Hi Valerie,
To my knowledge—and I should know since it is I who is talking up Tg(TMA)‚
the IPC is not on record in terms of a preferred test method——nor should it be.
One needs to understand what the Tg represents to determine which test method 
will give the better answer in a given case.
The Tg is a single-point locator for a temperature range (typically 50C or 
more wide) over which the resin changes from its 'glassy' state to a 'rubbery' 
state by a change in molecular/cross-linking structure. Essentially all of the 
resin's properties are different in the glassy and rubbery states—one is best 
served to think of entirely different materials.
Each of the three test methods [TMA, DSC, DMA] measure an entirely differtent 
property of the resin, with each of them being important for different 
purposes. They roughly relate to each other as follows: Tg(TMA) ≈ Tg(DSC)—10 to 
20°C ≈ Tg(DMA)—20 to 40°C.
For reliability issues, typically driven by differential thermal expansion, 
it is the Tg by TMA that is the most important.
The White Paper you are referring to is most likely the one that I have 
authored, titled: "Recommendations for PCB FAB Notes and Specifications in Printed 
Circuit Board Drawings for SnPb and Lead-Free Soldering Assemblies, the 
Qualification of PCB Shops and Activities to Assure Continued Quality." 

Werner

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