Surface mount technology soldering process used in PCB Fab Houses

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Printed circuit boards have found use in almost all the electronic gadgets that we see around the world. It is a new type of technology that has made the electronic industry take a new path to give satisfaction to its customers. This technology has brought about the miniaturization of electronic gadgets and improved the portability of the same devices. Today, most clients can walk around with their devices such as smartphones and tablets, which was not the case sometime back. Due to the high demand for electronic devices, the world has seen growth in the area of manufacturing due to the introduction of both small and large printed circuit boards fabrication houses. Both small and big manufacturing firms are trying their best to come up with quality work to meet clients’ demands. Though these firms can defer on how large their production is and maybe the quality of the produced boards, they both utilize almost similar technology and process of production. In this article, we are going to discuss the printed circuit board production fully.

We shall have a look at the various services that a good PCB fabrication house should offer to its clients. They include through-hole technology assembly, surface mount technology assembly, BGA assembly, assembly of rigid-flex PCBs, mixed assembly, etc. and they must also be compliant with the many set standards such as the class 2, IPC 610, and Class 3 standards. Now let us focus on the process of the surface mount technology soldering.

SMT soldering Process

The process is divided into several important steps that should be adhered to:

PCB design and output

The printed circuit board should be a copy of the designs produced using the design automation software. Such software includes KiCAD, Altium, Eagle, OrCAD, etc.

  • The designers have to inform the PCB manufacturers about their intention and when they are given a go-ahead, they have to submit the design files in the format that has been specified by the manufacturer.
  • The most common file that is used in the process of printed circuit board manufacturing is the Gerber file.
  • The Gerber file is also known as IX274X.

Take note that different Electronic Design Automation software will involve different steps in the production of the Gerber files.

  • The Gerber files carry very comprehensive data regarding the type of board being designed. They will have drill drawing, copper tracking layers, component notation, aperture, and many other options.
  • At this point, all the aspects of the printed circuit board design will undergo checks.
  • After proper checking and confirming that the design is of good quality, the designer will now forward it to the fabricators for the production process to start.
  • All PB manufacturing houses need to run a DFM check to ensure that the board complies with all the design rules.

 

The printing

This is the first step in the process of printed circuit board printing. It involves printing the solder paste on the solder pad of the printed circuit board. The process is made possible by the use of a solder paste printing press machine and the process is done in preparation of the board for welding.

Dispense of adhesives

This process is made possible by the use of the glue dispenser machine. The machine does the precise spot injection of liquids such as glue paints to the given spot of the product being manufactured. The main idea here is to fix the required components onto the PCB when the soldering process using the wave solder is done.

Placement of the components

After the confirmation that the printed circuit board has the current amount of solder paste placed on its surface, it is moved to the next step which is the placement of the electrical components.

  • A gripper or a vacuum nozzle is used to pick each component from where it is packaged.
  • Then it is passed through the vision system for checking before it is placed at a high speed in a location that is programmed for it.
  • When you search on the market, you will come across too many types of machines that have been designed for this process and it will now depend on the intention of your company to settle for which machine.
  • For example, when purchasing these machines, companies that focus on large production quantities will tend to focus on machines that can do large placement rates within a short period.

Pre-reflow AOI

AOI is an abbreviation for Automated Optical Inspection. In this stage, we are thinking about the inspection of the printed circuit board after the components have been placed.

  • Here, we have to confirm whether the connections of the components have been done correctly as described in the Gerber files delivered by the designer.
  • The component’s connection can influence the functionality of the whole board.
  • This is made possible by the use of the Automated Optical Inspection machine which will inspect whether the components are present, have well-connected polarity, and are of the correct expected value and type.

Reflow soldering

This is the process where electrical components are connected temporarily to respective contact pads before the entire printed circuit board is taken through a process of controlled heating to melt the solder and do a connection of the component permanently into the board.

  • Solder paste is a mixture of powdered flux and solder and has the characteristic of stickiness.

This heating process can be achieved by taking the PCB through a reflow oven. Sometimes the process can be achieved through the use of infrared lamb or even doing the soldering of each joint by utilization of the pencil exposed to hot air.

  • The reflow soldering has the advantage of being the most used method in soldering of the SMT components, although it is also usable in the THT components where a solder paste is filled on the drilled holes before the insertion of the components is done.
  • In THT, reflow is not generally used since the wave soldering method is cheaper for such components.
  • Sometimes you find we have boards that utilize both the THT and SMT components and therefore, in such a situation, we shall be forced to eliminate the wave soldering and utilize the reflow soldering method fully. This will drastically lower the cost of production and assembly.

Let us have a deeper look into the reflow process.

  • Soldering oven: the first step is the board is taken into the reflow soldering oven using a conveyor.
  • Pre-heat zone: this is the first part of the oven and it is where the board and its element temperature are raised systematically. The temperature rises slowly at a rate of 1 to 2 degrees Celcius until it hits a maximum of 140 to 160 degrees Celcius.
  • Soak zone: this is a zone where the PCB is placed for about 80 seconds at a temperature of about 150 degrees Celcius.
  • Reflow zone: here the board is then superheated to a temperature of about 220 degrees Celcius where this is done so that we can melt the tin that is mixed with the solder paste to bound the placed components into the PCB leads.
  • Cooling zone: here we freeze the solder to avoid the defects that may arise due to high temperatures.

