Solder paste printing technology and process parameter setting (1)

Abstract: With the high density of electronic assemblies and the miniaturization of components, fine-pitch leads have made higher demands on solder paste printing. In order to ensure product quality and develop a reasonable printing process, it is necessary to discuss the solder paste printing technology and process parameter settings.

Keywords: solder paste screen printing stencil printing rheological printability

1 Introduction

Surface mount technology (SMT) has become the most common technology in today's electronic assembly technologies. Solder paste printing is one of the key processes in the basic SMT process. Its quality directly affects the quality and efficiency of SMD assembly. With the high density of electronic assemblies and the miniaturization of components, fine-pitch leads place higher demands on solder paste printing. In order to ensure product quality and develop a reasonable printing process, it is necessary that the average size of the solder paste printing technology particles should not be greater than 1/3 of the screen mesh size. And process parameter settings are discussed.

2. Solder paste printing technology

In SMT production, there are two techniques for depositing solder paste onto the PCB pad: one is the printing technology based on screen and metal stencil printing templates. It is a widely used solder paste coating method in industrial manufacturing. Suitable for mass production. Another injection-coating technique for applying solder paste to injection systems is suitable for small batch production. This kind of system is controlled by the computer and can accurately deposit the solder paste. Compared with the former method, it can prevent solder paste from being wasted.

2.1 Screen Printing Technology

The screen printing mechanism consists of a PCB positioning system, a doctor blade system and a screen plate. The stencil is a key component of the screen printer and consists of a screen frame, a screen, and a mask pattern. The general mask pattern is made on the screen by an appropriate method, and the screen is tightened on the screen frame. Screen printing works as shown in Figure 1.

The role of the frame is to support and tension the screen, keeping the screen parallel to the workbench of the PCB clamping mechanism. Common frame materials include wood, anodized aluminum alloy, and stainless steel. In the premise of meeting the strength requirements, light alloys should be selected as far as possible in order to facilitate the operation. According to the stretching method, the frame has two types of fixed frame and self-tensioned frame. The fixed frame is fixed on the frame of the screen, commonly used adhesive fixing method, the screen is stretched directly to the frame with a stretching machine, so that the tensioned position of the frame and the frame form a whole, with the help of "screws "Adjust" or "stick frame" from the stretch net. This method is suitable for a variety of occasions, less batch printing.

The screen is the carrier of the mask and is also an important tool for controlling the amount of solder paste printing. It can determine the accuracy and quality of solder paste printing. The common material for the screen is stainless steel or single fiber polyester. In general, a layer of sensitized emulsion is applied to the screen to dry it into a photosensitive film. The negative film was then placed against the photosensitive film and exposed to ultraviolet light. The exposed portion becomes a permanent coating, and the unexposed portion is dissolved by the developer. In this way, a leak is formed in the adhesive portion where the solder paste is to be deposited, and the screen is formed after drying. The diameter of the wire and the size of the opening depend on the number of meshes, usually 80. The diameter of the filament and the thickness of the latex mainly determine the thickness of the deposited solder paste.

In general, the screen is only suitable for applications where the solder joint height is 300 μm or more. The solder paste has a viscosity of 450,000 to 700,000 CPS (measured with a Brookfield Viscometer). The scraper is recommended to use a rubber or polyurethane with a hardness of 70-90. Scraper, the average size of the alloy powder particles in the solder paste should not be greater than 1/3 of the screen mesh size.

Screen printing can not easily see the pads on the substrate, positioning with the pads on the substrate is more difficult and time consuming. Non-contact printing screen printing, prone to over-flow solder paste defects (Figure 2). Screen printing usually requires slower speeds than stencil printing and larger scratching gaps. At the same time, the use of lower viscosity pastes is desirable for ease of printing. In addition, when printing, it is necessary to adjust the screen and the work frame in parallel and keep a distance of 0.5mm. When the parallelism is within 0.05mm, the wiping gap can be adjusted to 0.76mm.

2.2 Stencil Printing Technology

Template stencil printing is a direct printing technique. It uses metal leaky stencils instead of screens in screen printing machines. The so-called leaky template is chemically etched on a piece of metal or laser stencil machine. Unlike a screen, the opening does not prevent the flow of solder paste. Therefore, the leaky template allows 100% of the solder paste to pass through. The net can only pass about 50% of the solder paste.

According to the etched material, the stencils can be divided into selective etched mesh/latex stencils, all-metal (stainless, brass, or nickel) leaky stencils, and flexible metal leaky stencils.

Fine-pitch solder paste printing usually uses metal stencil printing. Therefore, the drain hole of the drain plate controls the solder paste on the pad and print quality. The thickness of the stencil determines the thickness of the printed solder paste. The opening size of the stencil determines the area and shape of the solder paste pattern. Thicker stencils are not conducive to solder paste release, and are also prone to bridging due to thick solder paste. Too thin a template may affect the quality of the weld due to insufficient solder paste.

In general, the drain template is only suitable for applications where the solder joint height is 100-300μm or more, and the solder paste viscosity is 750000-1300000CPS (measured with a Brookfield Viscometer). A metal scraper or rubber with a hardness of 90 is recommended. In polyurethane scrapers, the average size of the alloy powder particles in the solder paste should not be greater than 1/3 of the screen mesh size. The opening ratio of the template is 1.5:1, and the printing area ratio PAR should be greater than 0.66, where PAR = L × W / 2 × (L + W) × T, L is the opening length, W is the opening width, T The height of the opening (ie, the thickness of the template). For the specific design of the template, see IPC-7525.