Analysis of SMT solder paste quality and testing
Keywords: solder paste, solder powder, flux carrier
The Quality and TesTIng of Solder Paste for SMT
Abstract: Because of the development of high density, high performance and miniaturizaTIon in electronic packaging, solder paste material and technology become more and more important. This paper discussed the quality of SMT solder paste, including powder preparaTIon, flux vehicle formulaTIon and basic testing.
Keywords: solder paste, solder powder, flux vehicle
Solder paste is a paste-like stable mixture composed of alloy solder powder, flux carrier, and the like. In the surface mounting technology, it acts as a bonding component to promote solder wetting, remove oxides, sulfides, trace impurities and adsorption layers, protect the surface from reoxidation, and form a strong metallurgical bond. Solder paste printing is the first process of SMT, which affects the subsequent process of patching, reflow soldering, cleaning, testing, etc., and directly determines the reliability of the product. According to statistics, 72% of electronic products' defects and failures are related to solder paste, so the performance of 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 environmental and health-damaging materials, the composition of solder paste And performance requirements are getting higher and higher. Under the constraints and promotion of market, environmental protection and legal factors, various organizations, scientific research institutions and companies at home and abroad have been increasingly researching and developing solder paste.
2 alloy welding powder
The key performance parameters of solder powder are shape, size distribution and oxygen content, which in turn depend on the milling technology. The manufacturing methods mainly include atomization methods (such as centrifugal atomization, ultrasonic atomization, multi-stage rapid cooling, etc.) and chemical electrolytic deposition . We use a simple fluid vacuum spray method, the basic principle is: under vacuum conditions, the Sn63Pb37 alloy solder bar is melted by induction heating, and then the metal stream is crushed by high-speed high-pressure jet nitrogen to atomize into small metal droplets. Then, it is rapidly cooled and solidified into a powder in a cooling medium, and finally classified and collected. The atomization method has a very fast cooling rate, greatly reduces the segregation of the alloy composition, increases the solid solution ability of the alloy, and the formed powder is uniform and fine. Due to the protective atmosphere, the oxygen content is low. This method also has the advantages of high sphericity, small size distribution range, and low pollution. The appearance of the solder powder prepared by different methods is shown in Figures 1(a) to (e). In Fig. 1 (a), the solder powder is loose and porous, and cannot be used. The shape of the solder powder is preferably spherical or spheroidal, as shown in Figure 1 (f) . Spherical solder powder has a small specific surface, low energy, is not easily oxidized during manufacturing, storage, and printing, and does not block the mesh during printing. Oxidation of the solder powder can cause defects such as poor solderability, bridging, solder balls, and the like. In Figure 1 (b), the size distribution of the solder powder is uneven, the ball is bonded to the ball, the shape is irregular, and the surface of the ball is not smooth. There are "small satellite" particles and holes, as shown in Figure 1 (d). Out of service. Surface defects of the ball may cause welding problems such as inconsistent heating rate during welding, residual gas in the holes, and solder splashing to form solder beads. It is known from the Stokes formula F=6pahv (F is the force of the spherical particle with a radius a moving at a velocity v in a medium with a viscosity of h), and the change in the size distribution directly affects the viscosity and rheological properties of the solder paste. This in turn affects the quality of the print. Figure 1 (c) is a relatively regular spherical solder powder, but still has some adhesion. Figure 1(e) shows the surface morphology of a regular solder powder in which the black is a tin-rich phase and the bright region is a lead-rich phase. Figure 1 (f) is a regular spherical solder powder of -325/+500 for AMT. The solder powder commonly used in SMT solder paste is smooth spherical, with a size distribution of 20 to 75 microns and an oxygen content of less than 0.3%.
