PCB Tech - Analysis of Green Paint Problem on PCB Board Surface

In order to speed up the printing speed and increase the production capacity when screen printing the green paint on the PCB surface, the operator will add a thinning solvent to dilute it in addition to the hardener. Common ones include "anti-white water" (BCS; Buty Cellosolve) and other solvents). Generally, suppliers of green paint on PCB boards recommend that only 15ml or 1-3% (weight ratio) be added to one kilogram of ink. After the viscosity is lowered, it is not only easy to push the print, but also at the processing site where the air conditioner cooperates with the air blower, the ink pile on the mesh cloth is also less likely to dry out.

After adding the hardener and diluent to the ink to be used when the can is opened, in order to make the overall viscosity uniform (150PS is appropriate), use a propeller-type or L-shaped paddle (ink tank rotation) agitator, or jump it up and down. Stir slowly with a shaker for more than half an hour. Sometimes in order to speed up work, it is inevitable to shorten the stirring or speed up the scraper, or add excessive solvents, or vigorously stir and not stand for defoaming, there will often be air bubbles on the edge of the line after the screen printing is hardened. Such residual bubbles are very obvious. Often rejected by QA or customers.

The PCB board will also produce air bubbles in the field screen printing. If the printing is too fast, air bubbles will be generated. The "leeward side" of the dense parallel line with the direction of the squeegee is also easy to trap gas. In severe cases, the "windward side" will also have air bubbles. bubble. Generally, the thickness of the shoulder on the leeward side is very thin, and even the lack of ink will appear on the substrate with the same spacing.

There are fewer bubbles in the dense line area. The diagonal web and the method of sticking the ink-blocking dot screen should be able to solve some of the air bubble problem. Bubbles often occur on the edge of the dense line area, and rarely on the open board surface. The magnified photos of the photopic and scotopic sections are used to support the above statement.

All are 100X photopic photography. Although the green paint on the PCB board surface is not green, it can still be clearly distinguished from other materials.

Enlarged 100X and 200X dark vision photography, the S/M layer is clearly green.

Generally, in the coating of liquid photosensitive PCB surface green paint (LPSM) by screen printing, bubbles often appear between fine lines. In the early days, when the formula of LPSM was not yet mature, a "dry film" photosensitive solder mask was also popular. In order not to be cut by the drop on the thick circuit, the thickness of this kind of dry film is about 4mil. The construction method is to apply a vacuum method to heat and press on the surface of the board, naturally no bubbles will be generated. The picture below is a slice made by the author 13 years ago, which is a DuPont product Vacrel 8040 for sodium carbonate resolution. The adhesion and surface flatness of this solder mask film are very good, but unfortunately it has already been eliminated from the mass production line due to poor adhesion, troublesome construction, and high cost.

However, the recent rise of thin P-BGA substrates requires that the thickness of the green paint on the PCB board surface be above 1 mil, and the plug holes must be flat and free of bubbles to prevent the "popcorn" downstream disasters from hiding in the dead corners. This "green paint on dry film PCB surface" of vacuum construction seems to have a chance to reproduce the world again.

The green paint on the surface of the 200X circuit PCB board is constructed by the electrostatic automatic spraying method, which not only reduces air bubbles, but also the thickness of the line shoulder is above 0.8mil, which is far beyond the specification. But the paint consumption is quite large.

The importance of the thickness of the green paint on the PCB surface

The two main tasks of the green paint on the PCB board are solder protection and wire protection. Because the green paint on the PCB board will be shipped with the PCB board, its quality is directly related to the downstream organization, and it also has a profound impact on the final product. In particular, the new BGA and CSP types not only need to eliminate air bubbles, but also require the thickness of the green paint on the PCB surface. In this way, the circuit board can be used in various harsh environments to prevent the wires from being infringed by external pollutants, and it will not cause a crisis in reliability (Reliability).

The thickness of the green paint on the PCB board surface has been clearly stipulated in the IPC-SM-840B specification. The minimum thickness for Class 3 high-reliability boards is 0.7mil. For Class 2 commercial boards (such as computers and computer peripheral products), The lower limit thickness is set at 0.4mil. But when the 840 was revised from the B edition to the C edition (1996.6), the original thickness specification was cancelled instead. The author's view on this cancellation should be based on the consideration of the increase in signal transmission speed. According to Maxwell's formula, the propagation speed of electromagnetic waves is proportional to the speed of light (C) and inversely proportional to the square root of the dielectric constant of the medium in which the waves propagate (Dieleatric constant; also known as the Relative Permitivity).

And the relative permeability of air and vacuum εr is the lowest, both are set to 1, so the speed of the electric wave in the air is equal to the speed of light. Generally, the εr of the green paint on the PCB board surface is about 3.0 (data measured at a frequency of 1Mz), so when the green paint on the PCB board surface becomes thinner (which can be close to air), the transmission of a square wave signal (also a kind of electromagnetic wave) Also faster. Therefore, the thickness of the green paint on the PCB surface of the high-speed computer PCB board is no longer emphasized.

However, when the thickness of the green paint on the PCB surface is insufficient, especially for power lines with larger currents and thicker line widths, the thickness of the two shoulders of the line should still be required to be 0.4mil, so as to avoid problems in bad environments and power-on work. Accelerate corrosion. After half a year of assembling the finished product, the power cord passed through the green paint on the PCB surface with insufficient thickness and was bitten by foreign objects in the harsh external environment. The thickness of the green paint on the PCB board surface on the circuit can be seen in the slice.

The green paint on the PCB surface is too thin to cause the copper wire to be corroded in the 10X picture, and the PCB board surface with the thick line is coated with green paint. The green paint on the PCB board surface is printed by scraping ink from left to right. Pay attention to the thicker situation before scraping than after scraping.

In the early PCB tin melting board, after downstream assembly, the molten tin layer on the top of the original wire flows again, causing the green paint on the PCB board surface to warp. On the right is the green paint printed on the bare copper wire (SMOBC) of the PCB board, and it is not affected by high heat.