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Printed Circuit Board (PCB) Solder Paste Inspection Techniques

Solder Paste Inspection, abbreviated as SPI, is a specialized process that evaluates/inspects solder paste deposit quality on PCBs. It ensures the correct deposition of solder paste on the board without faults. In most of the cases, the reason behind the improper functioning of the PCB is due to improper solder paste printing. Solder paste inspection is therefore an essential technique that ensures costs are kept as low as possible by detecting faults in the solder printing stage of the production line. Solder paste deposition during a printing process is an indispensable task to attach the components to a circuit board for maintaining a proper electrical connection.

WORLD’s Surface Mount Technology (SMT) inspection process utilizes both Automated Optical Inspection (AOI) and X-ray to visually scan the surface of the board using high-definition (HD) cameras and several light sources. These systems use the captured image and compare it with the ideal/conventional board image which is already saved in the system. A comparison is then made between the captured image and the ideal image to detect and highlight any defects or suspected areas.

World electronics uses AOI (Automated Optical Inspection) and X-ray for solder paste inspection and defects analysis on every PCB because we believe in quality.

Automated Optical Inspection (AOI)

AOI machine tests and evaluates PCBs for faults like presence or absence, component placement defects, dimensional defects, and surface defects. With the advancement of electronic contract manufacturing and circuit complexity, it has become unfeasible to manually perform SMT inspection. Thus, AOI plays an important role in boards after PCB assembly. The machine has the capability of detecting errors in the early stage of the manufacturing process to assure the PCB quality before moving to the next manufacturing step.

WORLD electronics has one AOI machine, made by Yestech. It uses a camera and light to scan the surface of the board for defects such as backward parts (polarity), shorts, opens, missing components, and incorrect parts, among others. The machine uses already installed SW and templates and compares them to the board it is scanning. If the scanned board doesn’t match with the template (sometimes by a certain percentage), the part and board will be flagged, and the defects will be highlighted.

This technique inspects the PCB by using captured images to check:

  1. Are components missing?
  2. Are components placed in the correct position?
  3. Are there defects?
  4. Are we able to validate the quality of the manufacturing process?

The AOI machine can inspect all size components such as 01005, 0201, and 0402s and packages like CSPs, BGAs, LGAs, PoPs, and QFNs. Initially, AOI machines were designed for 2D measurements/analysis (detection along X and Y coordinates only). With the addition of 3D systems, the measurement along Z coordinate has also become possible. At WORLD, we use a 3D AOI machine for measurement along all the axis. The ultimate aim of an AOI machine is to detect errors in the production line and immediately informing the same to AOI programmers (also known as feedback). Feedback to the AOI programmers is both helpful and necessary so that time could be saved during SMT inspection.

AOI Programmers

The Automated Optical Inspection machine runs on a dedicated program, developed by AOI programmers. The following are the situations under which feedback is generated:

SMT Inspection/Bare PCB Inspection via WORLD’s AOI Machine

WORLD’s 3D AOI machine tests PCB assembly for the following aspects:

X-Ray Inspection System/Automated X-ray Inspection (AXI)

A PCB X-ray inspection system, also known as Automated X-ray Inspection (AXI), uses X-ray radiation instead of visible light to identify hidden PCB defects such as:

The above-mentioned defects can’t be seen during the inspection by standard cameras or by the naked eye. X-ray inspection is a commonly used technique that can detect manufacturing defects.  The X-ray inspection machine can inspect the PCB from any angle.

WORLD electronics currently uses a 2D X-ray inspection system and are planning to upgrade it to a 3D AXI in the near future.

Structure/Principle of Operation for X-ray Machine

An inspection device based on X-rays consists of a chamber with a small gap where it is possible to insert and remove the PCB. The X-ray source is usually located at the bottom of the chamber. Within the chamber, a digital detector (phosphor screens) is positioned directly above the X-ray source. Using an edge conveyor belt, the PCB that needs to be examined enters the chamber and is located between the source and the detector. The backside of the PCB is exposed to the X-ray so that the X-ray can pass through the components.

While AOI produces full color images of the object surface, X-ray machine transmits X-rays through the objects and records grey scale images of the shadows. The image processing software then processes the image. It then detects the location and size/shape of expected features (for process optimization) along with the presence/absence of foreign objects or features (for anomaly detection).

