While it has been several years since manufacturers began moving to lead-free solder procedures, in part due to the European Union’s Restriction of Hazardous Substances Directive, some still believe myths that have long been inaccurate regarding the use of alloy joining materials that do not require flux and are based on lead and tin.
Temperatures Can Be Enough to Destroy Components
The first round of lead-free solder options to join metals and other materials were comprised of tin, silver and copper, which do have a slightly higher melting point of 217 degrees Celsius compared to existing solder’s 183 degrees Celsius. That disparity could cause problems regarding PC board damage.
However, newer products including several offered by S-Bond have significantly lower melting points that make it easier to join metals like aluminum. At the lowest temperatures, some materials can be joined at just 115 degrees Celsius.
Issues Regarding Silicon Will Require Other Materials
Heat pipes and vapor chambers are used to transfer and/or spread heat from concentrated heat sources such as high brightness light emitting diodes (LEDs) and high computing speed CPUs. These active thermal management devices are enclosures/tubes that have porous wick materials lining the walls that provide condensation surfaces and small connected pores that via capillary force, transfer condensed fluids that were originally vaporized at heat source surfaces. When the vapor is transported via convection to the cooler surface to condense, the fluid is then channeled back to the heat source surfaces in a continuous cycle, in effect pumping the heat out of the package without using external power surfaces. Figure 1 illustrates a vapor chamber used to cool a mounted CPU.
Thermal management is critical in the life and performance of such electronic components that all employ a variety of thermal interface materials (TIMs). With increased power and speed, the polymer-based TIMs being used today are limiting and metal bonding with solders is growing in application. Conventional Sn-Ag soldering temperatures can overheat the thermal fluids in heat pipes and chambers while Indium (In) solders are expensive and do not bond as well as active solders. Responding to this need, engineers at S-Bond® Technologies have announced its latest alloy, S-Bond® 140 as an effective TIM for bonding CPUs or LEDS to heat pipes and vapor chambers. The Bi-Sn-Ag-Ti alloy can wet and join to all metals including aluminum and to most ceramics and glasses. S-Bond® 140 is lead free, does not require plating and flux thus keeping electronic and LED packages clean.
Figure 2 illustrates a high brightness LED array that has been bonded to a Ni-plated copper vapor chamber with S-Bond 140 solder. This technique provides a high strength and high thermal conductivity metallic solder bond. Figure 3 is another example showing S-Bond 140 solder bonding copper heat pipe tubes (water as the phase change fluid) into aluminum slots to enhance the cooling from the heat pipe to the aluminum package base without plating and flux.. Normally when soldering heat pipes over 200°C, the water in the heat pipe goes to vapor and the resultant pressure distends/distorts the thin copper tube walls. Lower temperature metallic solders, such as S-Bond 140.
S-Bond active solder joining is emerging as an effective method to bond heat sinks to the back of High Brightness Light Emitting Diodes (HBLEDs). Active solders can wet and adhere to many of the thermally conductive ceramics (AlN, BeO, etc.) that are being used in HBLED’s and enable effective and thermally stable and conductive joints. (more…)