Techniques of PCB assembly

Techniques of PCB assembly

SMT has different types of techniques. These techniques can be classified according to assembly and soldering method.

Assembly method

SMT techniques are categorized into double-sided mix assembly and single-sided mix assembly.

Soldering method

Here, SMT are in two categories, which are: Wave soldering and reflow soldering

Some elements that influence the quality of soldering include:

Solder quality

PCB design

Flux quality

Equipment and administrating

Oxidation degree on soldered metal surface

Factors that influence reflow soldering quality

Several factors affect the quality of reflow soldering. These factors are explained below;

Technological requirement for setting temperature curve for reflow soldering

Soldering quality depends on temperature curve. Before 160 degree Celsius, one should control the rising rate of temperature to 2 degree Celsius per second. PCB and electronic components suffer heat when the temperature increases too quickly. This damages components and leads to PCB deformation. Meanwhile, that kind of high solvent evaporation speed makes metal powder spilled with solder ball.

The peak value of temperature should be more than melting point of alloy by 40 degree Celsius. A low peak value of temperature can result in incomplete soldering without producing a metal alloy layer. Sometimes, solder paste can fail to melt. A long reflow soldering time or high temperature value can make metal alloy extremely thick.

Soldering paste quality’s effect on reflow soldering technique

Statistics revealed that printing technique related problems account for 70 percent of surface assembly quality issues. During the printing process, edge subsiding and insufficient printing cause disqualification. PCBs having these kinds of defects need to receive work.  Certain inspection standards should be according to IPC-A-610C.

Technological requirement for SMDs

Some techniques need to meet certain requirements to achieve ideal mounting quality. Some of these requirements are accurate positions, ideal pressure, and accurate components. Certain inspection standards should working in accordance with IPC-A-610C.

Features of SMT Electronics

The through hole technology requires inserting pins of components into through-hole vias on PCBs. THT features high weight and large volume which make it difficult to assemble. Surface mount technology provided solutions to the issues of THT. SMT includes the following features;

Strong vibration resistance

Low rate of defect for soldering points

Electromagnetic and RF interference reduction

High assembly density

Automation accessibility

Surface Mount Electronic Components and Types

In terms of functions, SMT is similar to components for THT. However, the two techniques are comparatively better when it comes electric performance. Package types, use of components, and lead configurations make it hard to form a product.

For instance, they should tolerate high temperature and well-soldered to help products meet requirements. Surface mount technology has continued to evolve. This has helped to resolve several problems that arise from components standardization.

There are two major types of surface mount electronic components. They are passive and active surface mount electronic components.

Passive surface mount electronic components

Passive components don’t offer additional power benefit to the device. The shapes of these components are either cylindrical or rectangular. The weight of these components is much lower than their counterparts.

Surface mount capacitors and resistors are available in different sizes. This helps to meet the demands of different applications in the industry.

Surface mount resistor networks

These networks serve as replacements for a group of discrete resistors. It is a combination of different resistors. The body dimensions may change. Most times, they are available in 16-20 pins.

Surface mount discrete resistors

These discrete resistors are in two types. They are thin film resistors and thick surface mount resistors. Thin film resistors feature resistive element on a ceramic base. They also have soldered terminations on their sides. Thick surface mount resistors require screening a resistive film on an alumina surface. You can then get the resistance value by examining the variance between the compositions of resistive paste.

Active surface mount electronic components

For this type of component, they are various categories.

Ceramic leaded chip carriers

Ceramic leaded chip carriers are in postleaded and preleaded formats. The user fixes the leads to the leadless ceramic chip carriers’ castellations in the postleaded chip carriers. In the preleaded chip carriers, the manufacturer attaches the Kovar leads or copper alloy. The dimensions of leaded ceramic packages are similar to plastic leaded chip carriers.

Leadless ceramic chip carriers

These carriers don’t have any leads. They only feature castellations that offer shorter signal paths. These paths enable higher operating frequencies. Based on the packages’ pitch, the leadless ceramic chip carriers are in different categories. The commonest category is 50 mil. Others can be 20, 25, and 40 mil.

SMD Active Components for SMT

Plastic SMD packages are commonly used for nonmilitary applications. While ceramic packages have their own issues, plastic packages can be a better option.

Small outline transistors

Small outline transistors or SOT are four or three lead devices. The four lead devices are SOT 143 while the three lead SOT devices are SOT 23. These packages are suitable for transistors and diodes. SOT 89 and SOT 23 packages have become popular for surface mounting small transistors.

Small outline J packages

This package is like a combination of PLCC and SOIC. Small outline J package provides the benefits of both.

Small outline integrated circuit

This integrated circuit has leads on 0.050” centers. SOIC are suitable for housing several small outline transistors. Small outline integrated circuit needs careful handling to avoid any lead damage. SOICs have different body widths. The 150 mil is the one with less than 16 leads. The package with more than 16 leads is the 300 mil.

Fine pitch SMD packages

These packages feature a greater number of leads and finer pitch. They also have land pattern designs and thinner leads.

Plastic leaded chip carriers

This is a better option to ceramic chip carriers. These leads absorb the solder joint stress which helps to prevent any solder joint crack.

Ball grid array

The ball grid array is a package without any leads. BGAs are available in ceramic and plastic types. BGAs are ideal for self alignment when there is reflow.

Frequently Asked Questions

When should you use SMT electronics assembly?

Surface mount technology assembly is suitable for manufacturing smaller and lightweight electronics products. SMT assembly is a great technique for complex electronic devices. SMT is ideal for use in several applications. This technology is widely used in the electronics industry.

Does SMT require the drilling process?

Yes, SMT requires the drilling process. But, fewer holes are drilled into the PCB. This helps to reduce the cost of handling and processing. Through-hole technology requires more holes to be drilled. Therefore, SMT is a better alternative.

 Conclusion

SMT electronics are the commonest technique in the electronics world. SMT has helped in the production of small, and yet complex devices. It is no doubt that most devices today feature surface mount technology. The advent of SMT has led to the production of better and more effective electronic devices. SMT is now the order of the day in today’s electronic industry.