Printed Circuit Design & Fab - January 2008 - (Page 23)

noted that using IPC-4101 slash sheets for material choices is not sufficient to specify specific properties. The range of properties contained in these slash sheets is too broad for this purpose. It is therefore recommended that specific materials or their equivalents be specified for critical applications. The glass transition temperature should be determined using the Thermo-Mechanical Analysis (TMA) method, as per IPCTM-650, 2.4.24C3. This method is preferred over DSC4 and DMA5, two other methods used to determine the glass transition temperature, because thermal expansion of the PCB is a critical parameter and is given by TMA as a function of temperature. For all but thin PCBs, a minimum decomposition temperature, Td (determined as per IPC-TM-650, 2.4.24.66) as well as a maximum thermal expansion coefficient in the PCB thickness direction, CTE(z) (determined as per IPC-TM-650, 2.4.417) should be specified. CTE(z) values should be given separately for temperatures below Tg and above Tg, however, thermal expansion (TE in %) is often given in a range of temperatures from 50 to 260˚C; good TE values typically are 3.2% or less. Typically, the decomposition temperature is given as Td (5%) to a 5% weight loss. A decomposition temperature of Td (2%) to a 2% weight loss has been found to be a better indicator, but data based on this parameter is not yet widely available. Often, the time to delamination (T-288 or T-260) is specified, either in addition to Td, or instead of it. The T288 delamination time provides a more appropriate level of performance, given the process temperature required for lead-free soldering. Delamination time is sometimes combined with the requirement that the PCB needs to survive the process temperature for 4 to 5 excursions. Unfortunately, the data sheets from the various laminate/prepreg suppliers are typically neither consistent not complete. Moreover, where property measurements were performed by commercial laboratories (because the values given in the data sheets did not appear to be credible) the values of the CTE(x) and CTE(y) were found to be nearly double those given in the data sheets. When qualifying laminate materials, during the preliminary search, elimination and final selection process, the three properties that critical for the survival of the PCB and the PTH/via interconnect structure in a lead-free assembly are: ■ Glass transition temperature = Tg ■ Decomposition temperature = Td, and ■ Thermal expansion = TE These critical parameters can be combined in a Soldering Temperature Impact Index, STII, which is defined as STII = Tg/2 + Td/2 – (TE%(50 to 260˚C) x 10). For PCBs with thicknesses of 0.06 inches [1.5 mm] or greater, an STII value of 215 or larger is recommended. The STII concept is still not widely used, however, many, but by no means all, base materials offered by suppliers meet and exceed an STII-value of 215. The reason for the importance of these properties comes from the thermo-physical loading that the PCB interconnect structures, such as plated-through holes (PTHs), platedthrough vias (PTVs), buried vias and even micro-vias experience during the temperature excursions used in the leadfree soldering process. Failures such as via barrel cracking (FIGURE 1) and innerlayer post separation (FIGURE 2) can JANUARY 2008 be avoided by proper design. The resin recession, shown in Figure 1, does not have any real reliability consequences. In order to reduce the z-axis thermal expansion, various fillers are added to the basic PCB resin systems. In some cases, this reduces the strength of the material and leads to cohesive delamination failures as seen in FIGURE 3. Unlike adhesive failures resulting from a delamination at the copper-to-resin interface, cohesive delamination is limited to the dielectric material itself. Desmear Lead-free compatible, high-Tg (170-180˚C) materials often use phenolics as curing agents. The resistance of these laminate resins to desmear chemistries is substantially higher than the dicy-cured, high-Tg materials. Therefore, desmear procedures may require re-evaluation. Any new laminate material should not be assumed to be a drop-in replacement. If the desmear cycle has not been optimized, there will be poor hole wall topography and the potential for residual resin smear and plating defects. Design and Specifications Because of these issues, new PCB base materials designed for lead-free solder assembly need not only be properly selected according to their data sheet information, but also should have coupons tested in the product configuration to demonstrate their ability to meet quality and reliability requirements. 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Printed Circuit Design & Fab - January 2008 Contents Our Line Market Watch Around the World Happenings ROI PTH Reliability: Designing to Improve PTH Reliability EMC for the Real World PTH Reliability: The Survival and Long-Term Reliability of Lead-Free PCBs iNEMI's Emerging Technologies: Research Priorities for the Electronics Industry Printed Circuit Design & Fab Annual Buyers Guide Special Suppliers Section Guide to Products and Services Off the Shelf: Designcon Preview Marketplace Ad Index BGA Bulletin

Printed Circuit Design & Fab - January 2008

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