Due to some of my clients asked me some problems about the buildup (stack-up) frequently, I decide to write a article and hope it will be useful to all who don’t clear it.
PCB Stack-up is very important,especially for multi-layers.PCB engineers need to calculate the residual rate of copper & rubber content for inner layers first,then they will select suitable core and PP to meet the thickness of board.It will consider more if there is impedance control,it will be more complex.
All Prepreg and Core have their own theoretic thickness,but they are not the actual thickness while laminate. Below is one of our clients asked:
Why they different? Same prepregs but there are different actual thickness,Obviously,they are depend on the the residual rate of inner copper. Because when pressing the High temperature,rubber will be melt and flow into the area where no copper while laminate,thickness of PP will be smaller than the theoretic thickness.That’s why different residual rate of copper will cause different actual thickness.
However, the small different will not affect the wrap and twist of board .As you known, lamination select the symmetrical stack-up is good for the twist, and it will cause sloder better.
By the way, low RC of Prepreg is not allowed to use alone between inner layers,it will cause layers separate easily.And we will use more PP combination if inner layers are thick copper,because it need more rubber to fill.
Buildup for PCBs with impedance control is more complex.In order to meet both impedance and finish thickness,we need use special software Si9000 to calculate and select the right prepreg and core.Below is model for differential impedance:line width/space:4/6mil and the value is 100ohm.
PCBA123 who focous on PCB over 10 years hope this article is useful to you. If you have other questions about PCB, you can also talk to us, and we will do our best and be happy to share with you, our email address: [email protected] .
By the way, we are a professional PCB manufacturer from China with strong capability.We offer one stop PCB services worldwide, includes pcb assembly service.