How to choose suitable self tapping screws?

I： Used for metals (metal sheets, aluminum profiles, steel frames)
Thread style:
Mechanical Thread (Fine Thread): Similar in appearance to machine screws, with a finer pitch. Pre drilled bottom holes are required, which have high connection strength and are suitable for thick and tough metals.
Triangle thread: with a triangular cross-section, it has strong cutting ability and is suitable for thin steel plates, aluminum, copper, etc.
Fast thread: With a wide pitch, deep threads, fast screwing speed, and strong clamping force, it is currently one of the most commonly used types.
Key features:
Self drilling and self tapping: For metals, it is best to choose screws with drill bits. The tip of the drill bit can drill holes on its own without the need for pre drilling, greatly improving efficiency. The length of the drill bit determines the thickness of metal it can penetrate (such as commonly used 5mm, 10mm, 12mm).
Used for plastics (PVC, nylon ABS、 Fiberglass reinforced plastic
Thread style:
Spacing thread: The thread spacing is wide to reduce stress concentration on plastic materials and prevent plastic from expanding and cracking.
Large thread angle: usually 48 ° or 60 °, providing greater contact surface and retention force.
Key features:
Usually no self drilling is required, the tip is a slender guide tip for easy positioning.
Bottom holes must be pre drilled, and the diameter of the bottom hole is crucial (as will be described later).
Used for wood (solid wood, particleboard, density board, multi-layer board)
Thread style:
Coarse thread: The thread is deep and has a large spacing, which can strongly bite wood fibers and provide excellent clamping force.
Fine thread: Used for hardwood or areas that require finer connections.
Key features:
Common drywall nails (horn heads, fine teeth) are also widely used in wooden structures.
Fiberboard nails (double threaded) are specifically designed for particleboard and density board, which can effectively prevent board swelling and cracking.
Used for other materials (gypsum board, cement fiberboard)
There are specialized gypsum board screws (usually with fine teeth, horn heads, high-frequency quenched black phosphating treatment) and cement fiber board screws (with self drilling bits and anti-corrosion coatings).

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II： Determine the size specifications of the screws
1. Diameter: Refers to the nominal diameter of the threaded portion. Common ones include M3.5, M4.2, M4.8, M5.5, M6.3, etc. (metric). The larger the diameter, the stronger the tensile and shear forces.
2. Length: refers to the total length from the bottom of the screw head to the tip. The selection principle is that the depth of screw insertion into the connected material should be at least 2-3 times the thickness of the material to ensure sufficient biting force and strength. For example, to fix a 2mm iron plate onto a 10mm thick aluminum frame, screws with a length of at least 14mm (2mm iron plate+12mm aluminum frame bite) should be selected.

III： Choose the head style of the screw
1. Pan head/flat head: There is a flat transition between the head and the screw rod, which requires countersunk installation to obtain a flat surface.
2. Horn head/umbrella head: Most commonly seen in drywall nails and wood screws, the conical head can sink into softer materials on its own.
3. Hexagonal flange head/hexagonal washer head: The head comes with a gasket, providing a larger pressure bearing area and good anti loosening effect. It can be used for quick installation of wrench or socket.
4. Sink head: The top is a 90 ° or 82 ° cone, used in situations where the screw head needs to be flush with the surface.
5. Semi countersunk head: The top has a slightly decorative curvature, similar in performance to a countersunk head.
Dimension 4: Choose slot type (driving mode)
Cross groove (PH/PZ): The most commonly used, PZ (Pozidriv/meter groove) has better anti slip ability and greater torque than PH (Phillips/cross groove).
One slot: It is now less commonly used and prone to slipping.
Allen: Able to withstand high torque, not easy to slip, commonly used in situations that require high-strength fastening.
Star shaped slot (Torx): Highly recommended. It has the best torque transmission efficiency, is extremely resistant to slipping and damaging the screwdriver head, and is currently the preferred choice for mid to high end screws.
Composite groove type: such as hexagonal flange head with inner hexagon, providing multiple installation methods.