Reflow soldering of solder paste is the main board-level interconnect method used in SMT, which combines the required solderability extremely well. These features include ease of processing, wide compatibility with various SMT designs, and high reliability. Welding reliability and low cost. However, when reflow soldering is used as the most important SMT component-level and board-level interconnect method, it is also required to further improve solderability. In fact, whether the reflow soldering technology can withstand this challenge will determine whether the solder paste can be used as the primary SMT soldering material. Below we will explore several major issues that affect improved reflow soldering performance:

1. Fixation of bottom components

Double-sided reflow soldering has been used for many years. First, the first side is printed and wired, components are mounted and reflowed, and then the other side is turned over for PCBA processing. In order to save more, a certain process eliminates the reflow of the first side, but reflows both the top and bottom sides. A typical example is that only small components such as chip capacitors and chip resistors are installed on the bottom surface of the circuit board. . As the design of PCBs becomes more and more complex, the components mounted on the bottom surface are also getting larger and larger. As a result, component shedding during reflow becomes an important issue. Obviously, the component peeling phenomenon is due to the insufficient vertical fixing force of the melted solder on the component during reflow, and the insufficient vertical fixing force can be attributed to the increase in component weight, 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 after the improvement of the following three factors, the phenomenon of component peeling still exists, then SMT adhesive must be used. Obviously, the use of adhesive will worsen the effect of self-alignment of the component during reflow.

2. Undersoldering

Underfill is the formation of a solder bridge between adjacent leads. In general, all factors that can cause the solder paste to slump result in undersoldering. These factors include:
1. The heating rate is too fast
2. The thixotropic performance of the solder paste is too poor or the viscosity of the solder paste recovers too slowly after shearing
3. Metal load or solid content is too low
4.The powder size is awarded too widely
5Flux surface tension is too small.
However, slump does not necessarily cause under-welding. During reflow, the melted under-soldering solder may be disconnected under the impetus of surface tension. The phenomenon of solder loss will make the under-soldering problem more serious. In this case, an excessive amount of solder gathered in a certain area due to the loss of solder will cause the molten solder to become excessive and difficult to break.

In addition to the factors that cause the solder paste to slump, the following factors also cause common causes of underfill:
1. Too much solder paste deposited relative to the space between the solder joints
2. The heating temperature is too high
3. Solder paste heats faster than circuit boards
4.Flux wetting speed is too fast
5. Flux vapor pressure is too high
6. The solvent content of the flux is too high
7. Flux resin softening point is too low.

3. Intermittent wetting

The intermittent wetting of the solder film indicates that it is now on a smooth surface. This is because the solder surface can adhere to most solid metal surfaces and hide some unwetted under the melted solder cover. Point, so when the surface is initially covered with molten solder, there will be continued wetting. The metastable molten solder coating will shrink under the action of the minimum surface energy driving force, and after a while, it will gather into separate small balls and ridge-like protrusions. Continued wetting can also be caused by the gas evolved when the part comes into contact with the molten solder. Moisture released from thermal decomposition of organic matter or hydration of inorganic matter generates gas. Water vapor is the most common component of these gases. At the soldering temperature, water vapor has a strong oxidation effect, which can oxidize the surface of the molten solder film or the interface under some surfaces (a typical example is on the interface of the molten solder). Metal oxide surface). It is often the case that higher welding temperatures and longer dwell times will result in more severe intermittent wetting, especially in the base metal, and an increase in the reaction rate will result in a more violent gas release. At the same time, longer dwell times will also prolong the gas release time.

The above two aspects will increase the amount of released gas, and the method to eliminate the phenomenon of continued wetting is:
1. Reduce welding temperature
2.Reduce the residence time of reflow
3. Use a flowing inert atmosphere
4. Reduce pollution.

4. Low residue

For reflow processes that do not require cleaning, in order to obtain decorative or functional effects, low residues are often required. Examples of functional requirements include "exploring test surfacing layers through flux residues tested in circuits and "Electrical contact is made between the inserted connector and the through hole near the reflow soldering joint." A large amount of flux residue often leads to excessive residue coverage on the metal surface layer to be electrically contacted, which will prevent the establishment of electrical connections. In the case of increasing circuit density, this issue is getting more and more attention.

Obviously, a low-residue solder paste that does not require cleaning is an ideal solution to meet this requirement. However, the related reflow necessary conditions make this problem more complicated. In order to predict the welding performance of low-residue solder pastes in different levels of inert reflow atmosphere, a semi-empirical model is proposed. This model predicts that as the oxygen content decreases, the welding performance will rapidly improve 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 in the solid content of the flux. The model proposed by the experimental data is comparable and strongly proves that the model is effective and can be used to predict the welding performance of the solder paste and the material. Therefore, it can be asserted that in order to successfully adopt no cleaning in the welding process For low-residue solder, an inert reflow atmosphere should be used.

