Surface mount technology is not a zero-defect soldering process. Here, Juchuan Electronics (www.).smtworks.com.CN) will study the simple defects in surface mount technology (SMT) that cause PCB failure and try to figure out ways to avoid them. Reducing such defects is key to providing efficient PCB assembly services.
Before we discuss errors in SMT assembly, let's review the basics of SMT:
What is surface mount technology?
Surface mount technology (SMT) is a process in which electronic components are mounted directly onto the surface of a printed circuit board (PCB). Electronic components that are mounted in this way are called surface-mount devices (SMDs).
What are the advantages of surface mount technology?
SMTs have several advantages over through-hole mounting. SMT makes it easy to automate and increase component density on the board. It can also achieve higher circuit speeds while providing better high-frequency performance.
What is the difference between through-hole and surface-mount technology?
There are some differences between surface mount technology and through-hole technology. The main differences are cost, automation, board space, and component density. Through-hole components are more expensive to manufacture than SMT components and are not suitable for automation. Compared to SMT, through-hole technology also has higher limitations on board space and component density.
Troubleshooting Surface Mount Technology
SMT Error 1 Weld bridging or electrical bridging.
A solder bridging is a solder that crosses two conductors that should not be electrically connected, which can cause an electrical short circuit. These short circuits can cause the circuit to malfunction.
Potential causes: There are a variety of reasons that can explain the bridging;However, the most commonly recognized cause is an issue in the solder paste printing process. There may be some deviations in the printing arrangement or stencil layout of the PCB pad configuration. Excessive solder paste deposition can likewise lead to bridging. This can happen when the ratio of the stencil aperture to the pad is too high. The cold collapse rate of the solder paste also promotes bridging. An incorrect ratio of solder paste to flux weight can lead to slump. High temperatures and humidity can likewise cause the solder paste to collapse. The reflow curve can also increase bridging. We know that the purpose of the reflow process is to melt the powder particles in the solder paste. At the same time, it wets the surfaces that are joined together and finally solidifies the solder to form a strong metallurgical bond. The curve can be divided into four zones – a preheating zone, a soaking zone, a reflux zone, and a cooling zone. If the heating rate of the preheating area is too slow, it may be the cause of bridging. The part that comes into contact with the solder paste may tilt the deposit, allowing the solder paste to bridge. If the soaking time is too long, more heat will be injected into the paste, resulting in the thermal collapse of the paste. Inaccurate placement further narrows the gap between pads, increasing the chance of bridging. Too much component placement pressure can extrude the solder paste out of the pad.
Possible solution: The right application requires the right ratio of solder paste metal to flux by weight. In other words, the solder paste will not collapse. For example, spot solder paste typically has a metal content of 85-87%. If we use it for fine-pitch surface mount printing, the ratio will go down. Typically, 90% of the metal is used or should at least be used for stencil printing solder paste applications. An appropriate reflux profile is also important. Unless you are using automatic printer alignment, you should pay due attention to the alignment of the stencil holes with the pads. Ensure point pressure and accuracy of component placement. Reduce the template aperture size by 10%. Otherwise, the thickness of the stencil can also be reduced, which will reduce the amount of solder paste deposited.
SMT Error 2 Insufficient solder joints or electrically open circuit.
When two electrical connection points are separated, or when the area on the PCB interrupts the intended design of the circuit, it is called an electrical open circuit.
Potential causes. The solder paste printing stage of the surface mount process contributes the most to this defect.
Insufficient solder on the connector can cause the circuit to open. This can happen if the solder paste clogs the holes in the stencil.
Even if the amount of solder is sufficient, an open circuit can occur if it is not in contact with both the leads and the pads during reflow. This is called component lead coplanarity.
An open circuit can also be a consequence of the PCB manufacturing process itself.
Possible solutions.
First of all, the solution is to correct the aspect ratio. The aspect ratio is defined as the ratio of the width of the aperture to the thickness of the template. Solder paste clogging holes may be due to the aspect ratio being too small.
Extreme environmental conditions are strictly forbidden during the manufacturing process. Avoid solder paste contamination by controlling the environment.
Investigation of coplanarity is also very important when solving electrical open circuit problems.
Manufacturing must be inspected with a PCB manufacturer.
SMT Error 3 Solder Ball.
The development of very tiny spherical solder particles that are isolated from the body forming the joint. This is an important problem with no-clean processes, as a large number of solder balls can create a false bridge between two adjacent leads, creating functional problems for the circuit. For water-soluble procedures, solder balls are not so a concern, as they are removed periodically during the cleaning process.
Potential causes. Moisture contamination of solder paste is one of the main causes of solder balls. Saturation of moisture during reflow, leaving solder balls.
Lack of proper reflow curves can also lead to solder balls. The fast preheating rate will not provide enough time for the solvent to evaporate gradually.
Excessive oxides on the solder powder in the solder paste can also form solder balls.
Solder balls can be caused by poor solder paste print alignment and solder paste printed on the solder mask layer instead of the pad.
The solder paste applied to the bottom of the stencil during the printing process is also an attribute.
Possible solutions.
Coarser powder sizes are recommended as fine powder sizes have more oxides and are more prone to collapse.
