Reflow soldering of solder paste is the main board-level interconnection method used in the SMT assembly process. This soldering method combines the required solderability extremely well. These features include easy processing and wide range of SMT designs. Compatibility, high soldering reliability and low cost; however, when reflow soldering is used as the most important SMT component-level and board-level interconnection method, it is also challenged to further improve solderability. Above, the ability of reflow soldering technology to withstand this challenge will determine whether solder paste can continue to be the primary SMT soldering material, especially as the ultra-fine pitch technology continues to make progress. Below we will discuss several major issues that affect the improvement of reflow soldering performance. In order to stimulate the industry to study new methods to solve this lesson, we will briefly introduce each issue as follows:
Fixing of the bottom surface element
Double-sided reflow soldering has been used for many years. Here, the first side is printed cloth wiring, components are installed and reflowed, and then turned over to process the other side of the circuit board. In order to save more, this process is omitted. Instead of reflowing the first side, it reflows the top and bottom surfaces at the same time. A typical example is that only small components are installed on the bottom surface of the circuit board, such as chip capacitors and chip resistors, which are printed circuit boards (PCB). The design is more and more complicated, and the components installed on the bottom surface are getting bigger and bigger. As a result, the component falling off during reflow becomes an important problem. Obviously, the phenomenon of component shedding is due to the insufficient vertical fixing force of the molten solder during reflowing, and the insufficient vertical fixing force can be attributed to the increase in the weight of the component, the poor solderability of the component, the wettability of the flux or the insufficient amount of solder, etc. , Among them, the first factor is the most fundamental reason. If the components still fall off after the following three factors are improved, SMT adhesive must be used. Obviously, the use of an adhesive will make the self-alignment effect of the component worse during reflow.
Not fully welded
Undersoldering is the formation of solder bridges between adjacent leads. Generally, all the factors that can cause solder paste slump will lead to insufficient soldering. These factors include: 1. The temperature rise rate is too fast; 2. The thixotropy of the solder paste is too poor or the viscosity of the solder paste is restored too much after shearing. Slow; 3. The metal load or solid content is too low; 4. The particle size of the powder is too wide; 5. The surface tension of the flux is too small. However, slump does not necessarily cause under-soldering. During reflow, the melted under-soldered solder may be disconnected under the push of surface tension, and the phenomenon of solder loss will make the under-soldering problem more serious. In this case, the excess solder accumulated in a certain area due to the loss of solder will cause the molten solder to become too much and not easy to be disconnected.
In addition to the factors that cause solder paste slump, the following factors also cause common reasons for underfilled soldering: 1. Relative to the space between the solder joints, the solder paste is deposited too much; 2. The heating temperature is too high; 3. , The solder paste is heated faster than the circuit board; 4 the flux wetting speed is too fast; 5 the flux vapor pressure is too large; 6, the solvent composition of the flux is too high; 7, the softening point of the flux resin is too low.
Keep wetting
The intermittent wetting of the solder film refers to the current smooth surface (1.4.5). This is due to the fact that the solder surface can adhere to most solid metal surfaces, and there are some hidden under the melted solder coating. Points that are not wetted, so when the surface is initially covered with molten solder, there will be continued wetting. The metastable molten solder covering layer shrinks under the action of the minimum surface energy driving force, and after a while, it gathers into separate small balls and ridge-like bald objects. Continued wetting can also be caused by the gas released when the part comes into contact with the molten solder. The water released due to the thermal decomposition of organic matter or the hydration of inorganic matter will generate gas. Water vapor is the most common component of these related gases. At the soldering temperature, water vapor has a strong oxidizing effect and can oxidize the surface of the molten solder film or the interface under certain surfaces (a typical example is on the boundary of the molten solder. Metal oxide surface). The common situation is that higher welding temperature and longer residence time will lead to more serious intermittent wetting, especially in the base metal. The increase of the reaction rate will lead to more violent gas release. At the same time, a longer residence time will also extend the time for gas release. The above two aspects will increase the amount of released gas, and the methods to eliminate the continued wetting phenomenon are: 1. Reduce the welding temperature; 2. Reduce the residence time of reflow; 3. Use a flowing inert atmosphere; 4. Reduce the degree of pollution.
Low residue
For reflow processes that do not need to be cleaned, in order to obtain decorative or functional effects, low residues are often required. Examples of functional requirements include "detecting and testing surfacing layers by flux residues tested in circuits and Perform electrical contact between the plug-in connector and the through-hole near the reflow soldering point.” More flux residue often results in excessive residue coverage on the metal surface to be electrically contacted, which will hinder the establishment of electrical connections. In the case of increasing circuit density, this problem has attracted more and more attention.
