Surface treatment before solder mask

I. Introduction

At present, in the printed circuit board industry, there are more specifications to regulate the solder mask printing, from the previous IPC-SM-840C (the Amendment 1 of 1996.1 C and 2000.6), but the pre-welding treatment is also required, many The downstream customers began to re-regulate the thickness of the copper foil, and there has been no change from the current regulations to the current 1.1mil or more. Here, the work on the pre-weld treatment needs to be more stringent, and of course, the line width (from 0.3mm to 1mil of the BGA). More and more detailed, there is a big relationship, please look at the following sections, it is not difficult to understand the requirements:

Fig.1 and Fig.2: After the surface treatment of the solder mask is described, one side of the circuit is thin and the thickness is not uniform, resulting in an increase in impedance, which is extremely unfavorable and very unstable for the pulse wave of the panel signal. In today's world of products, high-current electrical products continue to increase, so some downstream customers began to pay attention to the thickness of copper foil, if necessary, must also implement high-voltage-resistant test;

Figure 3: The problem is that the production of bubbles is easy to occur at the corner during welding, and the reason for this is as follows: First, because the water at the corner of the line is not dried in time during surface treatment, the relationship between printing speed and ink . If bubbles occur at the corners on the side of the circuit, there will be peeling of the solder mask during high temperature soldering. This is why some printed circuit boards tend to have the solder mask peel off at the edges of the circuit during high temperature wave soldering.

Therefore, the process before soldering prevention has a considerable influence on the circuit and solder resist printing.

Second, the role of wire and welding

To understand the effect of pre-solder treatment on the printed circuit board, start here to understand the relationship between them, and there are two main components that make up the board:

First, the line: This is the part of the circuit board used to transmit signals. In different circuit boards, the materials used in the line are not the same, including: copper, silver, gold, nickel, carbon ink, etc. The most widely used is mainly copper. In the development, there are copper through holes, secondary copper, silver through holes, chemical nickel gold, carbon ink through holes, and so on.

Second, solder mask: It is mainly used to protect the circuit. In some articles, it is called green paint. Why is it emphasized here that it is called solder resist printing instead of green paint? Most of the solder masks we see in the market are green, but since SONY's SS-00259 specification in 2003 and European WEEE and ROHS were issued worldwide, the demand for harmful substances has been further improved. In particular, PBB, PBDE, and related halogen regulations have attracted the attention of many businesses. They have begun to use environmentally-friendly inks, followed by blue solder mask inks.

In order to cover the surface of the solder mask with high adhesion, the surface must be roughened, and the surface of the circuit must be dried in time to prevent oxidation, which will cause the solder mask to fall off after drying.

Third, the impact of surface treatment on printing

The general process of surface treatment is: heavy brush à light brush à washing à dry surface.

The copper foil or non-woven brush is used to grind the copper foil surface. The main parameter indicator is the grinding width. When the width is too large, copper powder is easily produced, and a small amount remains in the circuit. After soldering, slight short circuit occurs. The performance of the tester is general and will be difficult to find, causing direct loss to downstream customers. In addition, the phenomenon as shown in Figure 1 above will also be formed; when the width is too small, the surface treatment is incomplete, that is, there are impurities on the copper foil surface. The denseness of the solder layer can easily cause the solder resist to fall off during wave soldering.

The temperature and the speed of the surface drying also have quite important factors. It is necessary to set the parameters according to the size of the layout. In general, visual observation of the presence of no vapor on the surface of the substrate is a criterion. If water droplets or moisture remain on the board surface, first, the ink cannot adhere to the board during printing, and it is easy to peel off. Second, when the water adheres to the screen or the film, the pattern is abnormal (water patterns, ink diffusion, etc.) ).

Finally, the problem of pre-press processing has appeared in the industry. However, in the acceptance criteria of IPC, there are few definitions of undesirable phenomena. Therefore, we would like to discuss with you the impact on electrical performance.

Source: PCB Network City

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