PCB News - Understanding PCB microsectioning

Microsectioning is an extremely important link in the process of PCB manufacturing.It is an important means to detect circuit boards,find and solve problems,ensure the quality of the board, improve the yield of products,and improve the process and production process.

Micro-sectioning provides an important objective factual basis for discovering and solving problems. The correct production of micro-slices is related to whether the truth of the problem can be found, whether the PCB manufacturing process and production process can be improved, and the problem can be avoided again.

To find the truth, solve problems, and continuously improve the production process or production process, we must first understand and understand micro-sectioning. Really and correctly master the production method of microsections, so as not to be misled by some illusions.

The classification of micro-slices in PCB manufacturing

Circuit board anatomical destructive microsection methods can be roughly divided into three categories:

1.Microsection

Refers to the through-hole area or other plate area, after the cut sample is filled with sealant, the vertical section is made perpendicular to the board surface direction, or the horizontal section of the through hole is made cross-section (Horizontal section), These two methods are generally common micro-sectioning.

PCB longitudinal section microsection

2.Micro-cut holes

It is to carefully use a diamond saw blade to cut a row of through holes from the center into two halves,or use sandpaper to grind a row of through holes in half vertically and longitudinally. Under a 20X~40X stereo microscope (or called a solid microscope), observe the overall condition of the remaining half wall in a full field of view. At this time, if the back plate of the through hole is also ground to be very thin,the half hole of the translucent substrate can also be back lighted to check the coverage of the copper layer of the initial hole.

Cut the cavity with a diamond blade, and the two halves will immediately appear under the sun, and any PCB production defects will be invisible to the original appearance. If you want to learn more about the details, you can do technical and academic micro-sections. Direct observation with a stereo microscope after cutting a hole is more holistic than micro-sectioning, but photography requires the help of an electron microscope SEM to have a better effect.

3.Oblique slice

Fill the through holes of the multi-layer PCB board with glue, and perform oblique grinding of 45° or 30° in the vertical direction, and then observe the variation of the conductor lines on the oblique plane with a solid microscope or a high-power tomography microscope. In this way, the dual characteristics of straight cut and cross cut can be taken into account. However, this method of slicing has a certain degree of difficulty, and it is not easy to perform microscopic observation.

PCB microtome preparation process

1.Sample Acquisition

Use a special diamond saw blade to intercept the sample from any position of the PCB board,or cut off the unwanted part through the shearing equipment to obtain the required slices. During the cutting process, it is necessary to avoid getting too close to the edges of the holes to prevent deformation of the vias due to pulling.The best practice is to cut off a larger sample block first, and then use a diamond saw blade to precisely cut out the desired sample to minimize deformation that may be caused by mechanical stress.

2.Resin Encapsulation

The encapsulation process is designed to stabilize the sample and reduce deformation.A suitable resin material is used to fill the through holes and secure the sample plate. This step ensures that the walls of the holes to be observed and the plate are firmly clamped during the subsequent milling process to avoid distortion of the copper layer due to stretching.

3.Grinding process

Using sandpaper on a high-speed turntable, the sample is ground by its cutting force to the central cross-section of the through-hole, i.e. the plane where the center of the circle is located, in order to accurately observe the cross-section of the hole wall. A consistent direction of grinding should be maintained while grinding:

First, use 240# sandpaper to roughly grind the sample until it reaches the open position of the through-hole (a moderate amount of water is required to cool down and lubricate the grinding process).

Then switch to 600# sandpaper,grinding to the depth of 1/3 of the hole, and timely correction of the grinding process deviation.

Then use 1200# sandpaper to grind to 1/2 of the hole depth until the preset indicator line appears, and continue to correct the deviation.

Finally,the rough surface is removed by sanding with 2500# sandpaper to ensure that the desired smoothness is achieved at 1/2 of the hole depth.

4.Polishing operation

In order to clearly show the details of the slices,fine polishing is necessary to remove the scratches left by the sandpaper. The polishing procedure is as follows:

Prepare the polishing solution by mixing the polishing powder with water (about 4-5 spoons of polishing powder in 0.5 liters of water, shaking for 1-2 minutes).

Wet a polishing flannel and apply the polishing solution evenly to the flannel.

Keep the polishing direction in line with the direction of the holes and carry out the polishing process for 1-2 minutes.

5.Light Etching

The ratio of light etching solution is: 5-10CC ammonia + 45CC pure water + 2-3 drops of hydrogen peroxide. After the polished surface is washed and dried, it can be lightly etched. Light etching helps to differentiate between the various layers of the metal and its crystalline state. Using a cotton swab dipped in the light etching solution, gently rub the surface of the section for about 2-3 seconds and then dry it immediately to avoid oxidizing and discoloring the copper surface. A good light etch will give a vibrant copper color.

PCB microsectioning technology is widely used in the PCB and SMT industry, it can effectively monitor the intrinsic quality of the product, find out the truth of the problem, and assist in problem solving. It is suitable for PCB quality inspection and process improvement, electronic component structure analysis, PCBA soldering reliability assessment, solder joints on the tin pattern and defect detection.