(a) Extremely irregular solder powder (b) Irregular solder powder (c) Regular solder powder
(d) Surface hole of A ball in b (e) Regular solder powder surface (f) Regular spherical solder powder
3 flux carrier
No-cleaning requires the flux carrier to maintain the function of the traditional solder paste, but also has the characteristics of good volatility, less residue after welding, no corrosion, and the film is hard and inert. The specific gravity of the flux carrier in the solder paste is generally 10% to 20%, and the volume percentage is 50 to 60%. As a solder powder carrier, it functions as a binder, a flux, a rheology control agent, and a suspending agent. It consists of a film-forming substance, a solvent, an activator (surfactant, catalyst), a corrosion inhibitor, a stabilizer, an antioxidant, a thixotropic agent and the like. In order to achieve the no-clean effect, it is recommended that the solid matter, the solvent and the active agent account for 25%, 50%, and 10% of the flux carrier, respectively. The solid material is made of a thermosetting resin and a synthetic resin, and forms a hard and transparent film on the surface of the solder joint after soldering. Generally, a mixture of polyols should be used as a solvent. It is recommended to use a high (about 230 ?° C) low (about 160 ?° C) boiling point, high (30 ~ 1000 cps) low (3 ~ 20 cps) viscosity of the alcohol to prepare a certain viscous solvent, It has a corresponding volatilization rate over a wide temperature range, and the volatilization between 150 and 220 ?° C should be slow to fast. For low-boiling alcohols, due to better volatility, it tends to cause the solder paste to dry out and lose its viscosity, which shortens the working life. The high-boiling alcohol has thickening and "moisturizing" effect, which can improve working life and printing performance, but is easy to absorb moisture. Volatilization is not complete. The active agent uses an organic weak acid instead of a conventional halogen-containing active agent to achieve low post-weld corrosion. In order to compensate for the insufficient activity, a dicarboxylic acid having a molecular structure having an alkyl group and a hydroxyl group can be used. A thermal analysis of the solder paste with a flux carrier content of 9% (Fig. 2) revealed that it volatilized rapidly between 100 and 150 ?° C and also had a large volatilization near 212 ?° C. It shows that the different components in the preheating zone (125~150 ?°C) and the welding zone play the role of cleaning and activating the surface to be welded, and then volatilize, which meets the requirements of the typical reflow soldering temperature process curve in the industry. At 220 ?° C, the weight percentage is only 95%, the residue is less, and a layer of hard and transparent film, no need to clean.
4 basic performance test
4.1 Viscosity and its characteristics
The viscosity of the solder paste is mainly related to the powder content, powder size, and flux viscosity  (see Table 1). The viscosity requirements vary depending on the application method. The viscosity is too high and it will stick to the mesh; too low to conform to the shape and the components cannot be bonded. Solder paste is a pseudoplastic fluid with thixotropic properties. The viscosity changes with time, temperature, shear strength and the like. According to the IPC-TM-650 (2.4.34) test method, the NDJ-7 rotary viscometer is continuously rotated at 7.5 rpm for 2 minutes at 25?±1 ?°C, and the reading is stabilized. Several commonly used imported solder pastes are used. 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 very heavy, and the thixotropy of Figure 3 can be achieved by adding a certain amount of hydrogenated castor oil to break the hydrogen bond during stirring and shearing  Become thinner) effect. The thixotropic agent is typically controlled at around 7% to facilitate agitation, smearing, leveling, slumping, and cementing.
Table 1 solder paste metal content, viscosity, size distribution and use
Metal content viscosity
(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 quantitative dispenser injection
75% 200-300 40-75 needle transfer drip
After the solder paste is printed, under certain temperature and humidity conditions, due to gravity and surface tension, the pattern collapses and spreads from the initial boundary to the outside. This slump can cause solder paste to overflow, resulting in pin-to-pin bridging defects during soldering. According to IPC-TM-650 (2.4.35), the IPC-A-21 pattern was used for cold collapse and thermal collapse tests. The collapse of the residence at 25 ?± 5 ?° C, RH (50 ?± 10)% for 10 minutes is shown in Figure 4 (a). It was then placed on a metal plate at 150 ?± 10 ?° C for 10 minutes, and its collapse was shown in Figure 4 (b). Figure 4 (a) Only between adjacent patterns with a margin of 0.06 mm, Figure 4 (b) only has a bridge between 0.06 mm and 0.10 mm, indicating good resistance to cold collapse and thermal collapse, mainly due to thermosetting. The reason why the resin and the surface tension are relatively large alcohols.
4.3 fine pitch printing
Fine-pitch printing requires a solder powder size distribution of 20 to 36 microns, otherwise it will affect uniformity and resolution, and even block the mesh; at the same time, it must have good shape retention to prevent overflow and slump. Figure 5 is a printed pattern having 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. The two are even and complete, the resolution is good, and there is no phenomenon such as spreading and bridging.
4.4 solder balls
The formation of solder balls is closely related to solder powder oxidation, flux activity, solder powder size uniformity, solder paste moisture absorption, and impurities. Figure 6 is a solder ball result tested in accordance with IPC-TM-650 (2.4.43). 6(a) is a scanning pattern just printed on a non-wetting ceramic substrate; and FIG. 6(b) is a case where the no-clean solder paste is held at 208 ?° C for 20 seconds. Because of the surface tension and the wetting effect between the flux carrier and the solder powder, the effect of "retracting" is good, so the color is shallow; Figure 6 (c) is the corresponding situation of the common solder paste, the residue is more, so the color is darker; Figure 6 ( The condition of d) was completely unqualified, mainly in the air for 6 hours, resulting in the formation of large particle beads due to volatilization and moisture absorption.
In short, solder paste seems to be commonplace, but plays a decisive role in the electronics packaging industry. Design, preparation, application, and management should be given full attention. The development of new high-quality solder paste, and the active optimization of quality and technology have positive significance for China's electronic assembly industry.