by Kerstin
October 29, 2022
PCB knowledge
(0)
05 mins

SMT soldering – how to solder PCB

SMT soldering can be defined as the process for creating electronic and electrical circuits with the help of SMT or surface mount technology. The components are usually directly mounted onto the PCB surface. SMT is modern technology and a key driver in the process of miniaturization of electrical and electronic circuits that allows the creation of lightweight and small devices with enhanced capabilities.

In the current era, superfast devices are manufactured with the help of SMT soldering, giving a breakthrough in achieving never approached goals in almost every field such as military, aerospace, transport, biomedical, industrial, and domestic appliances and other engineering fields.

What does SMT stand for in PCB?

SMT is abbreviated as surface mount technology and SMT soldering is a bit different from that of through-hole technology. SMT soldering equipment is different from than that is used in other PCB technologies and requires enhanced experience and expertise. The pads used are often of Tin (Pb) of Gold (Au) plated in SMT soldering. Soldering pads is the name given to its traces.

What is SMT soldering?

SMT stands for surface mount technology and is originally known as planar mounting. SMT soldering is a technique through which electrical and electronic devices are directly installed on the circuit board’s surface. While the components that are mounted are referred to as SMD components.

What is flux in SMT soldering?

The basic function of flux is to react with the oxides present in PCB chemically for ensuring a fresh, oxide-free, and oil-free surface at the temperatures of soldering to enable the intermetallic bond to take place.

What tools are needed for SMT soldering?

The following is the list of tools that are necessary for SMT soldering.
Tweezer
Tweezer is an essential component while progressing SMT soldering as one cannot grab components with bare hands. Tweezer is also assisting in the alignment of the components accurately. It also eradicates any chances of static charge and its attributed damage to the PCB.
Flux
Flux is necessary for making SMT soldering easy, and neat, and offers a well-finished look. It is responsible for removing the oxidation in the board and is also preventing oxidation of the joints of solder along with ensuring better adhesion.
Soldering stand
A soldering stand is very important for holding the PCB at a specific position while holding the soldering iron. Numerous variants of the stand are available in the market. A few come with magnifying glasses too that help has a close look at the circuital board.
De-soldering pump
While SMT soldering is in progress, one may solder a component at the wrong place or put extra soldering wire along the component. In such scenarios, the de-soldering pump is of great importance as it sucks all of the surpluses melted soldering wire from the board.

What are SMT components?

Different types of components are used for the SMT soldering of PCBs. A few of the most widely used components are given below.

Passive components

Capacitors
Resistors
Inductors

Active components

Transistors and diodes

Integrated circuits or ICs

Electromechanical devices

Relays
Switches
Connectors
Micro-controllers
Micro-motors

Apart from the devices mentioned above in various categories several other components are also used during the SMT PCB creation and soldered through SMT soldering.

How to solder SMT components with a soldering iron?

The following are the main steps of the process of SMT soldering with a soldering iron:

Get a soldering iron and its stand.
Cut down the soldering wire into the required pieces.
Pick soldering wire pieces.
Place the piece of soldering wire on the soldering iron.
Turn ON the soldering iron for melting the soldering wire.
Place the component onto a circuit board.
Solder the component with the tip of the soldering wire.
Solder the entire components and then turn OFF the soldering iron.
Inspect the contacts on the printed circuit board.
Re-solder the components that are not accurately placed.

What is quality SMT?

There are numerous steps taken to ensure the SMT quality after components are placed onto the surface of the PCB with SMT soldering:

An integral part of the quality process is inspecting components mounted on PCB. When the inspection is successful then it illustrates that the product has cleared the first stage.
The next step is the inspection of the circuit at the site of manufacturing for detecting problems for preventing any occurrence of non-conforming products from entry to the supply chain.
The final inspection is undergone when the PCB is ready to be dispatched. The process is usually done with X-ray machines for detecting problems with the circuit board.
The records of inspection are filed at every step for ensuring that quality standards are followed.

How are PCB made with SMT technology?

The following are the main steps involved in the making PCBs through SMT soldering.

Material preparation

First of all, prepare the printed circuit board and check for flaws if any. PCBs normally have gold-plated, silver, or tin-lead copper pads and have no holes.

Stencil preparation

Stencil the gadget that is used for ensuring a fixed and rigid position for placing the soldering paste. The designated positions are being followed for the soldering pads on the surface of PCB.

Solder paste printing

Soldering paste is usually amalgamation of flux and tin. The soldering paste is used for making connections of the soldering pads on to the PCB. Stencil tool is used for the placement of soldering paste with the help of a squeegee through an angle in between 45o to 60o.

Placement of SMT

Afterwards the PCB is placed on to the automatic machine known as conveyer belt for placement of different components on to its surface.

Reflow soldering

When entire devices and components are installed on to the surface of the board, it is then sent to an oven for reflow soldering.
The next step is to process the PCB to a zone that is pre-heated where the temperature of components is raised gradually and simultaneously.
The printed circuit boards are then put over a zone known as SOAK where the temperatures range from 140o to 160o.
Afterwards, the printed circuit board is placed to the zone of reflow in which the temperature is further ramped-up to 1o to 2o every second in order to melt-down the tin in soldering paste.
The last step is to send the printed circuit board to cooling zone to ensure SMT soldering to freeze down in order to avoid the joint’s defects.

Inspection

In the end, perform PCB cleaning and checked for flaws. If there are any flaws, repairing the components is done. Afterward, X-ray machine tests and flying probe tests are performed.

Conclusion

SMT soldering has a principal role in the manufacturing of PCBs through state-of-the-art surface-mounted technology. Therefore, SMT soldering can be utilized for getting rid of all kinds of erroneous and low-quality PCBs instead of the old and traditional methods. For more information about SMT and SMT components, please check our other posts.