How To Improve SMT Assembly Quality?

Before starting the discussion on the improvement of SMT assembly quality, the basics of the SMT are discussed.

What is SMT?

There are a number of techniques to assemble the electronic components on the Printed Circuit Board (PCB).

Surface Mount Technology (SMT) is one of them. Surface Mount Technology is a method or technique of electronic assembly in which the electronic components are mounted on the face of PCB rather than inserting the components through holes, as with the conventional technique.

Benefits of SMT?

The reason behind the vast adaptation of the SMT is the benefits that it offers, these are listed below;

• The cost of manufacturing with the advent of SMT has reduced.
• The utilization of the space on PCB is efficient.
• The SMT has made it possible to build very complex electronic circuits easily and in a very smaller space.
• The higher level of automation has equipped assemblies with good repeatability.

Issues with SMT?

Despite the benefits of SMT, it also has some defects.

These defects are key factors in affecting the quality of SMT assembly. These defects are given below;

• Electrical-Bridging
• Solder-balling
• Tomb-stoning
• Solder beading
• Excessive filet
• Insufficient Solder-joints
• Non-wetting or de-wetting
• Orange Skinning

These are stated issues with SMT. Next, we are going to define the techniques in order to improve the SMT assembly.

How to improve SMT Assembly?

In order to improve the SMT Assembly, it is very important to first understand the issues and then applying the solutions to these issues.

So these issues are one by one stated below along with their solutions that will contribute towards improving the quality of SMT assembly;

• Electrical Bridging: the other name for Electrical Bridging is Solder Bridging. This defect occurs when an abnormal connection between the two pins or traces is made by the solder. This result forms a conductive path between two pins or traces resulting in the failure of PCB. The following figure shows the Electrical bridging

Figure 1 showing solder bridging in SMT Assembly

Moreover, looking for a solution, there is no hard and fast, and direct solution to eliminate the electrical bridging problem in SMT, but it can be stopped by following practices.

1. Change the design of your circuit board: this is not the possible solution always, but it works most of the time and when it is possible, it is one of the most effective approaches.
2. Apply the moderate solder paste: the solder bridging can be the result of excessive solder paste. So in order to overcome this issue, moderate paste application of the solder is going to help. Sometimes, the problem of solder bridging also occurs, because of the overheating of the solder paste. So the moderate volume of solder paste along with the controlled temperature is going to help. This in turn will help to improve the quality of SMT assembly.

• Solder-balling: most common type of defect that occurs in the SMT assembly process is a solder ball. They are located as nearly to traces as 0.13 mm and thus cause the conduction to occur between two traces. This is the result of a violation of the electrical clearance principle. When solder balling happens the reliability of the assembled PCB is compromised. So it is very important to avoid and eliminate it. Solder balling is shown in the figure below:

Figure 2 showing the solder bridging and solder ball

The solder ball has a resemblance to air or water vapors that try to escape from a thick paste and convert it into liquid.

When the vapor escapes too fast a small portion of solder paste is taken out and on cooling it becomes a solder ball.

So now having a clear understanding of the problem, the following suggestions are given to avoid and eliminate it.

1.  The PCB should not contain water and it should not be kept in a humid environment.
2. The pad sizes and the design should be correct according to the recommendation by the vendor.
3. The preheating temperature is increased when suitable.
4. The printing process should be followed by the baking of PCB. It tries to eliminate the water vapors if any.

• Tomb-stoning: it has other names like crocodile or Manhattan effect. In this effect the component has one end lifted or tilted from the pad of the PCB during the reflow soldering process.It mainly affects the surface mount components such as resistors and capacitors etc.To better understand, please have a look at the following figure:

Figure 3 showing the tomb-stoning in SMT Assembly process

The possible solutions to tomb-stoning are given below;

1. Pad size of the footprint: if the size of the pad for a small passive part is not correct, there is a possibility that the thermal mass of the solder joint will be affected. So pad that has less mass can result in the solder reflow as compared to the larger pads. The solution is to follow the industry standard and thus the SMT assembly process can also be improved. Apart from following industry standards, another option is to follow the recommendations of the manufacturer.
2. The footprint construction is also crucial for the elimination of the tomb-stoning during the SMT assembly process.

Solder beading: often confused with solder balls, solder beads belong to a specific category of solder balls. Apart from this, they occur in a very close range to the passive components like resistors, etc.

A simple example picture to clear the understanding is given below:

Figure 4 solder bead near the resistor

The solder beads are formed in several steps and cause the problem of coagulation of the contacts when scattering in very close proximity.

To improve the quality of the SMT assembly, the problem of solder beads must be resolved. It can be avoided by avoiding the following steps.

1. Excess of solder paste: In order to avoid solder beading, the solder paste should be carefully applied in the areas where it needs to be. Excess use will result in scattering of solder paste and thus forming the solder beads when these scattered beads are in close proximity to resistors and capacitors.
2. Solder pad replacement: Second most potential source of occurrence of solder beads is the replacement of the pad. Because, when the replacement takes, there are chances that the solder bead will get to a nearby element. So care must be taken otherwise the quality of the SMT assembly will be compromised.

1. Excessive filet: this happens when the solder joint on-chip components are above the height of the part with a convex meniscus and are referred to as excessive filet. It is caused during the separation of the board from the solder wave and is more common in nitrogen soldering. So avoiding excessive filet will help to improve the quality of the SMT assembly.

• Insufficient solder-joints: occurs when the electrically connected points are separated, or the area on the PCB turns open which meant for conduction. This is due to the insufficiency of the solder on the joints on the PCB. The possible solutions suggested for this are given below:

1. The aspect ratio should be corrected. The aspect ratio is defined as the ratio of aperture width to stencil thickness. The aperture clogging is possibly caused due to an improper (too small) aspect ratio.
2. The solder paste contamination should be avoided in a controlled environment.

• Non-wetting: when the liquid solder is not adhered to at least one of the components mounted on the PCB, it is referred to as non-wetting.

Now the possible solutions for non-wetting include;

1. The adapted surface finish should be of better quality and higher temperature resistance.
2. The total profiling time should be reduced before the reflow stage.
3. Every soldering task requires flux in a certain range,so proper flux is applied.

Conclusion: Surface Mount Technology (SMT) the assembly process has many advantages such as reduced cost, and better space utilization, etc.

Along with advantages, it also has certain disadvantages such as solder-balling, electrical bridging, solder beading, excessive filet, and more. Due to these issues, the circuit either does not work or yields wrong results.

So that is why, the reliability of the PCB designed is highly compromised, and the improvement of the SMT assembly process is required.

Thus, in order to improve SMT assembly, all the above-mentioned issues must be resolved by following the solutions suggested in the article.

For example, if the issue of Electrical bridging occurs try to eliminate it by changing the design of the circuit or by applying the moderate solder paste; because, the improperly designed PCB and excessively applied solder paste could cause this problem.

So the moderate volume of solder paste along with the controlled temperature is going to help.

This in turn will help to improve the quality of SMT assembly. Similarly,if other issues arise they can be treated as well.

Once the issues are eliminated, the SMT assembly process will be improved and reliability is also improved.