Inside the component, the different molecular densities attenuate the X-rays by different amounts, resulting in the creation of light and dark areas in detective media. Consequently, we can easily identify cracks, i.e. the attenuation level of the dense part of the solder ball is heavy compared to the attenuation level of the crack or air.  The crack tends to be dark due to varying attenuation levels, and the dense portion of the solder ball appears as a light in the image.

Conclusion of AOI and X-ray SPI

Solder paste inspection is a vital part in a PCB production line. It is crucial to identify solder paste defects in the early production cycle, otherwise, it can cost you both time and money. WORLD electronics’ AOI and X-ray machines use advanced SMT inspection techniques to offer quality and defects-free PCBs to our customers.

For more on WORLD’s capabilities visit our services page and follow us on LinkedIn.

Printed Circuit Board Bake-out Process and Quality Control

Eliminating moisture from the PCB is essential for any PCB manufacturer. This can be achieved by baking the PCBs. If the moisture is not removed properly, then it may lead to the dysfunction of the board while implementing it in the device. But, before moving further with this topic, let us first learn what is typically meant by “bake-out”? “Bake-out” is a process of removing volatile compounds from materials, using high-temperature (100 to 125°C) or vacuum, before placing the material in an environment, where the slow release of these volatile compounds would hamper the performance of a device in which these materials are used.

The bake-out process in PCB, by its name, refers to baking the PCB at a temperature of 100°C (or more – if required), following the guidelines described under IPC 1601, to eliminate outgassing of moisture from the copper plating in plated through holes (PTHs). This occurs during manual or automated soldering processes that cause pin-holes, blow-holes, and sunken solder-fillets. The process of baking should be done in a clean oven in order to prevent the PCB from any kind of contamination that can occur during the process. It is also recommended to place the boards in such a way that the air can circulate freely around them during the baking process. Since we want the moisture (and other solvents, possibly) to escape, the best way to do this is to have the PCBs in a rack, vertically oriented with some space in between them. If the boards are stacked on top of one another or flat on the base of the oven, etc., then it can be more difficult for the moisture to escape. Overall, it is highly advisable to prevent moisture from reaching the PCB which can be done via pre-baking.

How we bake PCBs at WORLD electronics

Approach: Our team determined baking the printed circuit boards at receiving, prior to assembly for a minimum of 4 hours at 125°C and a maximum of 16 hours would meet IPC-1601 standards (Printed Board Handling and Storage Guidelines). Moisture barrier bags in a range of sizes were ordered for storage of the circuit boards after completing the PCB baking process. Two system programs were created within our existing production software for levels of baking PCBs (low temp and high temp).

Results: Our standardized printed circuit board bake-out process has dramatically decreased production time and minimized PCB fall out which results in fewer problems, quicker QC inspections, and higher quality circuit boards without fear of latent failures.

Why PCBs are Baked?

Moisture could lead to various functional failures on a PCB. It can fester in the epoxy glass and leads to cracks in the board. It is also responsible for causing separation in the PCB layers, especially interfaces. The presence of the moisture could slow down the circuit speed and can increase the delay time. If such problems exceed up to a certain limit, the device might simply fail to work. The next reason for baking PCBs is to inhibit the delamination of boards that occur during soldering. It happens when moisture accumulates in voids or at the interface between the epoxy/laminate on poorly bonded multi-layer boards. During soldering, the moisture turns into steam/water vapor and expands which creates pressure that can cause the device to fail. Hence, one can say that the most common cause of delamination is moisture, and pre-baking the board before soldering will reduce the cause of it. The chronological order would be to first bake the PCB and then do soldering if you want your PCB to function properly. 

Delamination of PCB during reflow soldering.

The picture given above shows a similar example of an FR4-based PCB’s delamination during reflow soldering. It happens due to the moisture present within the FR4 material that expands causing internal layers to separate. The FR4 is a “hygroscopic” material that absorbs moisture from the atmosphere. Hence, once the PCBs are fabricated, they should be vacuum-sealed. Therefore, predominantly, PCBs are baked to remove moisture. Although, PCBs constructed from Epoxy Resin are not so hygroscopic and hence there’s no need for baking. However, if the boards are old, or whenever there is a suspect of moisture in the boards, it is recommended to bake before soldering.

PCB bake-out in a nutshell

Parameter Explanation
Baking To get rid of moisture
Benefits To improve functionality/to increase life-span of PCB
Baking Temperature 100-125°C
Disadvantage Higher production cost and time

 

At WORLD electronics, we offer pre-baked PCBs to suit all your requirements which will deliver outstanding performance for the intended application.