Kasey Ma,

   Outgassing of the solvents and volatiles in the adhesive is the likely
culprit, especially if your ceramic substrate is fully dense. Interesting
that it didn't happen during bonding though. During bonding did you heat the
subtrates to the bonding temperatures then bring them together?  If so, then
this would allow for escape of the evolved gases and therefore explain why
you didn't see any bubbles.

    During the reflow the gas can only escape by diffusing along the
interface or through the adhesive itself, otherwise it will accumulate and
form bubbles.  In addition to the partial vaccum, you might be able to
reduce this accumulation by reducing the heating ramp rate to allow time for
the gas to diffuse away from the interface slowly. I've used use ramps as
slow as 0.01 degC/min, but that was because my stuff had significant amounts
of gas evolution from a multitude of varying molecular weight components.
Using a ramp rate this slow will obviously make your reflow run very long,
but you might be able to reduce the total run time by using TGA analysis of
the adhesive to pinpoint the temperature range where outgassing is the
greatest.  This way you could only use the slow ramps during the range where
the outgassing is occuring.

Masa




> Date: Thu, 6 Feb 2003 07:51:15 -0800
> From: "Bill Moffat" 
> To: "General MEMS discussion" 
> Subject: RE: [mems-talk] ceramic bonding using polymer as intermediate
layer
> Message-ID:
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>
> It sounds like gas production during high temperature bonding.  I =
> suggest a partial vacuum heated gas process to allow bonding but =
> continuous removal of the generated gas.  Bill Moffat
>
> -----Original Message-----
> From: [email protected] [mailto:[email protected]]
> Sent: Monday, February 03, 2003 11:14 PM
> To: [email protected]
> Subject: [mems-talk] ceramic bonding using polymer as intermediate layer
>
>
> Dear folks,
>
> I have a problem with above bonding, i.e., ceramic is bonded with =
> another substrate using polymer as intermediate layer.  There is no =
> voids after bonding, but a lot came out after reflow process that at 230 =
> degC.  It seems ceramic that gives the problem.  I appreciate it very =
> much if anybody could comment it.
>
> Thanks & best regards,
> Kasey Ma
> Agilent Technologies Singapore
>
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>
> ------------------------------
>
> Date: Thu, 6 Feb 2003 07:57:22 -0800
> From: "Bill Moffat" 
> To: "General MEMS discussion" 
> Subject: RE: [mems-talk] Adhesion of parylene to electrodes
> Message-ID:
<240593772F9E5C4BB5BECE5FF0B937EC12D14C@exchange.yield.yieldengineering.com>
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>
> My company makes silane equipment to assist the adhesion of organics to =
> non organics.  I am not familiar with A-174 but if the vapor pressure is =
> in a reasonable range I could arrange for deposition of the silane in =
> our equipment.  Possible advantages:- with silicon wafers and HMDS the =
> preferred silane we dehydrate totally then apply the silane as a vapor =
> which gives a sealed hydrophobic surface that has been mailed back to =
> literally hundreds of customers with no degradation in the mail.  let me =
> know if I can help.  Bill Moffat
>
> -----Original Message-----
> From: [email protected] [mailto:[email protected]]
> Sent: Monday, February 03, 2003 7:05 AM
> To: [email protected]
> Subject: [mems-talk] Adhesion of parylene to electrodes
>
>
> Hi. We are studying DEP microfluidics at the University of
> Rochester. We are working on an experiment to observe the behavior of
> water actuated over two nearly parallel vertical electrodes coated with
> parylene. However, there have been some problems with the adhesion of
> parylene to steel electrodes.
>
> 1. We know the roughness of electrodes will affect the adhesion, but we
> don't know how smooth we should polish the electrodes, and what steps of
> cleaning or drying should be performed.
>
> 2. We are ready to try A-174 silane as a adhesion promoter, but we don't
> know some treatment details, for example, how long should the electrodes
> be kept in the solution, how much time can lapse between applying the
> adhesion promoter and the parylene coating treatment.
> Does anybody know something about the above questions or other better
> methods?
>
> We appreciate your suggestion and help!
>
>
>
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> End of MEMS-talk Digest, Vol 4, Issue 5
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>