Thanks, Guy. Some of these things you realize over time. I don't pretend
to be a thermal scientist, but some things you can't help but notice.
-----Original Message-----
From: Guy Ramsey [mailto:[log in to unmask]]
Sent: Monday, February 12, 2007 3:20 PM
To: Stadem, Richard D.
Subject: RE: [TN] FW: Report from a BGA
Another superb post. Thank you. I have seen the same thing. Most people
don't know this stuff. Most of the time it doesn't matter. But, when it
does they are dead. Knowledge is so important to process control.
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 Stadem, Richard D.
Sent: Monday, February 12, 2007 3:56 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
-----------------------------------------------------
|
