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Subject:
From:
Joyce Koo <[log in to unmask]>
Reply To:
TechNet E-Mail Forum <[log in to unmask]>, Joyce Koo <[log in to unmask]>
Date:
Mon, 15 Mar 2010 07:17:11 -0400
Content-Type:
text/plain
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text/plain (284 lines)
Move you components to corner. Or trim off the board to let it be non symmetric. If it is not high speed, make it second floor use socket (if you do have z height free space. 
--------------------------
Sent using BlackBerry


----- Original Message -----
From: TechNet <[log in to unmask]>
To: [log in to unmask] <[log in to unmask]>
Sent: Mon Mar 15 07:03:51 2010
Subject: Re: [TN] Vibration issue

Eric,
unfortunately, the board(s)are stacked, with round steel distances in between. The group is already trying kind of mechanical stiffener(s), but as you know, the vibration nodes are not always distinguished, instead they move to other places on the board. The equipment shall withstand 100 g's acceleration. 


 

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Eric CHRISTISON
Sent: måndag 15 mars 2010 11:09
To: [log in to unmask]
Subject: Re: [TN] Vibration issue

Inge,

If you want to add damping to the board or perhaps change it's resonant frequencies there are a few things you can do.

You can change how you support the edges of the board. Are all four edges restrained or just three? If it's the latter then restraining the free edge will make a big difference. Also do the restraints act like simple supports i.e. can the edges rotate, if so then restraining them more firmly will make a difference.

If you want to reduce the amplitude that the board flexes then bond a stiffener across the board using an adhesive with a high loss tangent. 
Many years ago I did this on some big boards using VHB tape from 3M, it's a viscoelastic material and very good at soaking up energy if you apply shear to it. If you're not sure what I mean ask a tame mechanical engineer and they should be able to explain what I'm on about.

Regards,



On 13/03/2010 12:56, Inge wrote:
> We have tried different ways of moving studs, bolts and washers, but 
> the result was that we only got resonances in other positions.
> Agree, coating underneath package is negative, Need something like 
> Steve Gregory's stuff.
>
> /Inge
>
> --------------------------------------------------
> From: "Tan Geok Ang" <[log in to unmask]>
> Sent: den 12 March 2010 17:23
> To: "Inge" <[log in to unmask]>; <[log in to unmask]>
> Subject: RE: [TN] Vibration issue
>
>> a) Change in Mechanical design and support will able change to the 
>> resonant freq. on the TQFP (as mentioned in your email). Best 
>> solution in reliability.
>>
>> b) Stake/Pot the corners of the TQFP (3M 2216, 2 parts material).
>>
>>
>> Ensure that no coating underneath the TQFP if it is conformal coated.
>>
>> Is the "open" always in around the same corner? Sometime different 
>> types of washer and holding/supporting points might help.
>>
>>
>>
>> ________________________________
>>
>> From: TechNet on behalf of Inge
>> Sent: Sat 3/13/2010 6:15 AM
>> To: [log in to unmask]
>> Subject: Re: [TN] Vibration issue
>>
>>
>>
>> Odd idea, ol' biker. Will sleep on your proposal.  Don't know if that 
>> will stop leads from resonating. The high speed camera may show.
>> Anyway, must find a very secure way, because you will fly with that 
>> box one day...top engineering solution needed, no place for amateur 
>> trials.
>> /Inge
>>
>> --------------------------------------------------
>> From: "John Burke" <[log in to unmask]>
>> Sent: den 12 March 2010 13:35
>> To: "'Inge'" <[log in to unmask]>; <[log in to unmask]>
>> Subject: RE: [TN] Vibration issue
>>
>>> Actually if you have an open via field behind that QFP and open via 
>>> holes you could under fill it from the back of the board "as is" 
>>> with a little ingenuity - minimal tooling, say a 3/8 inch aluminum 
>>> "nozzle" on a liquid dispenser with a circular recess machined into 
>>> the end face for a
>>> 1/4 inch
>>> ID O ring machined to half O ring thickness as the depth - contact 
>>> the back of the board in the via field behind the QFP, apply light 
>>> pressure referenced to the top of the QFP and apply pressure to the 
>>> liquid
>>> dispenser...8-)
>>>
>>>
>>> John Burke
>>> (408) 515 4992
>>>
>>>
>>> -----Original Message-----
>>> From: John Burke [mailto:[log in to unmask]]
>>> Sent: Friday, March 12, 2010 1:23 PM
>>> To: 'Inge'; [log in to unmask]
>>> Subject: RE: [TN] Vibration issue
>>>
>>> Inge, the mass of the package is probably not helping here. Have you 
>>> tried an approach such as using a low height standoff on the part 
>>> and then effectively under filling the assembly?
>>>
>>>
>>> John Burke
>>> (408) 515 4992
>>>
>>>
>>> -----Original Message-----
>>> From: TechNet [mailto:[log in to unmask]] On Behalf Of Inge
>>> Sent: Friday, March 12, 2010 10:48 AM
>>> To: [log in to unmask]
>>> Subject: [TN] Vibration issue
>>>
>>> Hi all,
>>> we are making a  very vital part for a aircraft builder. Problem 
>>> with TQFPs, some 200 legs. There are resonance peaks that kill the 
>>> package legs.
>>> Redesign is not to think of from the customer, so we must do 
>>> something on inside of the box.  One solution (not popular) is to 
>>> use a string of a glue along the leg rows in order to lock them.  
>>> Another is to place a vibration dampening mat on one side of the 
>>> board. That one is more tricky , but there is a US  company with 
>>> lots of stuff for that purpose.  Anyone with experience from this 
>>> area? I should add that not all legs are broken, just one or two 
>>> after 100 hs of random vibration, but that is of course not 
>>> acceptable.  BGAs would solve the problem, but there is no time for 
>>> redesign.
>>> I'm sorry for coming with a  serious message at this holy time of 
>>> the week.
>>> Thanks
>>> /Inge
>>>
>>>
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-- 
Eric Christison Msc
Mechanical Engineer
Consumer&  Micro group
Imaging Division

STMicroelectronics (R&D) Ltd
33 Pinkhill
Edinburgh EH12 7BF
United Kingdom

Tel:	+44 (0)131 336 6165
Fax:	+ 44 (0)131 336 6001




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