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March 2014 • The PCB Magazine 21 liferation of IMS manufacturers. The big issue in the market now is one of long term reliability. There are plenty of low prices for these materials out there, but no one knows how long PCBs made from these low- price materials will function in the field, or at times if they are even true thermally conductive substrates. Given the investment being put into the switch from traditional to LED lighting, saving a few pennies per PCB could be false economy if they fail in the field. Emphasis, for both the IMS manufacturer and the PCB producer, has to be put on the equation of cost vs. reliability. Further, given the disparity in in-house test methods and the current lack of international standards as outlined above, as a PCB manufac- turer, supplying to an emerging market (LED lighting) with product life expectations in ex- cess of 50000 hours, at a usage of 10 hours a day, for nearly 14 years, you need to be abso- lutely sure of your material manufacturers cre- dentials. Put simply, there is no point crying over the cost of product failure or loss of reputation, when to save a few pennies a supplier for IMS has been chosen purely on price! All professional producers are generally geared to the provision of test and reliability data in line with industry recognised standards and test procedures, where available, from the likes of IPC, ISO, ASTM and UL. Along with the significant investments in product devel- opment and manufacturing infrastructure, this means the professional IMS manufacturer will not necessarily be the lowest price, but will in- voke the price vs. reliability/reputation analysis and become very competitive in terms of cost. PCB Fabrication Considerations Thermal Prepregs ML Lamination The heavy loading of these materials, with filler, leads to "low-flow" prepreg characteristics therefore there is a need for rapid temperature rise and early application of lamination pres- sure. It is possible to encapsulate 2 oz. copper tracks however, heavier copper weights will present some challenges. Machining Due to the type and content of filler in IMS, there is a tendency for rapid tool wear. Undercut, diamond-coated drill bits are pre- ferred; spindle speed will be around 80% of that used for FR-4. Two-flute end mills give best finish on alu- minium, and adjust cutter depth at 50% tool life to compensate for local abrasion by filler. Lead-Free Soldering and Thermal Cycling Insulated metal substrates are generally compatible with lead-free soldering processes however; construction has a significant influ- ence on reliability. Woven-glass reinforcement of the dielectric reduces its coefficient of ther- mal expansion both in the XY plane. Resin chemistry determines characteristics, such as T g and T d , which, likewise, affect reli- ability under thermal stress and thermal shock conditions. Surface treatment of aluminium prior to bonding is critical, both to achieve high bond strength and to avoid high-resistance shorts from metallic debris. Summary There continues to be increasing interest in cost-effective thermal management of elec- tronic devices and PCBs, and thermally con- ductive printed circuit substrates, or insulated metal substrates, are becoming the solution of choice When specifying or pre-selecting materials, think in terms of actual thermal impedance of an IMS material, rather than judge everything on the thermal conductivity figures—watts per metre Kelvin. Choose your materials to suit the applica- tion. And importantly, do your own qualifica- tion tests, and don't take data sheet values too literally. PCB ian mayoh is the european technical support manager at Ventec europe. to contact the author, click here. THERMALLY CONDUCTIVE SUBSTRATES & THERMAL MANAGEMENT continues