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 

---------------------------------------------------
Technet Mail List provided as a service by IPC using LISTSERV 1.8e
To unsubscribe, send a message to [log in to unmask] with following text in
the BODY (NOT the subject field): SIGNOFF Technet
To temporarily halt or (re-start) delivery of Technet send e-mail to [log in to unmask]: SET Technet NOMAIL or (MAIL)
To receive ONE mailing per day of all the posts: send e-mail to [log in to unmask]: SET Technet Digest
Search the archives of previous posts at: http://listserv.ipc.org/archives
Please visit IPC web site http://www.ipc.org/contentpage.asp?Pageid=4.3.16 for additional information, or contact Keach Sasamori at [log in to unmask] or 847-615-7100 ext.2815
-----------------------------------------------------