While it is a good idea to use a scrap PWB or CCA to create and certify your profile for that particular assembly, for process control of the oven you want a fixture that is more robust and will not change after several (or each) trips through the oven.
Trying to use a PWB as a process control fixture is not a good idea, as both the board and the TC connection deteriorates or changes after each pass through reflow, no matter which attachment method is used, including the ones listed in IPC 7530.
I use a pallet made of Durostone CAS761, and on it I have several small metal strap sets that I use to clamp the business end of the thermocouples, and these are distributed around the pallet. The strap sets are simply small pieces of metal screwed to the board, onto which another metal strap is screwed that clamps the TCs down so the TC weld makes good mechanical contact.
It does not matter that the process control fixture does not represent any of your production CCAs. You are simply using it with a given profile to verify that the oven remains the same from week to week, in other words nothing has changed, all of the heater elements are working properly, conveyor speed is the same (length of temperature curve from beginning to end remains the same), all fans are working, etc.
Clamping the thermocouples between two metal straps allows them to be easily removed/replaced without affecting the performance of the process control fixture itself. The integrity of the process control fixture is protected. If a scrap board is used, the TC connection is usually suspect because the epoxy, high temp solder, tape, etc. does not usually hold up very well, and the readings can vary with every pass.
The process control fixture is cheap to make and can be used to check the convection and vapor phase reflow ovens and the hot air rework machines (Airvac, Oki, Metcal, etc.)
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Brian Stumm
Sent: Wednesday, June 10, 2009 10:39 AM
To: [log in to unmask]
Subject: Re: [TN] Profiling compound?
Whenever possible we use Chipbonder adhesive and keep the TC Assembly
handy for future verification of oven performance.
On Wed, Jun 10, 2009 at 8:17 AM, John Queen<[log in to unmask]> wrote:
> For PB none of them are ideal, we don't use thermo couples on our boards due
> to the PPT of the component but if you need to use a TC glue is probably
> preferred and consider the percentage of deviation that the TC reports.
>
> john
>
> -----Original Message-----
> From: TechNet [mailto:[log in to unmask]] On Behalf Of Kane, Amol (349)
> Sent: 10 June 2009 14:16
> To: [log in to unmask]
> Subject: Re: [TN] Profiling compound?
>
> I have obtained repeatable results by using Al tape to stick the
> thermocouple to the board and using Kapton tape over it to maintain adhesion
> at high temps.
>
> Regards,
> Amol
>
> -----Original Message-----
> From: TechNet [mailto:[log in to unmask]] On Behalf Of Braddock, Iain (UK)
> Sent: Wednesday, June 10, 2009 5:34 AM
> To: [log in to unmask]
> Subject: [TN] Profiling compound?
>
> Hi all,
>
> What is the preferred medium for bonding thermocouples to boards for
> PbFree thermal profiling - forced convection!
>
> CIA,
> Iain
>
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