How do CTE and Tg Relate to Reliability? - Thinking Out Loud

Well, they relate, but lower CTE and higher Tg are not necessarily always the best. If we assume that the reliability issue is stress due to different thermal expansion between copper and the dielectric, leading to cracks, then we have to look for a solution that minimizes the difference in expansion between copper and dielectric over the range from room temperature (e.g. 20C) to the solder shock temperature, let’s say 288C in the case of lead-free processing.

With copper having a CTE of approximately 17ppm/C, copper will expand (if unrestrained) 4,556ppm when heating from 20C to 288C (i. e. 268x17).

Let’s look at three cases:

Case 1:

Room Temp (C)	ΔT (RT to Tg)	Tg	ΔT (Tg to SST )	Solder Shock Temp (SST)
20	150	170	118	288
	CTE below Tg (α1)		CTE above Tg (α2)
	35		70
	Expansion (ppm) from RT to Tg		Expansion (ppm) from Tg to SST		Total expansion (ppm) from RT to SST
	5,250		8,260		13,510

Case 2:

Room Temp (C)	ΔT (RT to Tg)	Tg	ΔT (Tg to SST )	Solder Shock Temp (SST)
20	150	170	118	288
	CTE below Tg (α1)		CTE above Tg (α2)
	20		100
	Expansion (ppm) from RT to Tg		Expansion (ppm) from Tg to SST		Total expansion (ppm) from RT to SST
	3,000		11,800		14,800

Case 3:

Room Temp (C)	ΔT (RT to Tg)	Tg	ΔT (Tg to SST )	Solder Shock Temp (SST)
20	130	150	138	288
	CTE below Tg (α1)		CTE above Tg (α2)
	20		70
	Expansion (ppm) from RT to Tg		Expansion (ppm) from Tg to SST		Total expansion (ppm) from RT to SST
	2,600		9,660		12,260

And the winner is: Case 3 (expansion closest to that of copper), even though Case 3 has a lower Tg than Case 1 or 2. Comparing Case 1 and 2: Case 2 has a lower CTE below Tg than Case 1, but leads to a larger expansion differential than Case 1. The lesson here is that we have to consider both CTE below Tg and above Tg.

Title: CTE, Tg and reliability
By: Roberto Tulman
Posted: September 5, 2008 4:50 AM

Arithmetics alone is not enough here.
The copper in-plane with the layers is not relevant as CTE of the dielectric is constrained by the glass cloth.
The copper plated inside the holes has a minimum elongation factor of 15%...
In the Lead-free world it will be more important to look at the Td than at the Tg.
Roberto Tulman
www.eltekglobal.com

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