How to make PCB reliable for 500-1000g shock test

[angy]

I have a Mil STD PCB with SMD components mounted on it.It was tested for 500G Shock test and dry solder problem on one of SMD resistor (2512)package occured.It was replaced with new one. Again 500G test was performed this time some other SMD resistor was the cause(maybe dry solder) .It was resoldered again.It will now be the last trial.
So i want to make sure everything is O.K.So i need suggestion as to what steps are needed to make sure it passes the tess this time? Also It will be helpfull if someone can recommend some adhesive to be used in component against shock and vibration

 

[ravindragudi]

First of all I would caution you against using same PCBA in shock test over and over again. You may start seeing cumulative failure effects !!

PCBA on its own cannot be guaranteed for pass condition for shock tests. You may also be well aware that the housing and any shock absorbing methods in the frame are important. Given that you are observing some limited failures of components getting off from solder pads, I think some sort of adhesive for the parts would help pass the test.

Again the solutions come with their pros and cons on maintenance aspect of the PCBA. Doing a complete PCBA conformal coating will help to some extent. It again need to be tested and seen how effecting this will be. For parts with their heights as a concern in shock and vibration tests, RTV can be applied locally to the pads of the parts or completely surrounding the part also.
There is another way to help withstanding the shock test for components is that, using the through hole parts. I know this will be time consuming, costly and other process related problems.

 

[angy]

Hii Ravi
Thanks for the valuable advice and some of them we have followed like conformal coating,RTV silicone on SMD electrolytic capacitor .My question is, will putting adhesive on SMD component would be of help?If you can provide particular Part No for the adhesive will be helpful to me.

 

[ravindragudi]

Dow Corning® 734 Flowable Adhesive Sealant
Loctite SI 5145 RTV SILICONE
I have used these for almost the same purpose to withstand in adverse shock and vibrations.
Share more info/ pictures if possible to look into the PCBA mounting in the housing.

 

[KlausST]

Hi,

to lower the force on the PCB/parts you may consider to completely fill your case with some silicone gel (silgel 612). Silgel 612act like a mechanical damping fluid.
The solid case should be filled with a minimum of air gaps.


I tried to find a link for a document in the internet but failed ... maybe you find it worth to try it anyway.


Klaus

 

[Mattylad]

I wonder why (AFAIK) the military & space still use PTH components on their boards?

 

[ads-ee]

I wonder why (AFAIK) the military & space still use PTH components on their boards?

The last time I worked on a mil product they were designing PCB with nearly all SMT components. Only a few large electrolytic capacitors were still PTH mounted. Shock and vibration were a big issue and there was a lot of inspection being done for solder joint quality (on non-RoHS solder). I'm sure shock and vibration will have a field day on the less flexible RoHS solder. I'm sure the military will experience this problem in the long run when systems that were supposed to be installed for 20 years start to fail due to cracked, whiskered (though conformal coating should help a lot), or otherwise failed solder joints.

 

[Mattylad]

Don't the MOD etc have an exception to ROSH ?
Or has that been removed now?

 

[ads-ee]

If you can't buy lead mil parts then you're stuck with switching to RoHS.

 

[dick_freebird]

I might suggest that you have this final test article
inspected by someone who knows MIL grade inspection.
If these were cold-joint failures then that ought to
have been visible to a trained eye.

Large-body capacitors in surface mount, I could see
being a big cantilever g-force problem. Might be better
(if board redesign comes up) to use more, smaller ones
(if shock/vibe is more of a problem than solder joint
yield and reliability statistics, anyway).

 

[angy]

If you can't buy lead mil parts then you're stuck with switching to RoHS.

What is ROHS complaint and how it helps in MIL STD PCB?What does it mean when a SMD component is ROHS complaint?

 

[FvM]

The discussion is about leaded solder joints being more robust than lead-free. Recent components are designed to be processed with lead-free solder according to RoHS regulations.

There's however an error of thinking in the previous discussion. Most lead-free components can be still used with leaded solder if necessary, with the exception of BGA ICs.

 

[ads-ee]

Here is an interesting discussion on Sn v.s. Sn/Pb coated leads.

 

[angy]

I have taken Loctite SI 5145 into consideration.
ADEKIT A 211 BG is also an option only limitation is its limiting "working temperature"


Anymore lists of Adhesive for Bonding/Staking electronic components.

 

[marce]

I haver had numerous SMD boards pass shock and ballistic tests, any ferrite cores were impregnated, components were generally tin finish, all BGA s re-balled, the only components that were problematic were SMD electrolytic so these are often replaced with a tant. or similar. Mil spec is different and more tolerant than designs for space, one of the main problems with space designs is the extreme temperature range experiences and the generally much more hostile environment... Hence the preference for true hermetically sealed ceramic packages, PTH is preferred due to the added lead compliance, some SMD components are approved, the rest of the complex circuitry is bonded chip in hybrid modules.....
In this case it is NOT the components that are at issue but the assembly process... It is this that needs sorting dry joints are unacceptable.

 

[angy]

I haver had numerous SMD boards pass shock and ballistic tests, any ferrite cores were impregnated, components were generally tin finish, all BGA s re-balled, the only components that were problematic were SMD electrolytic so these are often replaced with a tant. or similar. Mil spec is different and more tolerant than designs for space, one of the main problems with space designs is the extreme temperature range experiences and the generally much more hostile environment... Hence the preference for true hermetically sealed ceramic packages, PTH is preferred due to the added lead compliance, some SMD components are approved, the rest of the complex circuitry is bonded chip in hybrid modules.....
In this case it is NOT the components that are at issue but the assembly process... It is this that needs sorting dry joints are unacceptable.

Yes I agree it might be a poor workmanship .Will have to resolder again for proper soldering