SMT solder paste quality and testing

Abstract: With the development of electronic packaging to high performance, high density and miniaturization, the materials and technologies of solder paste are extremely important. This article discusses solder paste quality, solder carrier requirements, and basic performance testing of solder pastes for surface mount technology (SMT) solder pastes.

Keywords: Solder paste, solder powder, flux carrier

1 Introduction

Solder paste is a paste-like stable mixture composed of an alloy solder powder, a flux carrier, and the like. In the surface mounting technology, it plays a role in the fixing of components, promoting solder wetting, removing oxides, sulfides, trace impurities and adsorption layers, protecting the surface from re-oxidation, and forming a solid metallurgical bond. Solder paste printing is the first process of SMT. It affects subsequent patch, reflow, cleaning, testing and other processes and directly determines the reliability of the product. According to statistics, 72% of defects and failures of electronic products are related to solder paste, so the performance of the solder paste is crucial for SMT. With the rapid development of fine pitch (FPT), ball grid array (BGA), no-clean (NC), 0201 and other technologies, as well as restrictions or prohibitions on certain materials that damage the environment and health, the composition of the solder paste And performance requirements are getting higher and higher. Under the restraint and promotion of market, environmental protection, and legal factors, research and development of solder paste have become more and more in-depth at various domestic and foreign organizations, research institutes, and companies.

2 alloy welding powder

The key performance parameters for solder powders are shape, size distribution, and oxygen content, which in turn depend on milling technology. Its manufacturing methods include atomization (such as centrifugal atomization, ultrasonic atomization, multi-stage rapid cooling, etc.) and chemical electrolytic deposition [1]. We use a simple fluid vacuum spray method. The basic principle is: In the vacuum condition, the Sn63Pb37 alloy solder rod is melted by induction heating, and then the molten metal flow is atomized into small metal droplets by high-speed and high-pressure jet nitrogen gas. , and then rapidly solidified into a powder in a cooling medium, and finally classified and collected. The cooling rate of the atomization method is extremely fast, which greatly reduces the segregation of the alloy composition, increases the solid solution capacity of the alloy, and makes the formed powder uniform and fine. Due to the protective atmosphere, the oxygen content is low. This method also has the advantages of high ball rate, small size distribution range, and small pollution. The solder powder morphology prepared by different methods is shown in Figure 1 (a) to (e). The solder powder in Figure 1(a) is porous and porous and cannot be used. The shape of the solder powder is preferably spherical or ball-like, as shown in Fig. 1(f)[2]. The spherical solder powder has a small specific surface area and low energy, and is not easily oxidized during manufacture, storage, and printing, and does not block the mesh during printing. Oxidation of solder powder can lead to poor solderability, bridging, solder balls and other defects. In Figure 1(b), the size of the solder powder is unevenly distributed, there is a bond between the ball and the ball, the shape is irregular, and the surface of the ball is not smooth, there are "small satellite" particles and holes, see Figure 1 (d), also Out of service. The surface defects of the ball will cause welding problems such as inconsistent speed of heating during welding, residual gas in the hole, splashing of the solder, and formation of beads. According to the Stokes formula F=6pahv (F is the force of a spherical particle with a radius of a moving in a medium with a viscosity of v in a medium of viscosity h), the change in the size distribution directly affects the viscosity and rheological properties of the solder paste. This in turn affects the print quality. Figure 1(c) shows a more regular spherical solder powder, but there is still some adhesion. Figure 1(e) is a regular solder powder surface morphology in which black is a tin-rich phase and a bright region is a lead-rich phase. Figure 1 (f) is AMT's -325/+500 mesh regular spherical solder powder. SMT solder paste commonly used solder powder is a smooth sphere, size distribution of 20 ~ 75 microns, oxygen content of less than 0.3%.

3 flux carrier

No-clean requirement The flux carrier maintains the traditional solder paste function, but also has the characteristics of good volatility, less post-weld residue, no corrosion, and hard and inert film. The proportion of the flux carrier in the solder paste is generally 10% to 20%, and the volume percentage is 50 to 60%. As a solder carrier, it functions as a binder, flux, rheology control agent, and suspending agent. It consists of film-forming substances, solvents, activators (surfactants, catalysts), corrosion inhibitors, stabilizers, antioxidants, thixotropic agents, and the like. In order to achieve the no-clean effect, it is recommended that solids, solvents, and active agents account for about 25%, 50%, and 10% of the flux carrier, respectively. The solids are made of thermosetting resin and artificial synthetic resin. After welding, a hard and transparent film is formed on the surface of the solder joint. In general, a mixture of polyols should be used as a solvent. It is recommended to use high (about 230°C) low (about 160°C) boiling point, high (30~1000 cps) low (3~20 cps) viscosity alcohols to prepare a certain viscosity solvent, So that it has a corresponding evaporation rate over a wide temperature range, and the evaporation from 150 to 220 °C should be slow to fast. For low-boiling alcohols, due to better volatility, it can easily cause the solder paste to lose its viscosity and shorten its working life. High-boiling alcohols have a thickening and “moisturizing” effect, which can improve the working life and printing performance, but it is easy to absorb moisture. Volatilization is also incomplete. The active agent uses an organic weak acid instead of the traditional halogen-containing active agent in order to achieve the characteristic of low corrosion after welding. To compensate for the lack of activity, dicarboxylic acids having a molecular structure with an alkyl group and a hydroxyl group may be used. Thermal analysis of the solder paste with 9% of the flux carrier mass (Fig. 2) found that it evaporates rapidly between 100 and 150 °C and also volatilizes around 212 °C. It shows that the different components in the preheat zone (125~150°C) and the welding zone respectively play the role of cleaning and activating the surface to be welded, and then volatilize, meeting the typical reflow temperature process curve requirements in the industry. At 220°C, the weight percentage is only 95%, the residue is less, and it is a hard, transparent film that does not require cleaning.

