There are two main categories of PCB material: organic substrate materials and inorganic substrate materials, and the most used is organic substrate materials. The PCB material used for different layers is also different, for example, 3-4 layers are prefabricated composites, and double panels are mostly glass-epoxy materials. Materials used to make printed circuit boards (PCBs) include a set of insulating/dielectric and conductive materials used to construct circuit board interconnects. A wide variety of materials are available to meet different performance and budget requirements. The type of material used to manufacture PCBs is a key factor in the durability and functionality of PCB components. Choosing the right PCB material requires an understanding of the available materials and their physical properties, as well as how they align with the desired functionality of the board.
1. Select materials according to SMT requirements
In the process of lead-free electronic assembly, due to the increase in temperature, the degree of bending of the printed circuit board when heated increases, so it is required to use a plate with a small degree of bending as much as possible, such as FR-4 and other types of substrates. Due to the influence of the expansion and contraction stress of the substrate after heating on the component, it will cause the electrode to peel off and reduce reliability, so the material expansion coefficient should also be paid attention to, especially when the component is larger than 3.2×1.6mm. PCB used in surface assembly technology requires high thermal conductivity, excellent heat resistance (150°C, 60min) and solderability (260°C, 10s), high copper foil adhesion strength (1.5×104Pa or more) and flexural strength (25×104Pa), high conductivity and small dielectric constant, good blankability (accuracy ±0.02mm) and compatibility with cleaning agents, in addition to smooth appearance, no warpage, cracks, scars and rust spots.
2. PCB material thickness requirements
The thickness of printed circuit boards is 0.5mm, 0.7mm, 0.8mm, 1mm, 1.5mm, 1.6mm, (1.8mm), 2.7mm, (3.0mm), 3.2mm, 4.0mm, 6.4mm, of which 0.7mm and 1.5mm PCB thickness is used for the design with gold finger double panel, 1.8mm and 3.0mm are non-standard sizes. Printed circuit board size from the production point of view, the smallest single board should not be less than 250×200mm, the general ideal size is (250~350mm) × (200×250mm), for the long side less than 125mm or the wide side less than 100mm PCB, easy to use the panel method. Surface mounting technology specifies a bending amount of 1.6mm substrate with a thickness of 0.5mm for upward warping ≤ and 1.2mm for downwarping ≤. The generally allowable bending rate is below 0.065% according to the metal material divided into 3 types, typical PCB shown; According to the structure of soft and hard divided into three types, electronic plug-ins are also developing towards high foot count, miniaturization, SMD, and complexity. Electronic plug-ins are pinned on a circuit board and soldered to the other side, a technique called THT (ThroughHole Technology) plug-in technology. This drills holes for each pin on the PCB board, illustrating the typical application of the PCB.
3. PCB drilling requirements
With the rapid development of SMT chip technology, multilayer circuit boards need to be conducted, which is guaranteed by electroplating after drilling, which requires various drilling equipment. In order to meet the above requirements, at present, PCB CNC drilling equipment with different performance is launched at home and abroad. The production process of printed circuit board is a complex process, which involves a wide range of processes, mainly involving photochemistry, electrochemistry, thermochemistry; There are also many process steps involved in the production and manufacturing process, taking hard multilayer circuit boards as an example to illustrate their processing processes. In the whole process, drilling is a very important process, the processing of the hole occupies the longest time, the position accuracy of the hole and the quality of the hole wall directly affect the subsequent hole metallization and placement and other processes, but also directly affect the processing quality and processing cost of the printed circuit board, CNC drilling machine principle, structure and function The common methods of drilling on the circuit board are CNC mechanical drilling method and laser drilling method, etc., at this stage, the mechanical drilling method is the most used.
4. Four main features (from IPC 4101 – Specification for Rigid and Multilayer PCB Materials) The variety of PCB material is critical to help define the properties of the base material.
(1) CTE – Coefficient of thermal expansion is a measure of how much a material expands when heated. This is crucial on the z-axis. Typically, the expansion is greater than the decomposition temperature (Tg). If the CTE of the material is insufficient or too high, failure may occur during assembly because the material expands rapidly above Tg.
(2) Tg – The glass transition temperature of a material is the temperature at which the material changes from a rigid glassy material to a more elastic and bendable rubber-like material. At temperatures higher than Tg material, the expansion rate increases. Materials can have the same Tg but have different CTEs. (A lower CTE is desirable).
(3) Td – The decomposition temperature of the laminate material. This is the temperature at which the material decomposes. Reliability is compromised, and delamination can occur when the material releases 5% of its original weight. Higher reliability PCBs or PCBs operating under demanding conditions will require TD greater than or equal to 340°C.
(4) T260/T288 – Delamination time at 260°C and 280°C – cohesion failure of laminate material due to thermal decomposition (Td) of epoxy resin matrix when the PCB thickness is irreversibly changed. To choose the most suitable laminate material for your PCB, it is important to know how you want this material to perform. One of the purposes of material selection is to align the thermal properties of the laminate material tightly with the components that will be soldered to the board.