Removing excess copper

In the process of preparation of the copper tracks and soldering, there is poring of excess copper and other material. This has to be removed to avoid the effect it can induce on the functionality of the printed circuit board.

  • We need to cover the needed copper with resistance and harden it then allow the board to proceed to the stage that follows which is the removal of the unwanted copper stage.
  • A copper solvent is used to remove the excess copper that is exposed while the needed copper remains protected by the hardened photoresist material.
  • After the process, the hardened photoresist is washed away and the needed copper is exposed.
  • The washing away is done with a different type of solvent to make it possible.

Post reflow AOI

Again, we have to take the board through an AOI inspection to ensure that no single error has been made during the soldering process.

  • This has to be done to ensure that the quality of the joint is up to satisfactory standards and that no defects can be detected and also ensure that the whole joints have been soldered during the process of reflow.
  • Remember reflow process is sometimes accompanied by poor joint connections or incomplete connections.
  • In the process, the components may encounter misplacements leading to shorts within the circuit.
  • Therefore, it is a very significant practice to always take the board through an inspection process to ensure that such errors are eradicated and confirm that the board being manufactured is of quality standards.
  • This process can be carried out using several methods as discussed below:

Manual checks:

  • This process is done by visual inspection, where our experienced experts have to check for any noticeable defects to ensure that the printed circuit board is up to standard.

Automated x-ray inspections

  • This type of inspection employs the same knowledge as the AOI whereby they utilize the principle of image capturing.
  • The only existing difference is that AXI utilizes the X-rays while the AOI utilizes light to carry out the process.
  • Every material has a different rate of X-ray absorption. From experience, we can notice that materials with heavy elements are capable of absorbing more portions of X-rays compared to those that are made out of lighter materials.
  • Therefore, heavy materials will have a better darker display as compared to the lighter materials.
  • This one is used when the components being inspected have hidden elements for example in BGAs pins tests.
  • The image below has a comparison between the AOI and the AXI on how they perform matters inspection.

Automatic optical inspections:

  • This one will find useful when the defects on the board are somehow easier to discover.
  • Such defects will include solder shorts, open circuits, insufficient solder, and solder bridges.
  • This makes utilization of light energy to carry out the process.

ICT

This is the most important equipment that is used in the electrical tests of PCBs.

  • The device has a bed nail that comes into contact with the components that have been mounted on the board and then a specific amount of current and voltage is used to carry out the possible defects on the components such as parameter deviations, misplacements, displacements, solder joint bridging missing components, shorts, and open circuits.
  • This method is good for mass production and simple PCBs because of the high-speed features it gives.
  • Flying probe test is another good example of the ICT methods of PCB tests and this utilizes a large amount of flying probes to achieve its goals of carrying out printed circuit board electrical tests.
  • Flying probe method is widely used in the assembly of the surface mount devices.

Conformal coating

In this process, we shall involve a thin polymeric material that is applied on top of the PCB to protect the components.

  • The thickness of the polymeric film is between 25 to 250 um.
  • It is purpose is to offer protection to a given board against dust, moisture, extreme temperatures, and chemicals spills.
  • If you don’t protect your PCB components, then expect several failures within a short period meaning the functionality will be altered.
  • In the modern manufacturing industry, most companies have opted to use transparent coating material such that it can allow easier inspection of the components.

Final inspection and testing of functionality

At this level, we need to test whether we can achieve the objectives of the design. An electrical test has to be carried out by a team of qualified experts to ensure that the functionality has been achieved as expected.

  • The customers have to provide the test software and sometimes the manufacturing firm can construct test fixtures according to customers’ definitions.
  • The PCB is simulated through the normal working conditions while the testers do the monitoring of the printed circuit board’s features and characteristics

Cleaning: washing and drying.

The production process has a lot of activities that are carried out by the machines. This machine utilizes oil as lubricants and they can also carry with them a lot of dirt which ends up getting into contact with the printed circuit boards being manufactured.

  • A good example is during the process of adding soldering paste, you will notice that some visible content of the flux is left behind during the process of manufacturing humans can transfer oils and dirt into the PCB while handling them.
  • This is what needs to be removed during this process to ensure that the PCB is very clean and ready for packaging.

Packaging and shipment

All the produced boards need to be packaged so that they can be delivered to respective clients. Packaging can be done in various ways one being anti-static packaging which can only be done if requested by the customers.

Shipment is done through various shipment agencies such as DHL, EMS, FedEx, and UPS. The customers are notified anytime the shipment has begun.

The extra components after the design process are also returned to either the customer or where they were ordered depending on the customer’s idea.

  • There are very significant factors that you have to consider when doing the SMT placements of the components such as the spacing and tolerances.
  • The most important factor is the coefficient of thermal expansion. This should not be too big between the materials making up the PCB since you will experience cracking of the solder joints.
  • Another factor that determines the components placement process is the cost and size factor. Components that absorb more current requires greater electrical and thermal considerations. Such components will need ground flow so that it can be able to manage the heat flowing in them.

Conclusion

This is all about the surface mount devices soldering process. It is a comprehensive process that starts with the PCB design and output, the printing, dispense of adhesives, placement od components all the way to packaging of the finished product.

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