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Lead-free: Upgrading Thermoplastics for RoHS
December 31, 1969 |Estimated reading time: 4 minutes
lead-free soldering is a necessity for many manufacturers, but issues, such as the need to re-engineer to accommodate board components that can withstand higher temperatures, have risen. this article highlights the benefits associated with upgrading thermoplastic resins for connectors to a high-heat-resistant formulation.
By Steve Fournier, Winchester Electronics and Jamie Tebay, GE Plastics
When re-engineering to accommodate electronics and board components that can withstand the higher heat of lead-free soldering, one electronics manufacturer* met a new challenge - upgrading thermoplastic resins for connectors from a moderately heat-resistant one to a high-heat-resistant formulation. This company designs and manufactures technical interconnect products, including a range of electronic connectors for consumer and business electronics and highly specialized military and medical interconnects (Figure 1).
Minimizing Tooling Changes
The product modification initiative began more than two years ago in anticipation of the July 2006 RoHS deadline. The project quickly moved from a simple shift in resin to one that had to bring many variables under control, and ensure that components were heat resistant. While the key driver was resistance to production temperatures, it became clear that other needs were important, including conformance to additional regulatory and application requirements - fire retardancy; smoke generation; and military, aerospace, and medical specifications and standards - as well as tooling costs. Economics and operational needs brought about a strong focus on tooling. It was estimated that retooling costs would have been as much as $2 million, and it would have required up to a year to develop and qualify new molds. Several design programs would have been passed on during the conversion period - potentially losing business.
Several tooling variations and techniques, which were developed during the tin/lead solder era, were for molding polybutylene terephthalate (PBT) resins. However, PBT cannot withstand the temperatures found in infrared (IR) reflow soldering. Many company products must meet tight tolerances for the location of pins and sockets, and the PBT matrix, tended to soften and distort in the higher IR heat.
Tooling and Resin Shrinkage
Tooling determines molded-part geometry; the tighter the tolerance on any given dimension, the greater the importance of consistent resin behavior. The final geometry of an injection-molded part is the result of a complex mix of resin conditions, for example, moisture content during molding, precision of formulation with resins and fillers, injection pressures, temperatures, time of residence in the mold, and more. This becomes more complex with some common fillers, such as glass fibers that, depending on flow in the mold, can become strongly aligned during molding. Parts with strongly aligned fiber reinforcement typically exhibit different shrinkage in the direction of resin flow in the mold compared to shrinkage perpendicular to flow direction. All of these shrinkage factors are taken into account in the design of tooling, and every mold cavity is enlarged to ensure the geometry of the molded part.
Figure 1. Series of connectors with thermoplastic pin-and-socket carrier.
If shrinkage is ignored, there are a number of resins that can handle IR reflow temperature, including polyimides (PAs), polyetherimides (PEIs), and others. But these resins in conventional formulations have shrinkage factors significantly different from PBT. Therefore, a high-heat alternative was needed to meet higher soldering temperatures. Therefore, a custom compound** that was a polyimide variant containing polyphthalamide (PPA) with flame-retardant (FR) compounds formulated from polyimide 6 and 6,6 (PA 6 and 66), and PPA base resins was developed. The compounds were designed primarily for the electronic component industry because of their insulating properties.
PPA is a semi-crystalline, aromatic PA that exhibits relatively low moisture absorption, and inherently heat and chemical resistant, and rigid. Because it can be processed with hot-water molding technology, which avoids the energy penalties of hot-oil production, it also is easy and economical to mold.
Design of Experiment
PPA alone exhibits shrinkage significantly different than that of PBT. PPA also is not inherently flame-retardant. It was clear that compounding would be required to specific tooling constraints, as well as regulatory and engineering requirements for a range of connector applications. After several iterations of designs of experiment (DoEs) and multiple trials using tooling, a final composite that is flame-retardant and shrink-matched to PBT was developed*** (Table 1). This formulation also meets the requirements for halogen-free parts and exhibits low viscosity, which is important for thin-wall molding characteristics of connector designs, and has a heat-deflection temperature (HDT) of 230°C for 30-60 sec. and 260°C for 10 sec. The compound also has a specific gravity that is 13% lower than that of PBT. This translates into more parts per pound of compound - offering a potential long-term process cost saving. The replacement material also has higher flexural and tensile modulus than PBT, which improved mechanical properties, allowing greater durability of the parts in demanding environments.* Winchester Electronics, Wallingford, Conn.** LNP specialty compounds, GE Plastics, Pittsfield, Mass.*** Starflam UF-1006 HW Z270 specialty compound, GE Plastics, Pittsfield, Mass..
Steve Fournier, director of engineering, Winchester Electronics, may be contacted via e-mail: s.fournier@winche-sterelectronics.com. Jamie Tebay, Americas product manager, GE Plastics-LNP, may be contacted via e-mail: jamie.tebay@ge.com.