Learning Forum

[umesh_maheshwari]

I have made a 12 layers PCB which has uniform layer copper content (20%) and pattern from top to bottom layers.

I used trace mapping technique for PCB modelling followed by modal analysis.

The result of modal analysis and actual vibration test results are as follows:

• modal analysis first mode frequency : 447 Hz
• actual vibration test frequency observed : 345.6 Hz

 

Layers thickness details are as follows:

PCB dimensions: 125x100x2.3773 (all are in mm).  Bare PCB Mass: 55gm

 

I can share the PCB file if required.

 

Please help which boundary conditions will leads to simulation results close to vibration test results.

 

 

[Ashish Khemka]

Hi,

Based on the analysis result it looks like the structure is stiffer than in the real life scenario. Can you compare the setup with the test model? What may cause the stiffness of the structure to increase?

 

Regards,

Ashish Kumar

[umesh_maheshwari]

I have one more doubt. 

For this analysis , i have used FR4 properties given in Ansys mechanical database. FR4 is an othrotropic material but i found one for observation that shear modulus is calculated assuming it as isotropic material.

Please see the properties sheet attached from ansys database.

My PCB is made of Isola 370HR material for which i don't have mechanical properties hence i used ansys FR4 properties.

Can someone provide me the mechanical properties of Isola 370HR?

Thanks in advance.

 

[umesh_maheshwari]

@ Ashish kumar

 

Actual test setup photo is attached here.

 

Simulation model is like as below:

Following boundary conditions are taken for simulation:-

1. Bottom Face of all 4 aluminium spacer are given as fixed boundary condition.

2. M3 screw is modelled as beam element (radius:1.5 mm) from pcb hole top edge to spacer bottom inner edge. Total length of spacer is 10mm having ID :3.5 mm and OD:7 mm and made of aluminium alloy 6061.

3. Frictional contact boundary condition is taken between Spacer top face and PCB bottom face and friction factor value : 0.2.

These boundary conditions are increasing the stiffness of my PCB ? Then what will be the correct boundary conditions for above test?

Request you to comment on this and help if possible.

 

 

 

[Sebastian Hoetzel]

Hello Umesh,

Please check your material data of the laminate. Here is a datasheet of 370 HR.
https://www.isola-group.com/wp-content/uploads/sites/2/data-sheets/370hr-laminate-prepreg.pdf?t=460332920

The construction of glass fiber has also influence on the stiffness of the board. I think this could be the reason why your results do not match the test data.

Best regards

Sebastian 

[umesh_maheshwari]

Sebastian Sir

I have used the properties given in 370HR datasheet but in that shear modulus values are missing. 

As you have already mentioned that construction of glass fibre i.e. glass style influnece the stiffness then how to know the mechanical properties for each glass style?

My PCB board layer stack is as attached.

 

It is made from two different forms of glass style with core and prepreg materials. How to get the properties of these, I used ansys sherlock for natural frequency analysis as Isola FR4 properties are defined but are different from datasheet values. Again no information about piosson's ratio and shear modulus in ansys sherlock also.

The natural frequency observed using sherlock was also 415Hz which is also nowhere close to vibration result.

ODB file is attached.

https://drive.google.com/file/d/1Ix8UD5NDJlOYKx6i85TeL6S8kuwadTf9/view?usp=drive_link

Request you to suggest me reagrds to calculate mechanical properties of these laminate whether i will take test coupon containing only FR4 material or i should include the copper layers also?? 

 

[Sebastian Hoetzel]

Hello Umesh,

Here is a paper which discusses different modeling and calibration posibilities:
https://ieeexplore.ieee.org/document/9939509

I hope this helps to get better material data for the pcb.

Best regards

Sebastian