SMT ASSEMBLY
The assembly processes in the electronics sector
SMT technology (Surface Mount Technology) concerns the assembly and soldering of all the components that are placed on the surface of the Printed Circuit Board (PCB) without crossing it from one side to the other.
SMT is the one that has seen the widest development in recent years because, in general, it allows for greater miniaturization and allows the production of more complex boards with smaller dimensions.
It also allows to occupy both sides of the PCB extensively, further increasing the possibility of placing components in the available spaces.
In most cases and in almost all target sectors, the surface-mounted components are assembled following an automatic process, i.e. carried out by machinery, which allows optimisation of productivity and repeatability of operations.
The main principle is to obtain a deposition of the solder paste in the positions in which the components will be placed before placing the components on them and then subjecting the assembly to a thermal reflow cycle in which the paste, composed of metal alloys atomized and flux melts, soldering the components to the printed circuit board.
To correctly develop this process, ideally simple, it is necessary to take care of many important details, act in compliance with the standards and have a process validated and kept under constant control.
The steps of automatic SMD assembly are:
Screen printing
In this phase the solder paste composed of atomised metal alloy and flux binder is deposited on the PCB in the positions where the components will then be placed. The printing takes place through a laser-engraved silk-screen stencil realized using the PCB construction drawings – the Gerber files – which allow the PCB pads to be reproduced with great precision in the form of openings.
Using an automatic screen printer, which is able to control the tension and centering of the stencil and the extrusion pressure of the scraper, it is possible to obtain the formation of extremely precise solder paste deposits which are a fundamental requirement for the success of compliant welding during the thermal cycle.
To optimize the results of screen printing and therefore of welding, the stencils must also be designed and manufactured with appropriate measures of the openings and the thickness of the material to perfectly adapt to the type of machinery and materials used; the company must therefore train competent personnel to drafting of construction specifications to be sent to those who build the stencils.
Pick & place assembly
At this stage the screen printed PCB is ready to receive the surface mount components. The assembly of the components takes place with the “pick & place” which is a high-tech machinery designed to pick up the necessary components directly from the packages and place them on the PCB in the desired positions (place).
Using robotic heads that move on a reference plane, the components are taken from their tapes and reels and positioned on the PCB according to the XY coordinates entered during the programming phase of the machine.
The programming is carried out by a specialized operator who assigns the coordinates and the rotation of components for each position foreseen on the printed circuit according to the design files.
Thermal cycle of reflow in the oven
Once the treatment in P&P has been completed, the boards have all the components positioned above the solder paste and are therefore ready for the thermal process of reflow in the oven.
The first phase of the thermal cycle is the pre-heating which serves to gradually bring the board closer to the melting temperature and to activate the chemical substances that help the deoxidation of the surfaces and are inert at room temperature.
After the pre-heating, the actual reflow process begins. Lead free alloys melt at a temperature of about 217 ° C and it is therefore necessary to make sure that each component reaches this temperature so that the alloy passes into a liquid state, “wetting” all the surfaces and then solidifying in the cooling phase and forming the bond between component and printed circuit.
All these three phases: pre-heating, melting and cooling are fundamental for the correct formation of the solder joints and are therefore regulated by a thermal curve called “soldering profile” which must be studied and set for each product by an operator specialized in the soldering process.
The soldering coordinator is also responsible for the choice of raw materials and for the harmonization of the entire process.
AOI control
At the end of the work, a fundamental phase allows the control of the entire assembled board: the Automatic Optical Control System (AOI).
An electronic board is made up of a very large amount of components and, even more so of soldered connections, some of them are very small ones: a single critical point may be enough to affect the whole operation.
The AOI system captures images of each product by taking multiple photographs with its 8 cameras at various resolutions.
The acquired images are then processed by a very powerful software that compares them with the library of corrected images according to the IPC standards both as regards the quality of the solder joints, as well as for the positioning and polarity.
In this way the machine can alert if there are any points to be checked in any tab of the processed batch.
During the first production the performance of the machine is checked by qualified operators who can intervene by widening or narrowing some parameters where the board has particular morphologies.
The verification of the cards reported during production is performed by a qualified soldering operator trained on IPC standards who has the task of evaluating the defect, repairing the product and possibly giving back feedback for the improvement of the control programs.
Having an automatic optical control line not only allows you to keep the quality of the assembled products under control but it is also a great help for keeping processes under control.
In fact, if the data are analyzed continuously, it makes it possible to correct any process drift well in advance, highlighting the most recurring types of errors, thus promoting the maintenance of a stable process that tends not to produce errors.
