Flexible circuit board material
Electronic products play an important role in our life. Flexible circuit boards exist in mobile phones, digital cameras, cars and computers we use every day.
In 1898, Albert Hanson of Berlin, Germany, put forward the concept of flexible circuit board for the first time in his patent, describing how to coat a piece of paper with paraffin wax to make a flat conductor.
Although waxed paper has enough flexibility, today's FPC basically uses polyimide or polyester film as the base material. Polyimide material is the preferred material for almost all chips due to its non-flammability, geometric stability, high tensile strength and the ability to withstand welding temperature.
Polyester has a low dielectric constant and absorbs little moisture, but it is not resistant to high temperature. The melting point of polyester is 250 ℃ and the glass transition temperature (Tg) is 80 ℃, which limits their application in a large number of end welding applications. In low-temperature applications, they are rigid, but the cost is low. They are suitable for use on products such as telephones and other products that do not need to be exposed to harsh environments.
Type of flexible circuit board
Basic type
1. Single-sided FPC
• Common types * in today's production
• * Common and * suitable for dynamic deflection applications
2. Double-sided FPC
• There are two conductive layers
• Production with or without electroplated through-hole
3. Multi-layer FPC
• Multi-layer FPC
• With three or more conductive layers
• The circuit layers are interconnected by electroplated through holes
4. Rigid and flexible circuit board
• Hybrid structure, which is composed of rigid and flexible base plates with selective formation pressure
• Conductive connection with metallized holes
5. Dual channel/backplane flexible circuit board
• Only one conductive layer
• After treatment, it is allowed to contact the wire from both sides
• Commonly used for integrated circuit packaging
Why metallographic preparation?

Due to the miniaturization and lightweight development of the size and weight of electronic samples, they are widely used in consumer civil appliances, medical devices, automotive electronics and other industries. In order to meet the needs of these industries, the design and processing of circuits are also continuously improved. The production improvement of electronic products is a process in which multiple processes cooperate with each other. The quality of the products in the previous process will directly affect the production of the products in the next process, and even directly affect the quality of the final products. Therefore, the quality control of key processes plays a vital role in the quality of final products. As one of the detection methods, metallographic section technology plays an irreplaceable role in this field. The metallographic preparation of the cross section of the circuit board is used to evaluate the quality of the lamination system and the coating structure, such as plating through holes, welding cracks, voids, etc., to determine whether it meets the performance specification requirements. In specific cases, more targeted solutions can be adopted. For example, observe the cross section at the center of the FPC through-hole, or gradually thin the sample and observe the mass of each layer. The more information collected in the analysis process, the greater the possibility of further improvement of the product. In the process of analysis, professional equipment, such as IsoMet 1000 precision cutting machine and AutoMet series semi-automatic grinding and polishing machine, can greatly simplify the preparation process and improve the preparation efficiency.
Preparation process
1 Determine the observation position of the circuit cross section. The use of sharp cutting equipment, such as razor blades and diamond blades, requires smooth cutting of the circuit.
2 Fix the sample between two slides to ensure that the sample is flat and vertical. Use epoxy resin for bonding, clamp and place in an oven at 100 ° C for about 10 minutes or until the resin is cured.
3 Select the appropriate inlay according to the * high temperature that the sample can bear without deformation. Select EpoThin 2 or EpoxiCure 2 epoxy resin with good edge protection and low exothermic temperature.
4 Put the inlay mold into the SimpliVac vacuum inlay machine, and use the fixture (such as UniClip sample support fixture) to hold the sample, so as to provide the support force when pouring the epoxy resin, and ensure that the sample is vertical, so as to ensure that the grinding surface is perpendicular to the flexible circuit board in the slide.
5 Use SimpliVac vacuum inlay machine for cold inlay to ensure good adhesion and permeability between sample and resin. After the sample is completely cured, remove the inlay abrasive.
6 Complete the preparation of sample metallographic section according to the preparation scheme in the following table.
7 Clean and observe the cross section of the sample. Measure and record the average thickness of the coating, and complete all other project evaluations according to the customer's SOP.
Summary
Summary 1: In order to ensure sufficient grinding time, the sample surface can be observed under the microscope after each step. When uniform scratches are observed by the microscope, and the scratches will not be refined if grinding continues, it indicates that the current step has been completed, and the sample can be thoroughly cleaned and the next step can be entered.
Summary 2: Automatic preparation can remove materials more stably and improve the consistency of sample preparation results. Manual preparation is difficult to ensure that the sample surface is subjected to uniform pressure. The uneven force will cause the sample to tilt, and the measured values such as coating thickness or IMC thickness will be much larger than the actual data due to the sample tilt. These key data will lead to the failure of project judgment due to unreliable preparation factors.