5. Gaps

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 coplanarity of leads
3. Insufficient wetting
4. Solder loss.

This is caused by solder paste slump on pre-tinned printed circuit boards, wicking of leads or vias near solder joints. The problem of coplanarity of leads is the new, lighter 12 mil (um) pitch. In order to solve this problem, a method of pre-coating solder joints with solder before assembly is proposed. This method is to expand the size of the local solder joints and A controllable local soldering area is formed along the bulging solder pre-covered area, thereby compensating for changes in the coplanarity of the leads and preventing gaps. The suction effect of the leads can be reduced by slowing the heating rate and making the bottom surface more than the top surface. More heat to solve, in addition, the use of slower wetting flux, higher activation temperature or can delay the melting of solder paste (such as solder paste mixed with tin and lead powder) can also minimize the core The wicking effect, before covering the circuit board with a tin-lead coating layer, covering the connection path with a solder mask can also prevent wicking caused by nearby vias.

6. Solder ball

Solder ball formation is the most common and difficult problem. This refers to the solidification of solder into pellets of various sizes not far from the main solder pool during the reflow process; in most cases, these pellets are made by Composed of solder powder in solder paste, solder ball formation can cause problems such as circuit short circuit, electrical leakage, and insufficient solder on the solder joint. With the development of fine-pitch technology and soldering methods that do not require cleaning, the use of SMT process of solder ball formation.

The causes of solder ball formation include:
1. Grease stains caused by improper circuit printing process;
2. Excessive exposure of solder paste to an oxidizing environment;
3. Excessive exposure of solder paste to humid environment;
4. Improper heating method;
5. The heating speed is too fast;
6. The preheating section is too long;
7. Interaction between solder mask and solder paste;
8. Insufficient flux activity;
9. Solder powder oxide or excessive pollution;
10. Too many dust particles;
11. In a specific reflow treatment, inappropriate volatiles are mixed into the flux;
12. Solder slump caused by improper solder paste formulation;
13. Open the package before using the solder paste without fully returning to room temperature;
14. Excessive printing thickness results in "slump" forming solder balls;
15. The metal content in the solder paste is low.

7. Solder beading

Solder beading is a special phenomenon of solder ball formation when using solder paste and SMT process. Simply speaking, solder bead refers to those very large solder balls with tiny solder balls attached (or absent). They are formed around components with extremely low stand-offs, such as chip capacitors. Solder beading is caused by flux exhaust. During the preheating phase, this exhaust effect exceeds the cohesive force of the solder paste. The exhaust promotes the formation of isolated agglomerates of the solder paste under the low-gap components. It melts during reflow. The isolated solder paste emerged from under the component again and agglomerated.

Reasons for welding beading include:
1. The thickness of the printed circuit is too high;
2. Solder joints and components overlap too much;
3. Too much solder paste was applied under the component;
4. The pressure on the component is too large;
5. The temperature rises too fast during preheating;
6. The preheating temperature is too high;
7. The moisture is released from the components and solder resist;
8. The activity of the flux is too high;
9. The powder used is too fine;
10. Metal load is too low;
11. Solder paste slump too much;
12. Solder powder oxide is too much;
13. The solvent vapor pressure is insufficient.
Perhaps the easiest way to eliminate solder is to change the stencil pore shape so that less solder paste is sandwiched between the low-footed components and the solder joints.

8. Raised fillet welds

Solder fillet lifting refers to the complete lifting of leads and solder fillet welds from QFP solder joints with fine circuit pitch after wave soldering, especially near the corners of components. One possible cause is sampling before wave soldering. Mechanical stress on the leads during inspection, or mechanical damage to the board when handling the circuit board. When sampling and testing before wave soldering, use a pair of tweezers to scratch the leads of the QFP component to determine whether all the leads are soldered during the soft baking. As a result, misaligned solder toe is generated. From the top to the bottom, it can be seen that if the underside of the board causes some secondary remelting on the interface of the weld zone / fillet weld, then lifting the leads and fillet welds from the circuit board can relieve the internal stress One way to prevent this problem is to perform a sampling inspection after wave soldering, rather than before wave soldering.

9. Poor 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. Tombstoning, also known as the Manhattan effect, Drawbridging effect or Stonehenge effect, is caused by uneven lubrication of both ends of the reflow element. Therefore, the unbalanced surface tension of the molten solder is applied to both ends of the component. With the progress of SMT miniaturization, electronic components have become more and more sensitive to this problem.

Reasons for this situation:
1. uneven heating;
2. Component problems: difference in shape, light weight, and difference in solderability;
3. The thermal conductivity of the substrate material is poor, and the thickness uniformity of the substrate is poor;
4. There is a large difference in the thermal capacity of the pads, and there is a large difference in the solderability of the pads;
5. Poor uniformity or poor activity of the flux in the solder paste. The thickness of the solder paste on the two pads varies greatly, the solder paste is too thick, the printing accuracy is poor, and the dislocation is serious;
6. The preheating temperature is too low;
7. Poor mounting accuracy and severe component offset.

