Functions of Holes on PCB Board
When designing electronic devices, it is necessary to focus not only on the design of the printed circuit board layout, but also take into account the mechanical and manufacturing aspects of the design process.
Fixing holes play an important role in PCB design. These holes, located on the technological field of the blank, are designed to ensure the correct location of the photomask of the printed circuit board pattern on its blank during exposure and the correct arrangement of the layers of the multilayer printed circuit board during pressing.
Let’s look at a few guidelines to follow when placing fixing holes on a PCB.
Fixing Holes (PCB Tooling Holes)
Although the creation and placement of fixation holes is one of the easiest operations in PCB design, they are essential in fabrication and assembly. There are three main factors to consider when placing fixing holes:
- Placement in panelization
- Hole diameter and coating
- Placement with fiducials
Boards or panels (blanks)
PCB manufacturers do not produce individual printed circuit boards, but entire panels. A panel (blank) is a sheet consisting of a dielectric base laminated on both sides with copper foil. Fixing holes are placed on the technological field of the panel (blank) to facilitate the manufacture of the printed circuit board.
Most manuals from PCB fabricators and assemblies clearly state that fixing holes must be placed on the PCB panel. Therefore, if you are designing your own panel, place fixing holes in the corners of the panel. It should also be taken into account that manufacturers can only accept a customer-designed panel if it fits their size.
Now let’s look at the differences between fixing holes and mounting holes on a PCB.
Differences between fixing and mounting holes
The main difference between fixing holes and mounting holes lies in the plating: fixing holes are through holes without plating, while mounting holes are usually plated with copper. Another difference is the diameter: fixing holes are generally standard sizes, while mounting holes depend on the mounting hardware used to secure the board to the case.
Fiducials are reference points located on a printed circuit board that allow screen printing equipment, surface mount equipment, and automated optical inspection equipment to see how the board is oriented in space. Test points should be placed at the 3 corners of the PCB layout, this allows you to accurately determine the orientation of the PCB. Fiducial marks are also recommended to be placed in the corners of the panel, slightly shifted from the fixing holes.
Placing fixing holes in the PCB
In order to place fixing holes on a PCB, in electronic design CAD, you need to use the “place a hole” tool and set parameters: diameter, coating, etc. Altium Designer contains a complete set of tools for designing and laying printed circuit boards. No matter what types of holes are used in a PCB, Altium Designer can help you create high-quality designs and get them ready for production.
During the corrosion process of printed circuit boards, the two-sided boards with a metallized hole need an even more special protection against corrosive attack, in relation to the single-sided boards . This occurs both with regard to the tracks, which are generally much thinner than in single-faced plates, and in the protection of the holes, which, because they are metallized, would be susceptible to corrosive attack.
Negative image – copper tracks
It is for this reason that the protection is done chemically through tin , which is applied in its liquid form by electrolysis and, with this, it can penetrate completely in all holes and in any point that is not protected by the film, applied in the process of image transfer . In the process, the circuit appears in negative, making it possible to apply tin in all points that must not be corroded .
Before corrosive attack, the film or paint is carefully removed. After removal, the plate looks like a copper bottom and pewter tracks and pads.
Tin tracks – copper bottom to be corroded
Another reason we have tin as a protector in the corrosion process is that it has chemical properties that make the corrosive, usually based on ammonia, attack only copper, without any type of attack on tin, which would not work with many others metals
Corroded plate with intact tin
It is extremely important that the corrosion has maximum speed, so that there is no time for the corrosive to attack the tracks laterally, since as the corrosion occurs, the corrosive comes into contact with the side of a track exactly in its thickness, which will not be protected by the tin.
Therefore, it is standard for the plates to be manufactured with ½ ounce copper, which greatly favors this speed of corrosion, which occurs only where there are no tracks. The tracks have their thickness increased to 1 ounce or more.
It is that before the application of the tin, they also receive by electrolysis, an additional copper layer, but that only increases the thickness of the tracks, pads and the walls of the metallized holes. , leaving only the points to be corroded with the original laminate thickness of ½ ounce. This is what ends up favoring a much faster corrosion. After corrosion, it is necessary to remove the tin that protected the copper.
Tin being removed
A few decades ago, this tin was not removed; it was submerged in reflow oils at high temperatures, making it shiny and ready for soldering. However, the presence of tin on the tracks also made it impossible for the anti-weld mask to adhere well, which ended up loosening during the soldering processes of the plates.
From the implementation of the HASL (Hot Air Solder Leveling) processes, which occurs after the application of the anti-solder mask, this tin that protects the copper in the corrosion process started to be removed, leaving the plate with the circuit only in copper and ready for the impressions of anti-solder mask, legend of components and the other finishes of the printed circuit board .
Corroded plate and tin removed
The corrosion process of double-sided plates needs to have all parameters very well adjusted, establishing the perfect relationship of density, temperature and PH of the corrosive, as well as the speed of the conveyor, in addition to the pressure and direction of the corrosion nozzles, which emit corrosive jets simultaneously on both sides.
For this, the corrosors are equipped with 2 sets of pumps and nozzle frames , which emit these jets from top to bottom and from bottom to top, with well-calculated sizing so that the attack occurs at the same intensity on both sides.
Corrosive spray jet
Any mismatch in these dimensions could cause excessive corrosion on one side and undercorrosion on the other side. Therefore, it is of fundamental importance to constantly observe the correct application of all these parameters.
Therefore, the maintenance and adjustment of our machines are carried out periodically, always observing the production and quality standards in tests that prove the efficiency of all the procedures carried out at FX PCB China.