Electronic circuit assembly process

Originally, all electronic circuits were assembled by hand using only a soldering iron. As technology advanced, components became smaller and more difficult to assemble by hand and the number of components that could fit on a single board increased. Therefore, the need for automatic PCB assembly developed.

At FADESA ENGINEERS we do much more than the electronic prototype for your device. We have the capacity for the design and assembly of electronic circuits. We have a SMD assembly department that allows the assembly of printed circuits to customer specifications.

Automated electronic circuit assembly system

Before getting into the subject, this is an approach to try to approach the concept of electronic circuit assembly, there may be variants and different methodologies, depending on the needs of the electronic board, the device and the machinery available to perform the electronic assembly.

The manufacturing line for prototypes of surface mount electronic systems on printed circuit boards(PCB) consists of: screen printing equipment for adhesive and solder paste; equipment for self-supply, positioning and coupling of components and reflow oven for lead-free soldering process.

Silk-screen printing system for the assembly of electronic circuits

This complex system, as its name suggests, is responsible for screen printing the PCBs.

In this first assembly process of electronic circuits, we can find minimum technical characteristics of the screen printing system, which can vary according to different factors. Normally, we find systems that have an adjustable magnetic base, a positioning of the base in three axes in horizontal plane (x,y,θ) and of the base in a vertical axis (z), with adjustable height to adapt to different PCB thicknesses, a vacuum system to hold the PCB during the stencil separation (Stencil), a double articulated spatula head with independent print force adjustment, the travel of the spatula head is also adjustable according to the print size, has double 300 mm metal spatula for fine pitch stencil printing, selectable print speed from 10 mm/s to 100 mm/s and vertical stencil separation speed of the PCB parallel and adjustable: 0.5 mm / s to 12 mm / s, a mechanical PCB clamping, to handle PCBs defines a minimum size ≤ 50 mm x 60 mm x 0.5 mm and a maximum size: ≥ 410 mm x 395 mm x 5 mm, usually have a vision system with dual color camera and two laser pointers for alignment with the template, the alignment range in x, y axes ≥ ± 6 mm and in θ ≥ ± 2.5 º, the printing accuracy ≤ ± 20 microns at 3 σ. These characteristics may vary according to the machine to be used.

Adhesive system for the assembly of electronic circuits

It is a machine highly recommended in the assembly process of electronic circuits. This machine is in charge of placing the adhesive paste where it is needed for the subsequent placement of the components. Depending on the machine used, it also has certain characteristics and requirements in the process.

Pick and place” system for the assembly of electronic circuits

The Pick&Place system is responsible for picking the components and placing them in the previously defined position. As we mentioned at the beginning of this post, with technological advances, components are becoming smaller and smaller, which makes it difficult to assemble them by hand. This automatic electronic circuit assembly service increases the precision of the assembly of components on electronic boards and stands out for its speed.

Like all machinery, depending on the one owned, it has minimum technical characteristics for the “pick and place” system. These technical characteristics will depend on several factors:

Depending on the number of feeders, they usually have intelligent feeders with automatic position and status recognition to the machine, also defining the range of component sizes handled. Others also have trays for single components.

To operate automatically, the machines have a top-down vision system for automatic fiducial research and recognition in the alignment process as well as a laser alignment system on the component path from the feeder to the PCB and a bottom-up vision system for component inspection prior to positioning.

The universal magnetic PCB fixture also allows offline programming. It has an interface software for data conversion from CAD programs (Layout) to positioning files, ability to check and correct the program without placing the components on the PCB (virtual debugging) and finally it has a PC integrated in the equipment for control and software as well as automatic tool change.

Soldering furnace in the assembly of electronic circuits

In order not to make this post too long, let’s summarize the function of this process. This oven is basically used to bake the solder paste so that the components are completely fixed to the PCB. Actually the machines destined to this process do not have much mystery.

Soldering in the assembly of electronic circuits

When we refer to soldering in electronic engineering, we mean, an electrical junction that conducts a signal and a power through two contact points. We can also define it as; a solder is a durable mechanical joint between the contact points of the electronic component and those of the printed circuit board. This solder is very resistant to aggressive environmental conditions that may occur such as vibrations, shocks, temperature changes, etc.

This mechanical bonding or soldering in electronic circuits is only achieved if the proper material is used. In this case the Intermetallic material used must be sufficient for the joint.

The amount of intermetallic material and mechanical strength are related. There is a minimum limit of intermetallic material, below which there is no bonding. In addition, there is also a limit above which the strength is maximum and if the amount of material is increased, the strength becomes lower and lower.

Therefore, it is very important that the welding machines produce the correct amount of intermetallic material for a good joint. These machines must be capable of reaching the necessary temperature to form a good joint.

Differences between wave soldering and selective soldering

Let’s start with wave soldering. In a wave soldering, when you solder the components that go through the PCB such as connectors, shields, sockets…, even though they have a higher speed in the process than in selective soldering, the following happens:

• There is no choice of temperature and all joints are produced at the same temperature, regardless of the thermal mass of the joint.
• The weld is produced in contact with air so that weld oxide is produced. This can be covered with waxes and equivalent materials, but it still contaminates the solder. One way to prevent this from happening is to encapsulate the chamber with nitrogen, but this requires high consumption compared to selective soldering.
• The cost in thermal energy is very high since a lot of thermal energy is required to keep the large amount of solder that is stored in the wave soldering process molten.
• To solder the components on one side, it is necessary to bath them with the molten solder, which can damage them. To solve this problem, protective masks can be applied to the circuit, but this would be much more expensive.

On the other hand, selective soldering has some advantages over wave soldering:

• The welding nozzle is programmable and speeds can be adjusted according to the size and thermal mass required by the components.
• There is no oxidation of the weld as it is not in contact with air and does not need nitrogen to prevent it.
• Less thermal energy is required because there is less welding volume.
• The components that go on the opposite side to the one being soldered are not exposed to overheating and we avoid having to protect them.

The above mentioned refers to a sequential and single nozzle selective soldering, but we have the option of high production selective soldering in which multiple nozzles work simultaneously. These nozzles are assigned a group of pins from each component and a unique stencil is created for each PCB.