4 basic performance test

4.1 Viscosity and its properties

The viscosity of the solder paste is mainly related to the powder content, powder size, and flux viscosity [3] (see Table 1). The viscosity requirements vary with the application method. Viscosity is too high and it will stick to the mesh; too low to be conformal and unable to adhere to components. Solder paste is a pseudoplastic fluid with thixotropy. Its viscosity changes with time, temperature, shear strength and other factors. According to IPC-TM-650 (2.4.34) test method, at 25±1°C, use NDJ-7 type rotary viscometer to continuously rotate at 7.5 rpm for 2 minutes to stabilize the readings. Several commonly used imported solder pastes The viscosity is in the range of 600~730Pa.s, suitable for stencil printing. Since the flux carrier material contains many hydroxyl groups, alkyl groups, and carboxyl groups, the hydrogen bond is heavy. By adding a certain amount of hydrogenated castor oil to break the hydrogen bond during stirring and shearing, the thixotropy of Fig. 3 can be achieved [4] (cutting Become thinning effect. Thixotropic agents are generally controlled to about 7% to facilitate mixing, missing prints, leveling, slump resistance, and adhesive elements.

Table 1 Solder paste metal content, viscosity, size distribution and use

Metal content Viscosity

(Pa.s) Size

(mm) Main use

90% 600-1000 40-75 General stencil printing

90% 400～600 20-36 fine pitch screen printing

85% 400～600 20-45 General screen printing

80% 300～400 20-45 Dosing Dispenser Injection

75% 200-300 40-75 needle transfer drop

Figure 3 Viscosity changes during solder paste use

4.2 Collapse

After the solder paste is printed, under certain conditions of temperature and humidity, due to the effects of gravity and surface tension, the pattern collapses and expands from the initial boundary to the outside world. This slump can cause the solder paste to flow, resulting in pin-to-pin bridging defects during the soldering process. According to IPC-TM-650 (2.4.35) using IPC-A-21 pattern, cold collapse and hot collapse tests were performed. See Figure 4(a) for the slump in the environment of 25±5°C and RH(50±10)% for 10 minutes. Then it was placed on a 150±10°C hot plate for 10 minutes, and it collapsed as shown in Figure 4(b). Figure 4 (a) Only between adjacent patterns with margins of 0.06 mm, Figure 4 (b) shows only bridges between 0.06 mm and 0.10 mm, indicating good resistance to cold collapse and hot collapse, mainly due to the use of thermosets. Resins and surface tension are the major causes of alcohols.

4.3 fine pitch printing

Fine-pitch printing requires a solder powder size distribution of 20 to 36 μm, which will affect the uniformity and resolution, and even block the mesh. At the same time, it must have good conformality to prevent overflow and collapse. FIG. 5 is a print pattern with a pitch of 0.4 mm and a thickness of 0.2 mm. FIG. 5( a) is a scanning pattern immediately after printing, and FIG. 5( b) is a case where FIG. 5( a) is held at 150° C. for 10 minutes. Both are even and complete, with good resolution, and there are no phenomena such as spreads and bridges.

4.4 solder ball

The formation of solder balls is closely related to the oxidation of solder powder, flux activity, solder powder size uniformity, solder paste moisture absorption, and impurities. Figure 6 is a solder ball test according to IPC-TM-650 (2.4.43). Fig. 6(a) shows a scan pattern immediately after printing on a non-wetting ceramic substrate; Fig. 6(b) shows a case where the no-clean solder paste is held at 208C for 20 seconds. Because of the surface tension and the wetting effect between the flux carrier and the solder powder, the “retract” effect is good, so the color is light; FIG. 6( c) is the corresponding situation of the ordinary solder paste, and there are more residues, so the color is darker; FIG. 6 ( The condition of d) is completely unqualified. It is mainly placed in the air for 6 hours. The formation of large beads is caused by volatilization and moisture absorption.

5 Conclusion

In short, the solder paste seems to be commonplace and plays an important role in the electronic packaging industry. Design, preparation, application and management personnel should pay full attention to it. The development and development of new and high-quality solder pastes and the active optimization of quality and technology are of positive significance to China's electronics assembly industry.