Obviously, low-residue solder paste without cleaning is an ideal solution to meet this requirement. However, the necessary reflow conditions associated with this make this problem more complicated. In order to predict the soldering performance of low-residue solder paste in different levels of inert reflow atmospheres, a semi-empirical model is proposed. This model predicts that as the oxygen content decreases, the soldering performance will improve rapidly, and then gradually stabilize The experimental results show that as the oxygen concentration decreases, the soldering strength and the wettability of the solder paste will increase. In addition, the soldering strength also increases with the increase of the solid content in the flux. The model proposed by the experimental data is comparable, and it strongly proves that the model is effective and can be used to predict the welding performance of solder paste and materials. Therefore, it can be asserted that in order to successfully use the cleaning process in the welding process For low-residue solder, an inert reflow atmosphere should be used.
gap
The gap means that no solder joints are formed between the component leads and the solder joints of the circuit board. Generally speaking, this can be attributed to the following four reasons: 1. Insufficient solder deposition; 2. Poor lead coplanarity; 3. Insufficient wetting; 4. Solder loss. This is caused by solder paste on pre-tinned printed circuit boards. Slump, lead wicking (2.3.4) or through holes near the solder joints, lead coplanarity problems are the new lighter weight 12 mil (um) pitch four-core flat integrated circuit ( QFPQuad flat packs) is a particular concern. In order to solve this problem, a method of pre-coating the solder joints with solder before assembly (9) is proposed. This method is to expand the size of the local solder joints and follow the drum The pre-covered area of the solder material forms a controllable local welding area, which compensates for changes in lead coplanarity and prevents gaps. The suction effect of leads can slow down the heating speed and allow the bottom surface to heat up more than the top surface. To solve this problem, in addition, the use of a slower wetting flux, a higher activation temperature or a solder paste that can delay melting (such as a solder paste mixed with tin powder and lead powder) can also minimize the wicking effect. Before finishing the circuit board with a tin-lead coating, covering the connection path with a solder mask can also prevent wicking caused by nearby vias.
Solder ball
Solder ball formation is the most common and most difficult problem. This means that the solder solidifies into pellets of varying sizes not far from the main solder pool during the reflow process; in most cases, these pellets are caused by It is composed of solder powder in the solder paste. The solder ball makes people worry that there will be problems such as short circuit, leakage and insufficient solder on the solder joints. With the development of fine pitch technology and soldering methods that do not require cleaning, people are more and more It is urgently required to use the SMT process without solder balling.
The reasons that cause solder balls (1, 2, 4, 10) include: 1. Oil stains caused by improper circuit printing process; 2. Excessive exposure of solder paste to an oxidizing environment; 3. Solder paste Excessive exposure to a humid environment; 4. Inappropriate heating method; 5. Heating speed is too fast; 6. Preheating section is too long; 7. The interaction between solder mask and solder paste; 8. Flux activity is insufficient 9. Too much oxide or contamination of solder powder; 10. Too many dust particles; 11. In a specific reflow treatment, inappropriate volatiles are mixed into the flux; 12. Solder caused by improper solder paste formulation Slump; 13. The solder paste is not fully restored to room temperature before opening the package for use; 14. The printing thickness is too thick, which leads to "collapse" to form tin balls; 15. The metal content in the solder paste is low.
Solder beading
Solder beading is a special phenomenon of solder balling when using solder paste and SMT process. Simply put, solder bead refers to those very large solder balls with (or no) tiny solder balls attached to them (11 ). They are formed around components with extremely low feet, such as chip capacitors. Solder beading is caused by flux degassing. In the preheating stage, this degassing action exceeds the cohesion of the solder paste. The degassing promotes the formation of isolated pellets of solder paste under low-gap components, which melt during reflow. The isolated solder paste came out of the component again and coalesced.
The reasons for welding beads include: 1. The thickness of the printed circuit is too high; 2. The solder joints and the components overlap too much; 3. Too much solder paste is applied under the components; 4. The pressure to place the components is too large; 5. The temperature rises too fast when heating; 6, the preheating temperature is too high; 7, the moisture is released from the components and the solder resist; 8, the activity of the flux is too high; 9, the powder used is too fine; 10, the metal The load is too low; 11, the solder paste slump too much; 12, the solder powder oxide is too much; 13, the solvent vapor pressure is insufficient. Perhaps the easiest way to eliminate solder is to change the shape of the stencil holes so that there is less solder paste between the low-foot components and the solder joints. Welding fillet welding lift
Soldering fillet lifting means that after wave soldering, the lead and solder fillet are completely lifted from the solder joints of the quad flat integrated circuit (QFP) with fine circuit spacing, especially near the corners of the component. The possible cause is the mechanical stress applied to the leads during the sampling and inspection before wave soldering, or the mechanical damage caused by the scanning when processing the circuit board (12). When sampling and testing before wave soldering, use a tweezer to pass through the leads of the QFP component to determine whether all the leads are soldered during reflow baking; the result is a misaligned solder toe, which can be Observing from top to bottom, if the heating of the bottom of the board causes a part of the secondary reflow at the interface between the welding zone and the fillet weld, then lifting the lead and fillet weld from the circuit board can reduce the internal stress. One way to prevent this problem is to conduct sampling inspections after wave soldering (rather than before wave soldering).
