Lead-free soldering is expected to become the new standard in the future. Different legislations or draft directives target the restriction or the ban of the use of lead in the world. As an example,the European Union adopted the Restriction of Hazardous Substances (RoHS) Directive in January 2003. It will come into effect on July 1,2006. Most of the lead-free solder alloys melt at higher temperatures than that of the eutectic SnPb solder. The change to higher temperature solder alloys will directly affect the temperature profiles for reflow soldering,wave soldering,rework and repair. Typical lead-free reflow profiles will reach peak temperature of 245°C to 265°C for up to a minute. De-soldering,rework and repair will reach peak temperature above 300°C for a few seconds. In parallel,the complexity of the boards is increasing,leading to thicker multilayer structures. Laminates will thus be submitted to higher temperature for longer time through multiple reflow cycles. It is critical to understand how these new technical requirements will have an impact on the thermal resistance of electrical laminates.
This paper aims to provide correlations between different techniques used to characterize the thermal stability of electrical laminates suitable for lead-free soldering.
• Thermo-Gravimetry Analysis (TGA) was used to measure the degradation temperature (Td) and the time to degradation at a given temperature (D-260,D-288,D-300);
• Thermo-Mechanical Analysis (TMA) was used to measure the time to delamination at a given temperature (T-260,T-288,T-300),the number of temperature cycles before delamination (Nd),and the coefficient of thermal expansion along the z-axis (CTE);
• Differential Scanning Calorimetry (DSC) was used to measure the glass transition temperature (Tg).
The thermal stability data of various epoxy systems will be described and correlated to specific applications needs. Various examples of epoxy systems will be chosen within the portfolio of The Dow Chemical Company.
Results suggest that conventional FR-4 resins might still be suitable for standard FR-4 applications that need only a few lead-free reflow cycles. When the number of cycles increases,enhanced resin systems must be considered to avoid in-process failure. Highly thermo-resistant products are suitable for complex multilayer build-up or for applications targeting high in-use temperature. In addition to thermal stability,other key laminate parameters for board reliability are adhesion and toughness.
Author(s)
Ludovic Valette,Bernd Hoevel,Karin Jestadt,Tomoyuki Aoyama