Learning Forum

[dariusz_kielbowicz]

Good morning,

I have a requirement for random/harmonic/shock + solder fatigue assessment analysis of a PCB.

I have watched this short reflow tutorial, but it does not include what to do with these static offset results downstream:

https://jam8.sapjam.com/groups/QxhZIS5hvjR1EWlg4pCOD2/content?folder_id=WJWzCotfkCVmkXXUSfI1wR

Problem #1

If I was analyzing just the bare PCB I would use the static structural glyph as a prestress condition for downstream modal / random / harmonic analyses. The issue is that I want to mount the pre-stressesed (after reflow) PCB into a "cold" (room temperature) not pre-stressed housing which will be the starting point of downstream linear dynamic analyses.

If I bond the PCB to the housing in bolt location and do the procedure as in the reflow tutorial omitting the housing from the thermal condition the result will be incorrect as the PCB will not be "free floating" but restricted by the hosing stiffness.

If I omit the housing from the thermal condition, define the housing-pcb connections as frictionless or no separation in static structural they will not be locked in linear dynamics (as per table below) and cause rigid body motion.

Can you help me model this "free in 1st step locked in 2nd step" approach?

 

Problem #2

How do I take into account the stress offset from the reflow process in sherlock analyses. In mechanical It will impact the prestressed modal analysis and linear dynamic results by changing the stiffness of the board but linear dynamics assumes that the random/harmonic stress still oscillate around zero which will not be the case on the reflowed board. Can sherlock read in the static structural stress state of the PCB/leads/balls etc. and take that into account in random/harmonic/shock/solder fatigue & reliability as a mean stress offset? If so I'd need a tutorial on how to do so.

 

As far as I know I have to have

1) A static structural modelling the reflow process in a "free" board state

2) Lock the board in place in the housing 

3) Have a 2nd static structural in which I will map the IcePak temperature field onto a "cold" board+ housing already pre-stressed by the reflow process.

4) Perform modal/random/harmonic/shock analyses in workbench

5) Read in the step 3 stress state in sherlock as a "base state" around which all the linear dynamics/solder fatigue stress amplitudes oscillate

6) Do reliability assessment for thermal cycling, modal/random/harmonic/shock

 

Additional questions:

Is this entire process doable in Sherlock/mechanical? I never saw reflow process being actually taken into account downstream impacting operational thermal cycling and linear dynamics in any tutorial on the learning hub. Can sherlock read in mean stress offsets for its reliability calculations or does it always assume that both solder fatigue and vibration loads oscillate around a zero-stress state?

Is it possible to continue the analysis as shown in the reflow tutorial with an IcePak thermal map so the starting point of the "operating" load cycle is the board pre-stressed with the reflow process? Below is a profile of the proposed process: the first two steps is the reflow then the thermal load is disabled at the 3rd step where the IcePak map should take over.

Is it possible to tell Mechanical to map IcePak thermal maps onto the 3rd step and continue on with the analysis? Additionally, the housing of the assembly would be not scoped for the reflow thermal condition, but I'd like to map IcePak temps on it too from the 3rd step.

 

An additional problem I am facing is that I should set the environment (stress free) temperature at the reflow temp of 230C but this causes the assembly temp to go back to 230 after I disable the 3rd load step in the thermal load. When I set the environment temp to 20C the results of displacement and stress are the same just instead of being at the 2nd step they occur at the 1st so it seems to solve the problem of temps going back to 230 after disabling the 3rd step of thermal condition but will this allow a correct continuation with IcePak maps?

 

As an alternate approach to having reflow+IcePak loads in one static structural I tried to build the following flow in Workbench and I discovered is that regular static-to-static submodelling does not allow me to carry over the static structural reflow stress state to a new static structural where I wanted to apply the IcePak thermal maps, is there a way around this?

 Thank You for any suggestions to for tackling this problem

[Ashish Khemka]

Hi,

For problem#1 - can you tu using a deformed model (Using the solved model as model for next analysis). You may try using joints (say a bushing joint) to have specific stiffness in the required direction.

Regards,

Ashish Khemka