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August 2013

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From:
Steven Creswick <[log in to unmask]>
Reply To:
TechNet E-Mail Forum <[log in to unmask]>, Steven Creswick <[log in to unmask]>
Date:
Tue, 27 Aug 2013 19:39:38 -0400
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text/plain (144 lines)
Wayne,

I found that flux underfills [no-flow underfills] worked great on LTCC
metallizations.  Never had the opportunity to work with OSP on copper.  The
post applied, capillary underfills also worked well mechanically, but added
the obvious advantages/dis-advantages to the process flow.

Sikama still makes conduction reflow units -
http://www.sikama.com/falcon-8500/  for example.  I took a 3 heated zone
unit and modified the nitrogen purge system to get sub 10PPM O2 without
flooding the room with nitrogen.  I used fixed ports in various locations
within the oven to measure real-time oxygen levels - just had to open the
appropriate port for the O2 sensor inlet.  From a cold start, we could be at
or below 10 PPM within 45 minutes.

Most of the daisy chain die I worked with were 125 or 254 mm square.
Typically with 100 um bumps on 200 um pitch.

In the early stages, we could place the chip, either with tacky flux, or in
no-flow underfill in one part of the building then physically walk it to
another part of the building for convection, or conduction reflow.
Surprisingly, we never had an incident, but always made me very nervous...

Temp cycle was normally -55+125°C, but sometimes -65+150°C, or the much more
benign -40+80°C

Distance to neutral point and CTE delta were the big factors.

Like Joyce says, if you could get the old high lead C4 bumps, that is
better.  Placed many low I/O C4 bumped chips on FR4 for telecom application.
That used the same flux underfill as used with LTCC.

Having an acoustic microscope helps a great deal when setting up the
process, especially with a capillary underfill.  I know that everyone
clamors over Sonoscan, but Sonix also made fantastic equipment.  I found
that most of the transducers were similar and that the software was the
major difference between the manufacturers, but that may have changed in the
intervening years.

.... My ruminations


Steve Creswick
Sr Associate - Balanced Enterprise Solutions
http://www.linkedin.com/in/stevencreswick
                         616 834 1883



-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Wayne Thayer
Sent: Tuesday, August 27, 2013 1:25 PM
To: [log in to unmask]
Subject: Re: [TN] Flip Chips

I forgot to mention that I have no nitrogen oven.  I am thinking of buying a
rapid response hot plate and putting that inside of a big vacuum bell jar.
If I go through several vacuum purge cycles I should be able to get to
100ppm O2 in less than a half hour.  Anybody done that?  Recommendations for
hot plates?  How about recommendations for O2 meters which go down to that
level?

Wayne

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Wayne Thayer
Sent: Tuesday, August 27, 2013 1:00 PM
To: [log in to unmask]
Subject: [TN] Flip Chips

OK, Inge, you've inspired me to share what I'm up to presently:

I'm trying to prove out an in-house flip chip attach/underfill process.
I'll get the die from Practical Components and/or Topline, most likely 10mil
pitch full array, although Practical also reps a PacTech 200x400 micron
array which sounds pretty good too.  Looks like SnPb would be special order,
so I'll do SAC305.

I'm disappointed to see that the boards they stock for these are all OSP,
which means that in initial development anyway, that I'll have to cut the
test board in sections.

Any advice from the "school of hard knocks" for me?  Preferred
fluxes/underfills?  Good stories maybe?

Wayne

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Inge Hernefjord
Sent: Tuesday, August 27, 2013 12:47 PM
To: [log in to unmask]
Subject: Re: [TN] test

What is (not) going on?  No conversation. No problems. No fun stories.
Nothing at all. Technet's death-throes?

Inge


On 26 August 2013 15:26, Steven Creswick <[log in to unmask]> wrote:

> Ah, but here in the mid-west, the cicada's are rubbing their legs 
> together ...
>
> Steve C
>
> -----Original Message-----
> From: TechNet [mailto:[log in to unmask]] On Behalf Of Inge Hernefjord
> Sent: Monday, August 26, 2013 6:21 PM
> To: [log in to unmask]
> Subject: [TN] test
>
> Silent as Nevada desert.
>
>
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