PCB and utilizes an algorithm such as pixel analysis and machine learning to enhance and identify defects. The defects are then mapped to incorporate into the final AOI report. The defects are classified based on size, shape and color and are displayed in the user interface.
Internal defects such as solder voids and incomplete solder cannot be detected by mere optical inspection machines. To address this limitation, PCB goes through real-time x-ray inspection to have an early detection of assembly issues. An x-ray inspection should be operated by a certified and trained personnel. Safety protocols should be complied due to potential hazard from x-ray. Absorption of X-rays vary from material to material due to the atomic weight differences, thus enabling to produce unique and sharp x-ray images. Solder joints have relatively higher atomic weight and appears darker during X-ray inspection. This mechanism allows manufacturers to check for air gaps on the interface of the components. X-ray inspections are always required in BGA PCB assembly process.
Changeover Process to Second Side of PCB
For double-sided printed circuit boards, circuitry and components are present on the top and bottom sides thus, the need for changeover of set-up. The other side of the PCB is re-passed into the SMT machines starting on solder printing until the end of the PCB assembly flow. Double-sided PCB is usually the favored option rather than going for a larger board. This is because the additional process for a second pass has lower assembly costs due to the automation of solder printing and reflow processes.
A manufacturing floor for the PCB assembly process strategically consists of one straight line of machines that are sequenced based on the assembly flow. Only X-ray machines are often separated from the layout with a chamber enclosure due to safety precautions. The objective in the design of an electronics board is to make a compact device— that is, to decrease the size and to increase the functionality of the boards. With this trend, printed circuit boards have become more complex with components on both sides. It is very important that the manufacturing quality should be kept secure in double-sided PCB assembly process. It is imperative to close the deal with a PCB contract manufacturer with the capability to manufacture high-quality boards according to requirements.