Compared with high volume PCBA, high mix low volume PCBA production efficiency is low, and the procurement cost is high. Moreover, it is difficult to achieve complete automation; the production plan and organization are more complex and usually encounter many difficulties, requiring PCBA manufacturers with rich experience.
EASHUB often uses the following methods to continuously improve low-volume PCB production and control the quality of PCB assembly. Next, let’s look at the steps that EASHUB takes to maintain PCB quality.

Low volume PCBA quality control steps
Low volume PCBA quality control can help reduce production costs and improve the accuracy of PCB development. To improve the qualification rate of SMT chip processing and the reliability of production sample quality, EASHUB will conduct PCB design, component procurement, process flow, and Production equipment and staffing are tracked and controlled throughout the process.
Among these control measures, SMT patch control is essential in the PCB manufacturing process. If there is a defect in SMT, let us cause bad results for our subsequent production. We can avoid it by ensuring that the SMT patch is qualified, so how does EASHUB make it?
EASHUB first uses inspection methods to control the quality of SMT patches, including incoming inspection, processing inspection and surface assembly board inspection.

SMT patch incoming inspection
Material inspection before SMT chip processing is a prerequisite to ensuring the quality of the chip. The quality of the PCB board, components, and SMT chip processing materials directly affect the quality of the PCB board. Therefore, we strictly check the below items according to the relevant regulations.

• The producibility design of the PCB board
• The solderability of the pads
• The electrical performance parameters of the components
• The solderability of the soldering ends
• The pins of the SMT chip processing materials, such as rod solder, flux, cleaning agent
• The quality of solder paste, patch glue, etc.
•  

PCB board incoming inspection
1) Check whether the layout of the pad pattern, size, silk screen, solder mask, and via hole on the PCB design meets the SMT PCB board design specification requirements. (For example: check whether the silkscreen is printed on the pad, whether the distance between the pads is reasonable, whether the via hole is made on the pad, etc.).

2) The external dimensions of the PCB should be the same, and the dimensions of the positioning holes and datum marks of the PCB should meet the requirements of the production equipment.

3.) PCB allowable warpage size:

   a) Upward/convex: max 0.2mm/5Omm length max 0.5mm/length direction of the whole PCB.

    b) Downward/Concave: Maximum 0.2mm/5Omm length and maximum 1.5mm/length direction of the entire PCB.

4.) Inspect the PCB for improper transportation or storage damage.

SMT chip processing components incoming inspection
For the incoming inspection of components, we can conduct a sampling inspection through the quality inspection part. The inspection items mainly include pin coplanarity, usability and solderability.

First, the solderability of components is usually detected by using stainless steel tweezers to clamp the component body and immerse it in a tin pot at 230±5℃ or 235±5℃, take it out after 3±0.5s or 2±0.2s, and then put it under a 20 times microscope.

Next, check the soldering end’s soldering condition, and ensure that more than 90% of the soldering end of the component is tinned to be considered qualified.

The SMT workshop can do the following visual inspections:

1) Check whether the components’ marked value, specification, model, precision, external dimension, etc., are in line with the production process requirements.

2) Use a magnifying glass or visual inspection to check whether the component’s soldering end or lead surface is oxidized or contaminated.

3) The pins of SOT and SOIC cannot be deformed. Therefore, for multi-lead QFP devices with a lead spacing of less than 0.65mm, the coplanarity of the pins should be less than 0.1mm.

4) For components that need to be cleaned, we must ensure the components marked do not fall off after cleaning and do not affect the performance and reliability of the components.

SMT processing inspection
SMT chip processing and welding inspection is a comprehensive inspection of welded products. Generally, the items that need to be tested include: whether the spot welding surface is smooth and clean, whether there are holes, holes, etc.;

• Whether the spot welding is crescent-shaped, whether there is more tin or less tin, or defects such as tombstones, bridges, moving parts, missing parts, and tin beads.
• Whether components have different levels of defects;
• Detect whether there are defects such as short circuits and conduction during welding;
• Check for colour changes on the surface of the printed circuit board.

