Printed Circuit Design & Fab - September 2008 - (Page 24)

ENvIRONMENTAL uPdatE FiGurE 2. Typical printed circuit board construction. FiGurE 3. Typical failures due to thermal cycling. FiGurE 4. Photomicrograph of a failed board. cal multilayer printed-wiring board, stresses due to differential expansion during processing in a lead-free assembly are amplified by changes in thickness and layer design. FiGurE 2 is a detail of a standard printed circuit board construction. Material Selection and Test It is always desirable to define the requirements of a laminate based on reproducible laboratory tests that can be performed on different test instruments with similar and valid outcomes. For lead-free assembly of green laminates, the electronics industry has identified several test methods that, when taken together, can evaluate the characteristics and robustness of laminates. These include the following: ■ The z-axis coefficient of thermal expansion (CTE) (lower is better) ■ Glass transition temperature Tg (higher is better) ■ Thermal decomposition temperature Td (higher is better) ■ Time to delamination at 260°C, 288°C and 300°C (T260, T288, and T300) (longer is better) ■ Toughness of the composite as a function of temperature ■ Resistance to cathodic/anodic filamentation (CAF) growth ■ Interconnect stress testing (IST) ■ Highly accelerated thermal shock (HATS) Quality and Reliability of Green Materials and Processes A number of accelerated fatigue and environmental tests are commonly used qualitatively to compare materials, designs and processes and to describe a material property over time. For a multilayer printed-circuit board, the integration of design, materials and value-added processes is reduced to the 24 reliability of the z-axis connections. Failure due to thermal cycling can occur in several geometric locations, as shown in FiGurE 3. When a circuit board is powered and then turned off to cool, the circuit board materials — glass, resin, copper and plating finishes — do not expand and contract at the same rate. The resin in the laminate material expands at a much higher rate than the copper in the plated through hole. The copper resists the movement of the resin in the laminate and is stretched slightly during each temperature excursion. Pure copper has very little elasticity and deforms with stress, and with this deformity, the stress may be relieved mechanically through cracks that form at weak points in the metal. A typical plated through-hole construction is shown in Figure 3. During a thermal stress, caused by either turning the circuit power on or off or sequential solder during assembly, the board will expand and shrink in each direction. Failures in the conductive pathways can result in lost or diminished function of the board. These failures can be in any of these forms: ■ Corner cracks ■ Barrel cracks ■ Foil cracks ■ Post separation More descriptive names for failures are often required to help distinguish their causes, but these are typically subsets of the names above. FiGurE 4 is a photomicrograph of a failed board which illustrates the issue. The development of materials for the most severe lead-free soldering conditions requires a balance of material properties that challenge even the best laminate material formulators. The key factors include: ■ The dimensional changes above and below the glass transition temperature (Tg) of the multilayer laminate needs to ■ ■ ■ ■ ■ ■ ■ be less than that associated with traditional laminate materials. A reduction in the x, y and z-axis laminate changes can significantly reduce the stress placed on the plated through hole during the hightemperature solder processes. Adhesion to plated through-hole barrels, foils and reinforcements has to be greater at extreme temperatures to resist delamination. In multifunctional phenolic cured laminates, standard hole wall texturing and plating processes are strained, often resulting in reduced hole wall adhesion. Increased, void-free adhesion of prepreg to the etched core, conductors and spaces. Resin systems need to have higher thermal decomposition temperature Td to prevent local void formation adjacent to the plated through hole. Resin systems need to be able to withstand higher temperatures longer. Resin systems need to be robust in the presence of moisture, either by not absorbing or not being affected by it. New materials need to continue to meet flame retardant requirements to UL 94V0. New materials cannot cause significant changes in the material and processing costs of the raw laminate and prepreg. Lead-free Assembly Impact on PCB Fabrication Customer requirements come to board fabricators in the usual formal and informal manners: by drawings, by inside data and specifications and in the boilerplate on purchase orders. Although fabricators build to print, there is a concern when the material set or design is not consistent with the assembly process, so additional care needs to be taken to make sure that the boards will survive after leaving the PCB fabricator. SEPTEMBER 2008 printEd CirCuit dESign & fAB

Table of Contents Feed for the Digital Edition of Printed Circuit Design & Fab - September 2008

Printed Circuit Design & Fab - August 2008 Contents Our Line Market Watch Around the World Happenings ROI Positive Plating Off the Shelf Marketplace Ad Index EMC for the Real World Final Finish Forum Design for Green: Laminates A Systematic Approach to Increasing Layer Count The NTI $100 Million Club Printable Nanocomposites BGA Bulletin

Printed Circuit Design & Fab - September 2008

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