Controlled CTE/SMT
One question most often asked is “why is the CTE of an MLB so important?” In the case of a ceramic chip carrier (CTE 6 ppm/°C) mounted on a conventional printed circuit board (CTE 17-18 ppm/°C), when the chip carrier is large enough, the mismatch in expansion will cause shear stresses within the solder joint that mounts the device on the board. A sufficient number of thermal test cycles (typically -65°C to +125°C) will eventually lead to work hardening of the solder and cracking of the solder joint itself. The resulting intermittent electrical discontinuity is entirely unacceptable in high reliability electronics applications regardless of their application.
In the case of stacked dies and direct chip attach, the requirements become even more critical since CTE values for die materials can be between 3-4.5 ppm/°C and the mismatch is even more critical than for leadless ceramic packages. A normal laminate with a CTE of 17-18 ppm/°C could result in excessive strain on solder joints and subsequent work hardening and cracking.
In addition to the above, newer lead-free solder systems may be considerably more “brittle” than traditional lead-tin, whereby a mismatch between the expansion of the device and that of the board may cause work-hardening and potential cracking after less thermal cycles, making CTE matching even more important. Arlon offers two choices for controlled CTE in finished PWB’s.
85NT is a pure polyimide laminate and prepreg system with a glass transition temperature of 250°C, reinforced with a non-woven aramid fabric. 85NT combines the high-reliability features of polyimide, (improved PTH reliability and temperature stability) with the low in-plane (X,Y) expansion of 7-9 ppm/°C and outstanding dimensional stability of the aramid reinforcement. 85NT meets the requirements of IPC-4101E/53.
55NT is a multifunctional epoxy resin laminate and prepreg system, reinforced with a non-woven aramid fabric. This system combines compatibility with leadfree processing, using a high- temperature epoxy resin of 170°C glass transition temperature, with the low in-plane (X,Y) expansion of 6-9 ppm/°C and outstanding dimensional stability of non-woven aramid reinforcement. 55NT meets the requirements of IPC 4101E/55.