Tombstoning
Tombstoning means that one end of a leadless component (such as a chip capacitor or resistor) leaves the substrate, and even the entire component is supported on one end of it.
Tombstoning is also called Manhattan effect, Drawbridging effect or Stonehenge effect. It is caused by uneven lubrication at both ends of the reflow component; therefore, the unbalanced surface tension of the molten solder is exerted on both ends of the component. With the progress of SMT miniaturization, electronic components are becoming more and more sensitive to this problem.
Reasons for this situation: 1. uneven heating; 2. component problems: shape difference, too light weight, difference in solderability; 3. poor thermal conductivity of the substrate material, poor uniformity of the thickness of the substrate; 4. difference in heat capacity of the pad Larger, the solderability of the pad has a greater difference; 5. The uniformity or activity of the flux in the solder paste is poor, the thickness of the solder paste on the two pads is different, the solder paste is too thick, the printing accuracy is poor, and the position is misaligned Serious; 6. Preheating temperature is too low; 7. Post-installation accuracy is poor and component deviation is serious.
Ball Grid Array (BGA) poor ball formation
BGA ball formation often encounters defects such as under-soldering, misalignment of solder balls, missing solder balls, and insufficient amount of solder. This is usually caused by insufficient fixing force or insufficient self-setting force on the ball during reflow. Insufficient fixing force may be caused by low viscosity, high blocking thickness or high outgassing speed; and insufficient self-setting force is generally caused by weak flux activity or low solder volume.
BGA ball formation can be achieved by using solder paste alone or by using solder balls with solder paste and solder balls with flux; the correct and feasible method is to use the overall preform with flux or solder paste. The most common method seems to be to use solder balls together with solder paste, and the ball formation process using tin 62 or 63 ball bonding has produced excellent results. In the case of using solder for tin 62 or tin 63 ball soldering, the defect rate increases with the decrease of the flux viscosity, solvent volatility and pitch size, and also with the deposition thickness of the flux, the activity of the flux and the welding The dot diameter increases with the increase of the spot diameter. In the ball soldering system that uses solder paste for high-temperature melting, no solder ball leakage is observed, and its accuracy varies with the thickness of the solder paste and the volatility of the solvent, and the activity of the solder , The size and solderability of solder joints and the increase of metal load increase. When using tin 63 solder paste, the viscosity, pitch and reflow section of the solder paste have almost no effect on the spheroidization rate at high melting temperature. In the case of requiring the use of a conventional printing release process, easy-to-release solder paste is essential for the individual ball formation of the solder paste. The ball-forming process of integral pre-forming is also very promising. Reducing the thickness and width of the solder link is also very important to improve the success rate of ball formation.
Pore formation
Porosity is usually a problem related to welded joints. Especially when applying SMT technology to reflow solder paste, in the case of leadless ceramic chips, most of the large pores (>0.0005 inches/0.01 mm) are between the LCCC solder joints and the printed circuit board solder joints At the same time, in the fillet weld near the LCCC castle, there are only a few small pores. The existence of pores will affect the mechanical properties of the welded joint, and will damage the strength, ductility and fatigue life of the joint. This is because the growth of pores will coalesce into extensible cracks and cause fatigue. The pores will also increase the stress and covariance of the solder, which is also the cause of damage. In addition, the solder shrinks during solidification, the delamination of exhaust gas and the entrainment of flux when soldering plated through holes are also the cause of the porosity.
During the soldering process, the mechanism for forming pores is more complicated. Generally speaking, the pores are caused by the exhaust of flux entrained in the solder in the sandwich structure during reflow. (2, 13) The formation of pores is mainly caused by The solderability of the metallized area is determined, and changes as the flux activity decreases, the metal load of the powder increases, and the coverage area under the lead joint increases. Reducing the size of the solder particles can only slightly increase the porosity. In addition, the formation of pores is also related to the time allocation between the coalescence of solder powder and the elimination of fixed metal oxides. The earlier the solder paste coalesces, the more voids are formed. Generally, the proportion of large pores increases with the increase of the total pore volume. Compared with the situation shown by the analysis results of the total pore volume, those instructive factors that cause pore formation will have a greater impact on the reliability of welded joints. , Methods to control the formation of pores include: 1. Improve the solderability of the component/shirt bottom; 2. Use a flux with higher flux activity; 3. Reduce solder powder oxide; 4. Use
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