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February 2007

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Subject:
From:
"Stadem, Richard D." <[log in to unmask]>
Reply To:
TechNet E-Mail Forum <[log in to unmask]>, Stadem, Richard D.
Date:
Tue, 13 Feb 2007 08:52:00 -0600
Content-Type:
text/plain
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text/plain (235 lines)
I use a high-speed Dremel drill. However, here is another free tip:

You can purchase the Dremel drill STAND for about $35 at any Menards,
Lowes, Home Depot, or other large home and building store. Mount the
drill into the stand (firmly) and bring the drill bit down so it is just
touching the board surface, then screw down the stop adjustment until it
stops the drill just at the surface when you rotate the Z-axis handle.
Make sure the drill bit is clamped very firmly in the chuck.
From there, if my ancient memory serves me correctly, for each full turn
of the stop adjustment you will increase the drill penetration exactly
.030". So if you turn the stop adjustment 2.5 turns, you will drill down
.075", which is the thickness of the standard .062" PWB plus .015" into
the solder joint. This is just an example.
Please double check the amount of z-axis displacement with your drill
stand before you try this. It is easy enough to determine. I scribed a
line on my stop adjustment so I can see exactly where I am. 

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Whittaker, Dewey
(EHCOE)
Sent: Tuesday, February 13, 2007 8:01 AM
To: [log in to unmask]
Subject: Re: [TN] FW: Report from a BGA

And you learned about this from word of mouth.
Dewey

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of John Burke
Sent: Monday, February 12, 2007 4:34 PM
To: [log in to unmask]
Subject: Re: [TN] FW: Report from a BGA

Yes I know the drill - it is a very small one..........8-)

I have carried out that type of profiling many many times.

I have found that the best type of drill to use is actually a dental
drill and hand piece. What do other people use out of interest?

John

 
 
John Burke
 
(408) 515 4992
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Guy Ramsey
Sent: Monday, February 12, 2007 1:39 PM
To: [log in to unmask]
Subject: Re: [TN] FW: Report from a BGA

We operate with the kind of knowledge that Richard and Werner have
shared along with the experience of measuring many different kinds of
assemblies, materials, with different methods of thermal transfer and
tools. When we get the luxury of following the "correct" way of checking
a board we also run thermal couples under parts, taped to the part, and
taped to the board to learn how these materials respond to our machines
and process methods.
Then
we make educated guesses, based on experience. 

You know the drill. 


Starting March 1, 2007 New Location
Guy Ramsey
R&D Assembly
1660 East Race Street
Allentown PA 18109-9580
(610) 264-1887
FAX (610) 264-1877

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Steve Gregory
Sent: Monday, February 12, 2007 4:09 PM
To: [log in to unmask]
Subject: Re: [TN] FW: Report from a BGA

What about us CEM's that rarely have the luxury of a spare bare board,
or spare BGA's that we can play with?

Sometimes we can convince our customers that we should have a spare PCB
and BGA's to be able to properly set up a reflow profile, but most of
the time we can't.

What I do in that case, is to try and find a scrap PCB that might be
close to the same thermal mass, and then I have some failed BGA's that
have been pulled off a board from rework, and just lay them over top of
my thermocouples, then do my plot.

I know it's not exact, but it's the best I can do sometimes. I just try
to ensure that I hit a high enough peak and stay liquidous for a decent
amount of time.

-Steve Gregory-

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Stadem, Richard D.
Sent: Monday, February 12, 2007 2:56 PM
To: [log in to unmask]
Subject: Re: [TN] FW: Report from a BGA

Werner, thank you for your note. I totally agree. 
I misread Inge's original email. I thought he said putting a
thermocouple on top of the SOLDER PAD was misleading. What he actually
said was putting a thermocouple on top of the component body was
misleading. That is true. 
Another note. I totally agree the best way to thermocouple a profile
test board is to drill up into the solder joint, insert the TC from the
bottom so the weld is inside the solder ball. However, I then backfill
the hole with thermally conductive epoxy with a minimum thermal
conductivity of 14 BTU/in.
hr. F. This helps reduce the insulative effect of the PWB just below the
weld inside the hole. After the thermally conductive epoxy is cured, I
then measure the electrical resistance of each of the thermocouple wires
to the solderball via. I should get less than 1 ohm. If I do, then I
know that the weld is making full mechanical contact with the inside of
the solderball and/or the pad. When the mole or data recorder runs
through the reflow oven, and I see a very solid line on the graph, I
know I am measuring the actual temperature of the solderball/pad
mechanical junction. If the profile line is not a solid line throughout,
but is "noisy" with little spikes in both directions, then I know I am
only measuring air temperature under the BGA, and I disregard that
particular reading. The attenuating effect of full metal contact is
easily seen.
 
Incidentially, I recently wanted to verify that a small bake oven was
getting a particular subassembly up to a certain temperature in a
pre-baking operation. When I suspended  thermocouples around the
component within the chamber (measuring the air temperature), the 4
thermocouples measuring the air temperature consistently measured 6
degrees C. less than the 5 thermocouples that I had embedded into the
subassembly using the method I just described. This was long after all
of the TC readings had flatlined. 
The calibration lab questioned as to how an object being warmed by the
air currents inside the oven could be hotter than the air currents
themselves.
The only explanation I could offer is that the thermal transfer
efficiency of air directly to a thermocouple weld is less efficient than
a piece of metal heated by the air making direct contact with the weld.
I proved this by taking the thermocouples measuring the air and
switching them with the thermocouples measuring the subassembly. The
measurements were exchanged, one for one. Again, the TCs measuring air
were all exactly 6 or 7 degrees cooler than those embedded in the
assembly. I then took one of the TCs measuring air and clamped it to the
metal rack in the oven. It went up 6 degrees.
 
The same phenomena holds true for a thermocouple weld sitting just above
a BGA pad, measuring air temperature alone. I notice it always measures
a few degrees cooler than a TC with a good mechanical attachment to the
solder or the pad, even when the solder is in liquidus.

________________________________

From: [log in to unmask] [mailto:[log in to unmask]]
Sent: Monday, February 12, 2007 1:30 PM
To: Stadem, Richard D.; [log in to unmask]
Subject: Re: [TN] FW: Report from a BGA


Hi Richard,
Ingemar is absolutely correctthe only way you know what the t's are at
the solder balls is to put thermocouples in them. It is indeed standar5d
procedure see IPC-7530, 7.9.5.
BGA solder balls get heated by conduction through the PCBmany people do
not realize thisand BGA component body is in fact a heat sink and a
shield preventing both convection and radiation heat transfer.

Regards,
Werner Engelmaier
?????
Engelmaier Associates, L.C.
Electronic Packaging, Interconnection and Reliability Consulting
7 Jasmine Run
Ormond Beach, FL 32174 USA
Phone: 386-437-8747, Cell: 386-316-5904
Werner Engelmaier 

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