September 12, 2023 at 4:18 pm
umesh maheshwari
Subscriber

Hi All

 

I have made a 12 layer PCB board having dimensions as follows: 125 mm x100 mm x 2.37 mm. Top and bottom copper layer is 1 oz whereas inside layers are 0.5oz. The PCB board has 4 nos. of diameter 3.4 mm holes whose center is at 5 mm inside from the edge. Each copper layer has 20% copper content and it is the same from top to bottom layer and dielectric material is 370HR (FR4). I did LLS for this board and the first natural frequency observed is 345 Hz.

 

I used trace mapping technique to simulate the same in ansys mechanical. I have few observations which are as follows:

 

Case 1:

 

Initially I imported the trace on a 3D layered PCB and holes were discarded but nodes at the center of hole location from top to bottom at all 4 places were taken as fixed support boundary conditions. The first natural frequency observed is 346 Hz which is close to the actual vibration result. 

 

Case 2:

 

PCB is modelled as shell (2D) and trace is imported  and again holes are discarded but node at the center of hole location at all 4 places is taken as fixed support boundary condition. The first natural frequency observed is 347 Hz which is also close to the actual vibration result.

 

Case 3:

 

PCB is modelled as shell (2D) and trace is imported. This time holes are present in the PCB board  , these holes edges are taken as fixed support boundary conditions. The first natural frequency observed is 426 Hz which is also nowhere close to the actual vibration result.

 

 

 

Case -3 is the actual representation of the actual vibration test then why fundamental frequency is not matching for this case whereas for Case-1 & 2 it is close to actal vibration test result.

 

 

 

I am not able to understand why such a thing is happening?

 

I know when I am adding holes and fixing holes edges as fixed boundary condition, this is increasing the stiffness of PCB , hence higher frequency for this case but this is the actual condition during vibration test then why such a difference is

occuring?

 

 

 

Please help.