PCB Manufacturing factory no inventory is very common, even a lot of regular ordinary FR4 sheet will also appear this situation. At this time, many board factories or market personnel will negotiate with customers in order to rush to meet the deadline, "can we change the same performance grade materials?" The customer will ask back, "what kind do you want to change? How to ensure performance match? " Often it's time for us to show up again!
The factors that should be considered in the selection of PCB boards are mainly internal and external factors. Let's take a look at the internal factors first. Internal factors are mainly internal characteristics of materials. Usually we need to focus on electrical performance, thermal performance and physical (Mechanical) performance. Here are some of the main features from these major points.
1. Electrical properties are mainly dielectric constant (Dk) and loss angle (Df).
DK is the abbreviation of dielectric constant, which is also called dielectric constant or dielectric coefficient in Chinese. It is a coefficient indicating the characteristics of insulation capacity, expressed in the letterε. In engineering applications, the dielectric constant is often expressed in the form of relative dielectric constant, rather than absolute value.
DF is the abbreviation of dispatch factor, which is called dielectric loss factor in Chinese. It is also called damping factor, internal distribution or loss tangent. It is the tangent of the phase difference angle between strain and stress period under the action of alternating force field. It is also equal to the ratio of loss modulus and storage modulus of the material (generally speaking, the energy lost in insulating plate in signal line And the ratio of on-line to medium energy. The loss is closely related to DK & DF. The following is the approximate calculation formula of dielectric loss.
A=2.3*F*Dk*Df
Among them, A is the loss of unit inch (dB/inch), F is frequency (GHz), Dk is relative dielectric constant, Df is loss factor;Of course, there are other factors that affect the final loss. This formula only evaluates the line dielectric loss per unit length from the perspective of the medium itself. We can also get the data of electrical characteristics through the material specifications. It can be seen that electrical characteristics have parameters of dielectric constant and loss factor, and other parameters such as bulk resistance and surface resistance, which we generally pay less attention to. These parameters only need to meet the specification requirements of IPC. Some special products may pay attention to these parameters, which is not the focus of our attention here.
II. Characteristic parameters related to thermal performance
For the definition of thermal performance parameters, mainly TG value, TD value, CTE and t288 / T260 / T300, we will introduce these parameters respectively.
TG value is the abbreviation of glasstransition temperature, i.e. glass transition temperature, which refers to the temperature of glass transition between glass state and high elastic state (commonly referred to as softening). In PCB industry, the glass state material generally refers to the medium layer composed of resin or resin and glass fiber cloth. The Tg of common TG plate is required to be greater than 135℃, medium TG is required to be greater than 150℃, high Tg is required to be greater than 170℃, and the higher Tg value is, the better heat resistance and dimensional stability are generally. Generally, there are different test methods for TG value, such as DMA, DSC and TMA. The test results of different test methods may be different, so we generally compare the TG values of different materials in the same test method.
TD value is the abbreviation of thermal decision temperature, also known as thermal decomposition temperature. It refers to the temperature when the base resin loses 5% weight when heated. It is a sign that the base material of PCB causes delamination and performance degradation when heated. Generally, TGA test method is used. The higher the TD value is, the better the material stability is (of course, relatively speaking).
CTE is the abbreviation of thermal expansion, also known as thermal expansion coefficient. Generally, the linear expansion coefficient is used to measure the performance of PCB board. It is defined as the ratio of the increase of length under the change of unit temperature to the original length, such as z-cte, which ultimately affects the change size of board thickness. The lower CTE value is, the better dimensional stability is, and vice versa.
T288 is a technical index reflecting the welding resistance condition of printed board substrate. It refers to the longest time that the substrate of printed board undergoes welding high temperature without blistering, delamination and other decomposition phenomena at 288℃, and the longer the time is, the more favorable for welding.T260 / T300 also means the same, but the temperature index is different, so it will not be explained here.
III. parameters related to physical (Mechanical) performance
Basically, it is water absorption, peel strength, bending strength and flammability. This part is not explained too much. You can go to Google search by yourself.
In addition to some of the parameters mentioned above, some of the material parameters may be more subtle and may not be found in some manuals, but they have to be considered, such as CAF resistance and CTI.
CAF is the abbreviation of conductive Anodic Deposition, also known as ion migration. It refers to the chemical reaction of electromigration of metal ions in the non-metallic medium under the action of electric field, so as to form a conductive channel between the anode and cathode of the circuit and cause the circuit short circuit. With the rapid development of electronic industry, electronic products are light, thin, short and small, the hole spacing and line spacing of PCB will become smaller and smaller, and the circuit will become more and more fine, so the ion migration resistance of PCB becomes more and more important.
CTI is short for comparative tracking lndex, also known as relative leakage index (or comparative leakage index, leakage tracking index). It refers to the maximum voltage value of the material surface that can withstand 50 drops of electrolyte (0.1% ammonium chloride aqueous solution) without leakage trace, unit: v.
I've talked about the internal cause. Let's talk about it. There are not many complicated academic explanations for these factors. We can see clearly some of the environmental requirements involved.
1. EU ROHS directive
It is mainly used to standardize the material and process standards of electronic and electrical products, so as to make them more conducive to human health and environmental protection. The purpose of the standard is to eliminate six substances in the electronic products of electric motors, including lead (PB), cadmium (CD), mercury (Hg), hexavalent chromium (Cr6 +), PBBs and PBDEs, and the content of Lead shall not exceed 0.1%. The standard has been formally implemented in July 1, 2006, so many PCB welding is also needed for lead-free, all of which are based on environmental considerations.
2. WEEE instruction
Directive on discarded electronic and electrical equipment
3. EU REACH regulation no SVHCs
It is a European Union regulation on the preventive management of all chemicals entering its market. It was officially implemented on June 1, 2007.
In addition to the above common environmental protection instructions and regulations, some countries and regions also have relatively strict restrictions on halogen materials, so many halogen-free materials come out later, and now many high-speed plates slowly come out corresponding halogen-free requirements, such as tu862hf, it958g, it988gse, etc.
The following figure 1 shows the signs of common environmental protection requirements.
After reading the above introduction, I don't know if you have some basis when replacing materials. Of course, there may be some differences between the manual of many materials and the actual situation, which requires you to collect materials and test and verify them at ordinary times. There is a saying that goes well: "master brings in the door, practice looks at the individual". I hope you can reach the following advertisement after reading the article I can only help you here.
