SMT Soldering
In the SMT production process, we all hope that the quality of the substrate will be in a zero-defect state from the start of the mounting process to the end of the soldering process, but in fact this is difficult to achieve. Due to the large number of SMT production processes, there is no guarantee that each process will not have a little error, so some welding defects will be encountered in the SMT production process.
These welding defects are usually caused by a variety of reasons. For each defect, we should analyze the root cause of its occurrence, so that the purpose can be clear and targeted.
Bridge
Bridging often occurs on ICs with dense pins or between chip components with small spacing. This defect is a major defect in our inspection standards and will seriously affect the electrical performance of the product, so it must be eradicated.
The main reason for bridging is due to excess solder paste or dislocation and slump after solder paste printing.
Excessive Solder Paste
Solder paste excess is caused by improper stencil thickness and aperture size. Normally, we choose to use a 0.15mm thick template. The aperture size is determined by the minimum lead or chip component pitch.
Misplaced Printing
When printing printed boards with pin spacing or chip component spacing less than 0.65mm, optical positioning should be used, and the reference point should be set at the diagonal of the printed board. If optical positioning is not used, printing misalignment will occur due to positioning errors, resulting in bridging.
Solder Paste Collapse
There are the following three phenomena that cause the solder paste to collapse
Printing Sag
Sag that occurs during solder paste printing. This has a lot to do with the characteristics of the solder paste, stencil, and the setting of printing parameters: the viscosity of the solder paste is low, the shape retention is not good, and it is easy to collapse and bridge after printing; if the stencil hole wall is rough, the printed solder paste is also easy Collapse and bridging occur; excessive scraper pressure will have a relatively large impact on the solder paste, the shape of the solder paste will be damaged, and the probability of collapse will also be greatly increased.
Countermeasures: choose solder paste with higher viscosity; use laser cutting template; reduce blade pressure.
Sag When Mounting
When the placement machine is placing SOP and QFP integrated circuits, its placement pressure should be set properly. Excessive pressure will change the shape of the solder paste and cause sagging.
Countermeasure: Adjust the placement pressure and set the drop position of the placement nozzle including the thickness of the component itself.
Sag When Welding Is Heated
Collapse also occurs when welding is heated. When the printed board assembly heats up rapidly, the solvent components in the solder paste will volatilize. If the volatilization speed is too fast, the solder particles will be squeezed out of the solder area, forming a slump during heating.
Countermeasure: Set an appropriate welding temperature curve (temperature, time), and prevent the mechanical vibration of the conveyor belt.
Solder Ball
Solder balls are also a common problem encountered in reflow soldering. Solder balls often appear on the sides of chip components or between fine pitch pins.
Solder balls are mostly caused by the scattering of solder caused by the rapid heating during the soldering process. In addition to the above-mentioned printing dislocation and sag, it is also related to the viscosity of the solder paste, the degree of oxidation of the solder paste, the thickness of the solder particles, and the activity of the flux.
Solder Paste Viscosity
For solder paste with better viscosity effect, its adhesive force will offset the impact force of the solvent discharged during heating, which can prevent the solder paste from slumping.
Degree of Oxidation of Solder Paste
After the solder paste is exposed to air, the surface of the solder particles may be oxidized, and experiments have shown that the incidence of solder balls is proportional to the percentage of solder paste oxides. Generally, the oxide of solder paste should be controlled at about 0.03%, and the maximum value should not exceed 0.15%.
Thickness of Solder Particles
The uniformity of solder particles is inconsistent. If there are a large number of particles below 20 μm, the relative area of these particles is large, which is easy to oxidize and form solder balls most easily.
In addition, in the process of solvent evaporation, these small particles are easily washed away from the pad, increasing the chance of solder balls being generated. It is generally required that the number of particles below 25um should not exceed 5% of the total number of solder particles.
Solder Paste Is Hygroscopic
This situation can be divided into two categories: the solder paste is taken out of the refrigerator before use, and the lid is immediately opened to cause water vapor to condense; the residual solvent is insufficiently dried before reflow soldering, and the solder paste causes the boiling and splashing of the solvent and water when the solder is heated. Solder particles are sputtered onto the printed board to form solder balls. Depending on these two different situations, we can take two different actions:
(1) Take out the solder paste from the refrigerator, do not open the lid immediately, but return to room temperature, and open the lid after the temperature is stable.
(2) Adjust the reflow soldering temperature curve so that the solder paste is fully preheated before soldering.
Flux Activity
When the flux activity is low, solder balls are also easily generated. The activity of no-clean solder is generally slightly lower than that of rosin-type and water-soluble solder pastes, and attention should be paid to the formation of solder balls when using them.
