PCB Stackup

PCB Stackup

The goal of any PCB stackup design is to select the material and specify the layer

Ordering such that it adequately delivers the required signal performance and power

Integrity at the lowest PCB cost.

PCB stack-up is an important factor in determining the EMC performance of a product.  A good stack-up can be very effective in reducing radiation from the loops on the PCB (differential-mode emission), as well as the cables attached to the board (common-mode emission). 

On the other hand a poor stack-up can increase the radiation from both of these mechanisms considerably.

Two Layer Stackup:

ASSEMBLY TOP

PASTE MASK TOP

SILKSCREEN TOP

BOARD FINISH

SOLDERMASK TOP

COPPER PLATING

TOP(Layer1)                                  

BOTTOM (Layer2)                           

COPPER PLATING

SOLDERMASK BOT

BOARD FINISH

SILKSCREEN BOT

PASTE MASK BOT

ASSEMBLY BOT

Four Layer Stackup:

ASSEMBLY TOP

PASTE MASK TOP

SILKSCREEN TOP

BOARD FINISH

SOLDERMASK TOP

COPPER PLATING

TOP (Layer1)                                   

GROUND(Layer2)                                   

POWER(Layer3)                                   

BOTTOM (Layer4)                           

COPPER PLATING

SOLDERMASK BOT

BOARD FINISH

SILKSCREEN BOT

PASTE MASK BOT

ASSEMBLY BOT

Six Layer Stackup:

ASSEMBLY TOP

PASTE MASK TOP

SILKSCREEN TOP

BOARD FINISH

SOLDERMASK TOP

COPPER PLATING

TOP (Layer1)                                   

GND (Layer2)                                  

SIG1 (Layer3)                                  

SIG2 (Layer4)                                  

PWR (Layer5)                                   

BOTTOM (Layer6)                            

COPPER PLATING

SOLDERMASK BOT

BOARD FINISH

SILKSCREEN BOT

PASTE MASK BOT

ASSEMBLY BOT

Electrical layers:

TOP                                   

GND                                  

SIG1                                 

SIG2                                  

PWR                                  

BOTTOM                          

Non Electrical layers:

ASSEMBLY TOP

PASTE MASK TOP

SILKSCREEN TOP

SOLDERMASK TOP

SOLDERMASK BOT

SILKSCREEN BOT

PASTE MASK BOT

ASSEMBLY BOT

NOTE:

•         ASSEMBLY & PASTE LAYERS (4 Layers) will not be on fabricated PCB, Those layers are being used only at the time PCB Assembly Process. Assembly layers will be converted in to printout and used to check the ref des of components while assembling, where as Paste layers will be converted in to stencils and used to put solder paste on the SMT foot print pad before doing reflow assembly.

•         All above layers will be taken as GERBERS from the design tool except for BOARD FINISH & COPPER PLATING layers, these layers will be taken care by the fabricators upon provided information.

•         These 8 Non electrical layers & Board Finish & Cu plating will be same for all type board stackup except ELECTRICAL LAYERS, say for 14 layer stackup, electrical layers will be different but it will have same other layers as like Two layer stackup, 4 layer and 8 layer stackup.

Typical PCB Stackup:

•         Stackup is symmetric about the center of the board in the vertical axis to avoid mechanical stress in the board under thermal cycling (By avoiding bow & twist of board)

•         Even number layers only will have symmetric stackup rather than odd numbers, let’s say 4, 6, 8 14 does but not 5, 7 or 9

•         Symmetry should be considered in copper thickness and in dielectric thickness from the central axis.

•         A signal layer should always be adjacent to a GND plane

•         Signal layers should be tightly coupled (close) to their adjacent GND planes. . So that, the return current will take less Inductive loop and then by reduces the EMI

•         Power and Ground planes should be closely coupled together. There by act as a parasitic capacitor and reduces noises.

•         High-speed signals should be routed on buried layers located between planes.  In this way the planes can act as shields and contain the radiation from the high-speed traces.

•         Multiple ground planes are very advantageous, since they will lower the ground (reference plane) impedance of the board and reduce the common-mode radiation.

•         The total number of layers required for a given design is dependent on the complexity of the design.  Factors include:  the number of signal nets that must break out from a BGA; the number of power supplies required by the BGA’s; component density and package types.