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September 2012

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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, 24 Sep 2012 13:05:55 +0000
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Silicon chip (or die) nowadays are not old IC anymore.  Low k dielectric, mems cavities, etc. etc.  if vendor ask you to bake, it got be a good reason for it.  

Joyce Koo
Materials Researcher - Materials Interconnect Lab
Research In Motion Limited
Office: (519) 888-7465 79945
Mobile: (226) 220-4760

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Stadem, Richard D.
Sent: Monday, September 24, 2012 8:54 AM
To: [log in to unmask]
Subject: Re: [TN] Bake time for boards with multiple solid copper planes

Hi, Steve,
Don't have to reach far.
It is well known that silicon wafers are extremely notorious for moisture adsorption, and not just on the surface.
In my case, the wafer or the chips alone were baked and weighed, prior to assembly. I have noted this several times, especially with wafers.
In most vendor spec sheets for silicon wafers and KGD (CSPs and flipchips), the vendor recommends a bake of 72 hours at 125 deg. C. if the chips are left outside of a drypack for any length of time.
The curve does indicate what you are saying, that a lot of the moisture is on the surface, but the curve also shows a significant amount of moisture removal over time after the initial steep drop in weight. Typically the moisture content drops by at least 20% in the first 8 hours, but more than 24 hours is typically required to go from 80% to the "magical" 20% of saturated moisture content, below which we experience no problems with moisture during reflow processes.

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Steven Creswick
Sent: Sunday, September 23, 2012 5:18 AM
To: [log in to unmask]
Subject: [TN] Bake time for boards with multiple solid copper planes

Richard,

"What I was especially amazed by was the amount of water a silicon chip will absorb. You would not think a piece of glass could add 10% of its dry total weight in water less than 8 hours later in a 40%RH atmosphere."

I had not experienced that in the past.  When baking hybrids, it was always observed that adhesives and platings were the major moisture absorbers.
Alumina, Silicon, and wires were deemed as surface adsorbers as moisture would flash off quickly under temp or vacuum.

I can readily understand how the surface of the chip will adsorb moisture, but die attach adhesives also are EXCEPTIONAL moisture absorbers as are most molding compounds, if speaking of PEMs.

Were you speaking of bare silicon chips & wafers?
Or chip on board products, or packaged die?

Reach way back into your databanks...  :-)

Steve C


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