Evaluation of halogen-free laminates used in handheld electronics
Lau, David Yuk Ho
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The purpose of this study is to examine the thermal and mechanical properties of various halogen-free laminates used in handheld electronic products and to correlate these properties to the manufacturing requirements and mechanical performance. Thermal properties determined for the laminates are the glass transition temperature, x,y,z-axis CTE, time to delamination at 260˚C and 288˚C, temperature to decomposition and interconnect stress test. SEM and EDS mapping analyses have been done on the laminates to determine the chemical composition and area fraction of the filler used in the epoxy. Three different fillers are identified from the laminates: Al(OH)3, Mg(OH)2, SiO2. Results show that the SiO2 fillers presented in the laminates reduce the z-axis CTE changes and increase the time to delamination. The x and y axis CTE is found to be lower than the z-axis CTE due to the glass fibers reinforcement in both the x and y directions. The temperature to decomposition is dependent on both the fillers and epoxy chemistry. Al(OH)3 starts to decompose above 200˚C and increases the rate of decomposition at 260˚C. Its decomposition also leads to an increase in z-axis CTE above 240˚C. Interconnect stress test results show that laminates with higher temperature to decomposition exhibit longer cycles to failure. Mechanical test results indicate that the halogenated laminates without fillers perform better than the halogen-free laminates with fillers. High fillers loading increase the flexural modulus and Vickers hardness properties of the laminates but decrease both the flexural strength and energy to fracture. Silica fillers in particular are susceptible to weaken these mechanical properties. Laminates without any fillers show plastic deformation of the epoxy matrix after fracture and result in a high energy to fracture. The adhesion of fillers to the matrix is important as the better the adhesion the higher the flexural strength is for the laminates. Results show that laminates with Al(OH)3 as the major filler have higher flexural strength than laminates with silica fillers, which also demonstrates that adhesion of the Al(OH)3 fillers are better than the silica. Laminates without any filler have higher copper peel strength than laminates with fillers.