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The irreversible development of modern electronics has been increasingly pushing PCBs towards such demands as miniaturization, light weight, high speed, better functionality and reliability, and longer lifetime, which results in the popularity of multilayer PCBs. Combined by a type of semi-solid adhesive which is called "prepreg", two or more single and/or double PCBs are stacked together to generate multilayer PCBs through reliable predefined mutual connection between them. There are three or more conductive layers in one multilayer PCB with two layers outside and one layer synthesized in the insulation board. With the increase of PCB complexities and densities, possible problems will take place such as noise, stray capacitance and cross talk when layer arrangement gets inefficient design.
Planning the good multilayer stack-up is the most important in determining the Electromagnetic Compatibility (EMC) performance of a product. A well-designed layer stack-up can both minimize the radiation and can stop circuit from being interfered by external noise sources. Well-stacked PCB substrates can also reduce signal cross talk and impedance mismatch issues. However, an inferior stack-up may get EMI (Electromagnetic Interference) radiation rising, because reflections and ringing in the system as a result of impedance mismatch can dramatically reduce product performance and reliability. This article then focuses on layer stack up definition and its designing rules and essential considerations.
What is stack-up?
Stack-up refers to the arrangement of copper and insulating layers that make up a PCB prior to board layout design. For multilayer PCBs, general layers include ground plane (GND plane), power plane (PWR plane), and inner signal layers. Figure 1 is a sample stack-up of an 8-layer PCB.
In accordance with this figure, it's obviously clear to indicate layer distribution in PCBs conforms to a symmetrical or balanced structure. Apart from 8-layer distribution, the spacing between layers should be taken seriously as well. To meet the requirement of miniaturization, minimum trace spacing must be obtained while planning layer stack-up. The space between layers can be either core or prepreg. Multilayer boards usually consist of at least one or more cores and prepreg. Cores are made up of a copper-plated glass-reinforced epoxy laminate sheets. The thickness of core is in the range from 0.1mm to 0.3mm.
Prepreg is the common term for a reinforcing fabric which has been pre-impregnated with a resin system. This resin system (typically epoxy) already includes the proper curing agent. The main function of prepreg is to stack all layers into a whole board by high temperature. The following table shows physical and chemical attributes of main categories of Prepreg, that is, 7628, 2116 and 1080.
|Type||Resion Content (%)||Resion Flow (%)||Gelation Time (s)||Thickness after solidification (mm)||Thickness (mm)|
Actually, thickness of each type of prepreg isn't always stable and some adjustments will possibly be made in order to meet the specific board thickness requirement. Some factors have to be taken into consideration when determining the count of prepreg including thickness of inner layer, product design thickness requirement or manufacturing technology requirement, features of prepreg, practical performance and the actual thickness after stack-up trial. Figure 2 below indicates a sample 4-layer PCB whose layer distribution and thickness are depicted.
In this example, the thickness of copper thickness is stable so that the thickness of prepreg and core should be adjusted in order to meet the whole thickness requirement. Two methods are available to be compatible with the adjustment mentioned above:
• Interlayer offset. Resin recession groove is used in board side design instead of choked flow pad. In regards with stack-up positioning, hot melting plus rivet plus dowel method can be used to solve stack-up offset problem.
• Stack-up measling. In the process of board arrangement, silicon pads can be added, together with the assistance of epoxy plate so that pressure will remain balanced, which can both help eliminating stack-up measles and effectively control the uniformity of board thickness.
Based on chemical and physical attributes of prepreg, you have an access to estimate thickness or copper weight of final circuit board through an easy calculation.
PCBCart's Standard PCB Layer Stack-up
PCBCart manufactures standard multilayer boards with layers in the range from 4 to 32 layers, board thickness from 0.4mm to 3.2mm, copper thickness from 18μm to 210μm (0.5oz to 6oz), inner layer copper thickness from 18μm to 70μm (0.5oz to 2oz), and minimal spacing between layers to 3mil.
Images below display PCBCart's standard stack-up capabilities and dimensions. Did not find the layer stack-up for your PCB boards? Inform us your PCB thickness and layer count using this form, and you'll receive corresponding stack-up very quickly.