Stencil Opening
Appropriate stencil opening shape and size will also reduce the generation of solder balls. Generally, the size of the stencil opening should be 10% smaller than the corresponding pad, and some stencil opening designs are recommended.
PCB Cleaning
The printed board needs to be cleaned after printing mistakes. If the cleaning is not clean, there will be residual solder paste on the surface of the printed board and in the vias, and solder balls will be formed during soldering. Therefore, it is necessary to strengthen the operator’s sense of responsibility in the production process, carry out production in strict accordance with the process requirements, and strengthen the quality control of the process.
Tombstone
During the reflow soldering process of the surface mount process, the SMD components will have the defect of desoldering due to warping, which is vividly called the “tombstone” phenomenon.
“Tombstone” phenomenon often occurs in the reflow soldering process of SMD capacitors and SMD resistors. The smaller the component volume, the easier it is to occur.
The phenomenon of “tombstone” occurs because when the solder paste on the pads at both ends of the component melts during reflow, the surface tension of the two solder ends of the component is unbalanced, and the end with the larger tension pulls the component to rotate along its bottom.
There are also many factors that cause tension imbalance. The following will briefly analyze some of the main factors.
Warm-Up Period
When the preheating temperature is set low and the preheating time is set short, the probability of melting the solder paste at both ends of the component is greatly increased, resulting in the unbalanced tension at both ends to form a “tombstone”, so it is necessary to set the preheating period process correctly parameter.
According to our experience, the preheating temperature is generally 150+10℃, and the time is about 60-90 seconds.
Pad Size
When designing chip resistor and capacitor pads, their overall symmetry should be strictly maintained, that is, the shape and size of the pad pattern should be exactly the same to ensure that when the solder paste is melted, the resultant force acting on the solder joints on the components is zero. Facilitates the formation of ideal solder joints.
Design is the first step in the manufacturing process, and improper pad design can be a major cause of component erection. Specific pad design criteria can be found in IPC-782, Surface Mount Design and Pad Layout Standards. In fact, too many pads over the component may allow the component to slip during solder wetting, causing the component to be pulled off the pad. one end.
For small chip components, designing a different pad size for one end of the component, or connecting one end of the pad to the ground plane, may also cause the component to stand up. The use of different pad sizes may result in unbalanced pad heating and paste flow times.
During reflow, the component simply floats on the liquid solder, reaching its final position as the solder solidifies. Different wetting forces on the pads can cause lack of adhesion and component rotation. In some cases, extending the time above the liquefaction temperature can reduce element erection.
Solder Paste Thickness
When the thickness of the solder paste becomes smaller, the tombstone phenomenon will be greatly reduced. This is because:
(1) The solder paste is thinner, and the surface tension of the solder paste is reduced when it melts.
(2) The solder paste becomes thinner, the heat capacity of the entire pad decreases, and the probability of the solder paste melting on the two pads at the same time greatly increases. Solder paste thickness is determined by stencil thickness.
Mounting Offset
Under normal circumstances, the component offset generated during placement will be automatically corrected during the reflow process due to the surface tension of the melting paste pulling the component. We call it “adaptive”, but the offset is serious, and pulling will cause Components stand up to produce a “tombstone” phenomenon. This is because:
(1) The solder end that is in more contact with the component gets more heat capacity and thus melts first.
(2) The adhesion between the two ends of the component and the solder paste is different. Therefore, the placement accuracy of the components should be adjusted to avoid large placement deviations.
Component Weight
Lighter components have a higher incidence of “tombstone” because uneven tension can easily pull the component. Therefore, when selecting components, if possible, components with larger size and weight should be selected first.
There are many solutions to these welding defects, but they often restrict each other. For example, increasing the preheating temperature can effectively eliminate tombstones, but it may generate a large number of solder balls due to the faster heating rate. Therefore, when solving these problems, multiple aspects should be considered and a compromise solution should be selected.
The reason for the formation of pores in SMT patch welding
In the welding process, the mechanism of forming pores is more complicated. Typically, porosity is caused by flux outgassing entrained in the solder in the sandwich-like structure during reflow. Porosity formation is primarily determined by the solderability of the metallization zone and varies with decreasing flux activity, increasing metal loading of the powder, and increasing footprint under the lead joint, reducing the size of the solder particles can only allow for an increase porosity.
In addition, the formation of pores is also related to the time distribution between the coalescence of the solder powder and the elimination of fixed metal oxides. The earlier the solder paste coalesces, the more pores are formed. There is also shrinkage of the solder as it solidifies, and delamination and entrainment of flux when soldering plated through holes are also causes of porosity.
Methods of Controlling Pore Formation in SMT Patches
1. Use flux with higher activity
2. Improve the solderability of components or circuit boards
3. Reduce the formation of solder powder oxides
4. Use inert heating atmosphere
5. Reduce the preheating degree of reflow lead
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