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

ENvIRONMENTAL uPdatE ceptable environmental risks: they are bioaccumulative and have been banned by RoHS and other environmental laws. Tetrabromobisphenol-A (TBBPA), the chief source of bromine in current PCB materials, reacts into the backbone of the epoxy polymer, but limits the temperature capability of laminate materials where it is incorporated. It is intended to evaporate and to decompose at temperatures that may allow flammable gaseous decomposition products to burn, and its presence interferes with combustion. This occurs above the melting point (180°C), and it is generally accepted that this occurs at a relevant rate above 230°C if unreacted. Since the lead-free assembly reflow processes can peak at 260°C or higher, the quantity incorporated into the formulation needs to be balanced with the functionality of the epoxy used. TBBPA seems to be guilty by association with “bad” bromine containing flame retardant additives rather than by a body of evidence. The perception that bromine is bad has led to the elimination of any source of bromine in halogen-free materials. In recent months, the EU has reversed its position on TBBPA after an exhaustive eight-year-long study concluded that TBBPA does not collect in the environment and is therefore not an environmental hazard. Halogen-free laminates have less than 900 ppm combined chlorine and bromine from any source. For the purpose of this definition, there are no restrictions on other halogens: fluorine, iodine or astatine. Fluorinated hydrocarbons, such as polytetrafluoroethylene (PTFE), are used in high-frequency applications but could be considered halogen-free in this definition. Iodine is rarely used as a flame retardant, and astatine rarely occurs in nature. Removing the bromine requires a change in the strategy of formulating core and prepreg materials to ensure flame retardancy. A common halogen-free strategy is to incorporate materials that foam the board when it is exposed to heat and begins to melt. The foam creates a glass or char that insulates and acts as a gas barrier. Phosphate esters and melamine polyphosphates are representative of proprietary flame retardants that are compounded into the halogenfree laminates. These flame retardant additives affect thermal, electrical and physical properties of the cores and prepregs produced. NanYa NPG-170TL, Hitachi BE 67G, Taiwan Union TU-642, Nelco N4000-7EF, Isola IS500 and other suppliers have representative halogen-free materials available that exhibit some combination of acceptable material properties. The Impact of the Design A PCB is an electrical component with resistance, capacitance and sometimes inductance. PCBs are designed to provide electrical interconnects between active and passive devices in a minimum amount of space. Control of the board’s electrical properties is important because it affects the function of the electronic circuits. Unfortunately, the green materials do not always have exactly the same electrical characteristics as the materials they are replacing. Altering line width and spacing, changing the distance between conductive planes or modifying the percentage of resin to glass in the cores and prepregs can accommodate some of the differences. Design actions optimizing electrical properties may not result in the best performance of thermal or mechanical properties. In a typi- APPLIED CIRCUIT SERVICES A proud Taiyo Partner. ➤ (LEFT TO RIGHT) JULIE FLANAGAN, Manufacturing Engineer DOUG AGHOIAN, President SEPTEMBER 2008 CircuiTree Ad AUG08_v1.indd 1 8/6/08 23 printEd CirCuit dESign & 8:31:39 AM fAB http://www.taiyo-america.com http://www.taiyo-america.com

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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