The reflow process should be selected based on the solder paste.
Interaction of solder paste with moisture and humidity should be avoided.
Check the minimum print pressure used.
Print alignment should be verified on a consistent basis before reflowing.
Ensure proper and frequent cleaning of the bottom of the formwork.
SMT Error 4 Tombstone.
A tombstone, sometimes referred to as the Manhattan effect, is a chip assembly that is partially or completely pulled off the pad to a vertical position where only one end is soldered. This is due to the imbalance of forces in the reflow soldering process. The components stand at one end as if they had come back from the dead. As a result, it looks like a tombstone in a cemetery. In fact, it is a defunct PCB design with an open circuit.
Potential causes. Uneven heating can lead to differences between the terminal blocks of the assembly. More precisely, if the heat is not evenly distributed, the solder will melt at different rates. Thus, one side refluxes before the other, causing the other side to stand upright.
Unequal heatsinks (i.e., ground planes) can take heat away from the pads if they are present within the PCB layer.
Sometimes due to temperature and humidity exposure on the solder paste, the solder paste is not sufficient to hold the component in place during reflow.
Excessive movement during and after the reflow operation can cause components to misalign, resulting in monuments.
The uneven placement of components on the pads before reflow soldering can lead to unbalanced soldering forces.
Possible solutions.
The component body must cover at least 50% of the two pads to avoid unbalanced soldering forces.
Ensure high component placement accuracy.
It is recommended to keep the warm-up temperature high so that there is little difference between the two ends when reflowing.
Movement is minimized during SMT assembly and minimized during reflow.
Minimal exposure to extreme environments, such as heat or humidity.
The extended soaking zone helps to balance the wetting force on both pads before the paste reaches a molten state.
SMT Error 5 does not wet or dehumidify.
A condition in which the fluid solder in a solder joint is not tightly adhered to at least one component. However, a condition in which the surface is in contact with a liquid solder, on which some or no part of the solder is attached.
Potential causes. Poor PCB finish can be one of the main reasons. Assuming that the base metal is visible, usually this is more difficult to weld, so non-wetting occurs.
This can also be due to the long soaking time during the reflux process. Resulting in the use of flux before soldering.
It is possible that during the reflow process, there is not enough heat, so the flux does not reach the proper activation temperature.
Possible solutions.
A better quality metal surface finish needs to be adjusted, such as a higher temperature-resistant OSP or ENIG.
Reduce the total analysis time before the reflux phase.
A flux for a given soldering task.
SMT Error 6 Beads.
The larger solder balls are located close to the discrete components and are very small from each other. This deformation is like a solder ball, but it is discrete because these solder beads are firmly attached to the discrete component rather than the multi-lead gadget.
Potential causes. Often, this problem is due to the deposition of too much solder paste.
Sometimes during the warm-up phase, the flux is deflated and the coalescing force of the solder paste is exceeded.
Excessive component placement pressure can also be a problem. This pushes the deposited solder paste onto the solder mask. Therefore, it is not possible to reintegrate into the joint.
Possible solutions.
Reduce the thickness of the template or reduce the pore size. A 10% reduction on the side where the beads appear should fix the problem.
Reduced component pick-up and placement stress.
SMT Error 7 Underfill and Undersolder.
The amount of solder paste deposited at the printing station is much smaller than in the stencil opening design, or after reflow, there is not enough solder to form a fillet at the component leads.
Potential causes. Formwork holes are sometimes clogged with dry slurry. This is one of the main causes of the problem.
It is very important to apply sufficient pressure over the entire length of the squeegee blade during the printing cycle. This will ensure that the template is wiped cleanly. Too much pressure can cause the solder paste to be scooped out, especially from larger pads, which can cause defects.
Because the blade speed is too high, the solder paste will not roll into the hole. The speed at which the scraper moves determines the available time for the solder paste to roll into the stencil holes and PCB pads.
When the viscosity of the solder paste or the metal content is too low.
Possible solutions.
Large openings can be split into smaller holes, and the scraper pressure can be checked if there is too much.
The formwork must be cleaned regularly and the slurry must be checked for expired or dry materials. In addition, adequate board support should be ensured.
Excessively high blade speeds are also undesirable and should also be controlled.
SMT Error 8 Cold solder joints or granular joints.
Some welded connections sometimes exhibit poor wettability and have a gray, porous appearance after welding. The dark, non-reflective, rough surface of the alloy should be bright and shiny, so it can be recognized.
Potential causes. One of the main reasons listed is that the solder does not absorb enough heat. This happens because the reflow solder is not having enough heat.
Many times, during the SMT soldering process, the flux doesn't seem to be up to the task. This may be due to inadequate cleaning of the components and/or PCB pads prior to soldering. Too much impurity in the solder solution can also lead to defects.
Possible solutions.
The maximum reflow temperature should be high enough for the material to reflow completely.
Components should not experience any type of movement during or after reflow.
Alloy analysis must be performed to check for contaminants.
With the miniaturization of PCBs, there are more and more problems related to PCB manufacturing. Tighter soldering process control is required for smaller components used on PCBs. Reducing common errors in SMT is an important step in improving the efficiency of SMT manufacturing, leading to more efficient PCB assembly services.
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