Troubled Techies

If I might make an observation - I think you could avoid most of  
these issues with Vapour Phase soldering.

Simple physics - a heated vapour at 225C will condense onto ALL  
surfaces including under components and Hey Presto - uniform  
soldering. We have many companies over here doing that even to the  
extent of doing away with x-ray analysis.

Unless I am missing something?

Hope it helps

Kindest regards,

Graham Naisbitt

[log in to unmask]



On 12 Feb 2007, at 23:33, John Burke wrote:

> 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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