In SMT chip processing, to ensure the soldering quality of the printed circuit board, we must focus on whether the reflow soldering process parameters are reasonable. If the parameters are set incorrectly, it cannot guarantee the soldering quality of the printed circuit board. Therefore, we must conduct two furnace temperature tests and one low-temperature test daily under normal conditions.

By constantly improving the temperature curve of welding products and setting the temperature curve of welding products, we will ensure the quality of processed products.

Light reflection distribution measurement method
The welding part detects the decoration, the light is incident inward from the oblique direction, the TV camera is installed above, and it is inspected. The most important thing about this detection method is understanding the SMT solder’s surface angle, especially the illumination luminosity information. We must obtain the angle information through various light colors. Conversely, if it is irradiated from above, the angle measured is the reflected light distribution, and it is sufficient to check the inclined surface of the solder.

Triangulation
That is the method used to check the three-dimensional shape. Although there are already triangle-based, the observed results will be different since the triangulation method requires using different light incidents and directions.

Surface Mount Board Inspection
In addition to SMT incoming inspection and processing inspection, EASHUB will also carry out a strict quality inspection after the SMT patch is completed In the low volume PCB assembly process. Our common inspection methods include the triangulation method, light reflection distribution measurement method, transformation angle inspection, focus detection utilization method, etc.

Transform angle check

This detection method must have an auxiliary system with changing angles. This system generally has at least 5 cameras and multiple LED lighting equipment, which can be inspected with multiple images, and the reliability is relatively high.

Focus detection method

For some high-density circuit boards, the above three methods are difficult to detect whether the product has defects, so we usually need to use the focus detection and utilization method. This method can achieve detection through the multi-segment focus method. For example, we can directly detect the solder surface’s height, set 10 focal plane detections simultaneously, obtain the focal plane by finding the maximum output, and detect the position of the solder surface.

EASHUB adopted the above detection method to ensure the excellent quality of SMT in the PCB assembly process. From this, we know that after the quality problem is found in the inspection of incoming materials, solder paste printing and pre-soldering tempering, it can correct according to the rework situation. Therefore, it is a low impact on the reliability of electronic products.

However, after SMT welding, the detection of problems will cause huge losses. Furthermore, unqualified post-soldering repairs need to be re-soldered after desoldering. In addition to working time and materials, it will also damage components and circuit boards. Therefore, the best thing is that the material inspection and processing inspection before the SMT patch can effectively reduce the defect rate, reduce the cost of rework and maintenance, and avoid the occurrence of quality hazards from the source.

Therefore, We can ensure the reliability of SMT patch quality through strict quality inspection. After the SMT inspection, we often use the following high mix low volume production and quality management systems to obtain maximum benefits.

Reduced scrap rates during commissioning
Due to the different production product types, it must frequently modify the equipment’s parameters, replace the tools and fixtures, and compile or call the numerical control program to produce better products. If it does not match, substandard products will be produced, resulting in product scrap and loss.

Most failed products in low volume PCB production are caused by product changeover and debugging equipment. Therefore, reducing the scrap rate during the commissioning process for the high mix low volume production is very important. Furthermore, EASHUB can reduce the loss as much as possible by establishing detailed work instructions and standard operating procedures for the commissioning stage.

Our production work instructions include the required CNC programs, fixture numbers, inspection methods, and adjusting parameters. Therefore, before low volume production, fully consider various factors and prepare operation instructions, which can improve the accuracy and feasibility of debugging, effectively reduce the time for model change and debugging, and improve equipment utilization rate.

Advanced management concept
If PCB production is only regarded as a production workshop and other parts are not involved, it will be not easy to establish an effective quality management system. Therefore, high-quality PCB production requires the cooperation of various departments, strict quality control is needed for each process, and it must not bring the one failed process to the next.

EASHUB adopts a prevention-oriented strategy through a strict whole-process tracking supervision and management system to ensure that each PCB production process, equipment operation specifications, and quality inspection measures are recorded.

Furthermore, the managers continuously implement advanced management concepts and follow the existing rules and regulations to improve the existing deficiencies.

From the above, although the quality control of products is more difficult due to the low volume PCB manufacturing characteristics, it still can ensure the product quality by establishing detailed operation instructions, introducing advanced management concepts, and optimizing production steps.