10. Ball Grid Array (BGA) Poor Ball Formation

BGA balls often encounter defects such as under-welding, misalignment of solder balls, missing solder balls, and insufficient solder volume. This is usually caused by insufficient fixing force or self-defining force on the ball during reflow. Insufficient fixing force may be caused by low viscosity, high barrier thickness or high outgassing speed, while insufficient self-defining force is generally caused by weak flux activity or low solder volume.

BGA ball formation can be achieved by using solder paste alone or using solder balls with solder paste and solder balls with flux. The correct and feasible method is to use the entire preform with the flux or solder paste. The most common method seems to be the use of solder balls with solder paste. The ball-forming process using tin 62 or 63 ball soldering has produced excellent results. In the case of soldering with tin 62 or tin 63 balls, the defect rate increases as the viscosity of the flux, the volatility of the solvent, and the pitch size decrease, but also with the thickness of the flux, the activity of the flux, and the soldering. The increase of the spot diameter increases. In the ball soldering system using solder paste for high temperature melting, no ball leakage is observed, and its accuracy depends on the thickness of the solder paste and the volatility of the solvent, and the activity of the flux. The size and solderability of the solder joints and the increase in metal load increase. When using tin 63 solder paste, the viscosity, pitch and reflow section of the solder paste have little effect on the ball formation rate at high melting temperatures. In cases where a conventional print release process is required, easy-to-release solder paste is critical to the individual balling of the solder paste. The integral pre-forming ball forming process is also very promising. Reducing the thickness and width of solder links is also important to improve the success rate of ball formation.

11. Void formation

Pore formation is often a problem associated with welded joints. Especially when applying SMT process to reflow solder paste, in the case of leadless ceramic chips, most of the large pores (> 0.0005 inch / 0.01 mm) are located between LCCC solder joints and printed circuit board solder joints. At the same time, in the fillet weld near the LCCC castle, there are only a small number of small pores. The existence of pores will affect the mechanical properties of the welded joint and will damage the joint's strength, ductility and fatigue life. This is because the growth of the 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 the damage. In addition, the solder shrinks when it solidifies, and the delamination of the plated through-holes and the entrainment of flux during welding are also the cause of the voids.

In the welding process, the mechanics of forming pores are relatively complicated. Generally speaking, pores are mainly formed by the metallization zone, which is caused by the exhaust of flux entrained in the solder in the sandwich structure during reflow. The solderability is determined and changes with the decrease in flux activity, the increase in the metal load of the powder, and the increase in the coverage area under the lead joint. Reducing the size of the solder particles can only increase the pores slightly. In addition, the formation of pores is also related to the time distribution between the agglomeration of solder powder and the elimination of fixed metal oxides. The earlier the solder paste coalesces, the more voids are formed. In general, the proportion of large pores increases with the increase of the total pore volume. Compared to the situation shown in the analysis results of the total pore volume, those enlightening factors that cause pore formation will have a greater impact on the reliability of the welded joint .

Methods to control pore formation include:
1.Improve the solderability of components / bottoms;
2. Use a flux with higher flux activity;
3. Reduce solder powder oxides;
4. Use inert heating atmosphere;
5. Slow down the preheating process before reflow.

Compared to the above, the formation of pores in a BGA assembly follows a slightly different pattern. Generally speaking, in the BGA assembly using tin 63 solder block, the pores are mainly generated at the board level assembly stage. On the pre-tinned printed circuit board, the pore volume of the BGA connector varies with the volatility of the solvent, metal composition and soft As the melting temperature increases, it also increases as the particle size decreases. This can be explained by the viscosity that determines the flux discharge rate. According to this model, a flux medium with a higher viscosity at the reflow temperature will prevent the flux from being discharged from the molten solder. Therefore, increasing the amount of entrained flux will increase the outgassing. The possibility of large porosity in the BGA assembly, without considering the solderability of the fixed metallization zone, the effect of flux activity and reflow atmosphere on pore generation seems negligible, The proportion of large pores will increase with the increase of the total pore volume, which indicates that the factors that cause pore generation in the BGA have a greater impact on the reliability of the welded joint than the situation shown in the total pore volume analysis result. This is similar to the case of void generation in the SMT process.

12. Summary

Reflow soldering of solder paste is the main board interconnection method in the SMT assembly process. The main problems affecting reflow soldering include: fixation of bottom components, undersoldering, intermittent wetting, low residue, gaps, solder ball formation , Solder beading, raised fillet welds, poor TombstoningBGA ball formation, void formation, etc. The problems not mentioned in this article include leaching, intermetallics, non-wetting, and distortion. Twisting, lead-free soldering, and so on. Only when these problems are solved, reflow soldering as an important SMT assembly method can continue to be successfully maintained in the era of ultra-fine pitch.