Design performance of multilayer PCB Board is similar to most of single layer and double layer PCB Board, it can avoid too much circuit fill in the very limited space, lead to impractical tolerance, high inner layer volume, even threaten the safety of product quality. Therefore, performance specification should consider complete evaluation include thermal shock, insulation resistance, resistance welding of inner layer circuit, below content discuss the mainly aspects during the process of multilayer PCB Board design.
1 Mechanical design factors:
Mechanical design include choose appropriate PCB Board dimension, PCB Board thickness, PCB Board lay-up, inner layer copper thickness, aspect ratio.
1> PCB Board Dimension
Dimension should be designed according to application, size of system cabine, production capability limitation of PCB Board manufacturer to choose the optimal selection. Big PCB Board has many advantages, such as less laminate, shorter circuit routes among different components, which will result in higher operation speed, moreover, each PCB Board can have more IN/OUT connection, therefore big PCB Board should be 1st choice among different kinds of application. For instance in the personal computer, most of motherboard we can see are all big PCB Board. However, design signal wire layout is not an easy task, since it needs more signal layer and inner layer connection or space, the difficulty level of heat treatment could be higher too. Designer should take various reasons into account, such as standard board dimension, dimension of manufacture facility and limitation of manufacture process. In IPC-D-322 has listed some guidance about how to choose the right dimension of Printed Circuit Board.
2> PCB Board thickness
Thickness of PCB Board is determined by different elements, such as amount of signal layer, thickness and quantity of power layer, hole quantity and diameter of through-hole as well as aspect ratio, lead length of automatic insert component and connection type. Total thickness of PCB Board is make up of conducting layer, copper layer, base laminate and pre-dip material. It is difficult to control the multilayer PCB Board, so +/-10% tolerance can be viewed as reasonable.
3> Lay-Up Structure
In order to decrease PCB Board twist into the minimum level so can have as flat as possible, the inner layer of multilayer PCB Board should be keep symmetrical. So the copper layer quantity should be even, and ensure copper thickness and copper circuit pattern symmetrical. Generally the material and structure of laminate (such as glass fiber) should parallel with the edge direction of PCB Board. Because after pressing the laminate will constrict which will cause the layout of PCB Board distorted, more liable to variability and low space stability.
However, through improve PCB design can make the bow and twist of multilayer PCB Board to minimum level. Through the dissipation of copper to ensure the structure symmetry, as well as the deep-dip material being separate evenly with reasonable thickness, through which can reach the purpose of decreasing bow and twist. PCB Board manufacture should start right from the core layer, till the outer layer of surface. The minimum thickness among 2 copper layer (dielectric layer) is 0.08mm.
According to the experience, the minimum distance between 2 copper layer, that is presoak material minimum thickness after laminate should at least 2 times of copper thickness. In other words, if the 2 adjacent copper layer thickness is 30μm, so the dielectric material thickness should be at least 2*(2*30μm)=120μm, this thickness will need to 2 piece of 1080 pre-preg material.
4> Inner layer copper
Most frequent used copper is 1oz, however, for the high intensity PCB Board, it is thickness is critical since it will has strictly impedance control. This kind of PCB Board will use 0.5oz copper, for power layer and ground layer, we should choose 2oz or even heavier copper. Etching heavier copper will decrease the controllability, it could bring difficulty to realize the tolerance pattern of circuit width and space. So it need special treatment.
5> Hole
According to the diameter of component lead or dimension of catercorner, plated through hole’s diameter normally remain among 0.028 to 0.01 inch, which can ensure the enough space and better soldering.
6> Aspect Ratio
“Aspect Ratio” is the ratio among PCB Board thickness and hole diameter. Normally speaking 3:1 can be viewed as the standard ratio, although 5:1 is also quite normal. Aspect Ratio can be determined through different elements like drilling, desmear or etch-back and plating.
2 Electric Design
Multilayer PCB Board is a high performance, high speed system. Refers to high frequency, time length of signal increase become shorter, therefore signal reflection and circuit length control become quite important. In the multilayer system, the requirement of component controllable impedance performance is very high, so electric design should follow above requirement. About elements decide the impedance value include laminate and pre-preg material dielectric, wire space of same layer, dielectric thickness and copper conductor thickness. In the high speed application, laminate sequence and signal network connecting sequence are paramount for multilayer PCB Board, dielectric parameter is critical to determine impedance, propagation delay and capacitor. The dielectric of epoxy glass laminate and prepreg can be modified through adjust content level of resin.
Epoxy resin dielectric parameter is 3.45, glass is 6.2. Through control the content level of these material, the dielectric of epoxy-glass can reach 4.2-5.3.
Relative low dielectric of prepreg material is more suitable for radio frequency and microwave circuit. In the radio frequency and microwave, lower dielectric parameter will has less signal propagation delay. In the base laminate, low dissipation factor can make the electric loss reach the minimum level.