CircuiTree - September 2008 - (Page 27)

that has driven OEMs to search for more environmentally friendly perceived materials for their products. The Various Flame Retardants of Use Halogen-based ﬂame retardants continue to be the workhorse for PWB applications. Generally, by weight, bromine is three times more effective as a ﬂame retardant than chlorine. Bromine’s effectiveness as a ﬂame retardant is due to its ability work in the gas phase generating hydrogen bromide that scavenges hydrogen-free radicals generated by the substrate, thereby effectively ceasing the chemical reaction of pyrolisis. Aside from the halogenated ﬂame retardants, there are many alternative types of ﬂame retardants; these can loosely be grouped into four categories as found in Table 1. This is by no means an exhaustive list of ﬂame retardants but a general list of compounds that have been found to work well in PWB applications. Table 1 Most Common Types of Alternative Flame Retardants Base Chemistry Phosphorous based Nitrogen based Metal hydrates Other inorganics Comments Can include both organic and inorganic materials Principally melamines and their derivatives Al and Mg based are most common Includes Zn and borate compounds “Phosphorous- and nitrogen-containing compounds are unique,” states Nikolas Kaprinidis, senior applications specialist for CIBA, “in that depending upon their structure, they can work by a variety of mechanisms to quench a ﬂame. These mechanisms include free radial scavengers in the gas phase, char formers in the condensed state and via intumescences (i.e., the blowing of an inert gas through the char and forming foam). Some phosphorous compounds even exhibit endothermic reactions thereby lowering the temperature of the ﬂame while still others offer superior properties of smoke suppression.” The metal hydrates principally work via endothermic reactions by releasing water and, as such, for PWB applications, their thermal reliability in the past has typically been somewhat borderline; but newer materials are emerging that do show better thermal robustness. The zincs and borates typically work as a synergist by recycling the main ﬂame retardant, but even they are under close regulatory scrutiny in the European Union. With such a vast variety of compounds to choose from, it is remarkable how few compounds can actually work in a PWB application. First and foremost it must work as a ﬂame retardant—easy enough to do. But just as important, the chosen ﬂame retardant must still maintain the resin matrix’s thermal reliability to survive soldering and reﬂow conditions and also be sufﬁciently bound into the matrix, or inert enough, to not leach out or break down during a variety of plating and etching processes that use a wide variety of processing chemicals. Another concern is the ﬂame retardant’s affect upon electrical properties, not only dielectric and dissipation factors but also with regard to electrical strength and voltage breakdown. is phosphorous. A standard FR-4 laminate requires roughly 20 percent by weight bromine to achieve V0 where as with phosphorous you might only require 5 percent by weight. And there are at least three means to get phosphorous into the PWB resin matrix; the phosphorous could be pre-reacted into the backbone of the resin, an additive approach could be made where an inert or reactive phosphorous-containing ﬁller/additive is added to the resin composition, or a combination of the two. The pre-reacted into the backbone approach offers the cleanest approach. There are generally less issues during lamination and better processing during drilling, plating, and fabrication. The downside is that some of the newer phosphorous-containing resins are anywhere from 100 to 500 percent higher in cost compared to traditional bromine-based systems. The alternative method is to use an additive approach, but, to date, the most cost-effective phosphorous compounds, that can still survive PWB processing, cost approximately four times that of TBBPA—still, for a standard mid-Tg product, that can add up to a 40 to 100 percent premium over TBBPA-based systems. The upside is that there is generally a reduction of CTE expansion due to less overall resin content. The downside is that with high ﬁller loadings you can have issues with drilling, plating, and resin wet out. Quite commonly, what are found in the market are halogenfree materials that use a combination of the previously mentioned approaches. In almost all instances, thermal expansion performance is enhanced and generally a reduction in overall cost is seen. But by using an additive approach there can still be a cost penalty felt by fabricators as they are forced to drill slower as these ﬁlled products tend to wear drill bits out faster. But with newer materials coming to market every day, there are actually ﬁllers and additives that are not only easier on drill bits but, in some instances, can actually extend drill bit life. Cost still is the main driver for the selection of halogen-free materials. But with the volume of halogen-free materials so low today, and with a huge volume potential to be realized, perhaps sooner than later these halogen-free materials could have cost parity with TBBPAbased materials. Material Performance and Selection When selecting materials for electronic application, the three main criteria for material selections are mechanical, thermal, and electrical properties. That is, can it be drilled, can it survive lead-free assembly, and what impact, if any, will there be on the circuit’s loss and permittivity? For better or worse, halogen-free materials have drawn higher scrutiny than traditional brominated FR-4 materials. Aside from the usual concerns of thermal reliability and having to change processing parameters, halogen-free materials have unfairly gained a reputation for having poor shelf life and excessive moisture pickup. The concern is that these newer, phosphorous-containing compounds are more hydroscopic than their brominated counterparts. But I wish to stress that it’s not necessarily the phosphorous, or the borates, or the nitrogen that are at issue but rather the design of the molecule itself that lends itself to hydrolysis, moisture pickup, or poor thermal stability. While it is true some halogen-free materials do have these drawbacks, not all suffer from short shelf life and moisture pickup; indeed, some halogen-free materials perform equal to or better than traditional FR-4 materials. At Isola we benchmarked numerous halogen-free materials against a traditional FR-4 laminate to measure the effects of aging and moisture pickup on these newer halogen-free materials. For all samples tested circuitree.com • September 2008 27 Cost Factors Cost is also a prime consideration for material selection. But as an industry, to date, we are nowhere near cost parity with TBBPA-based material. There are many variables that factor into to the cost equation. By far the most popular non-halogenated ﬂame retardant of choice http://circuitree.com

Table of Contents Feed for the Digital Edition of CircuiTree - September 2008

CircuiTree - September 2008 Contents My Line Industry Review Tech Talk Flexible Thinking New Halogen-Free Materials: Their Time Has Finally Arrived Asian Section IPC Issues PCB and Package Convergence Ask the Flexperts Market Outlook IPCA Showcase Technical Product Spotlights Classified Ads Upcoming Events Ad Index

CircuiTree - September 2008

For optimal viewing of this digital publication, please enable JavaScript and then refresh the page. If you would like to try to load the digital publication without using Flash Player detection, please click here.