by Edson Ito, Vectra & Zenite LCP technical marketing manager, Ticona Engineering Polymers

The electronic industry is driven by market trends with stringent environmental and energy efficiency regulations that call for the miniaturization and the convergence of the ubiquitous electronic devices that we all use today.

These ever shrinking devices, with their increasing number of inputs and outputs, depend on the high reliability of their assembled components that are critical to the final product quality.

However, miniaturization is creating daily demands on the most popular packaging choices — packaging that uses ultrathin parts and connectors and sockets with high pin density.

To create these high precision injection molded parts, the microelectronics industry is increasingly turning to a unique group of high performance plastics that enable them to completely fill small and intricate molds.

LCP — A Class of Resins for E/E Applications

One such material solution for electrical and electronic applications involves a class of resins known as liquid crystal polymers (LCP). This family of high performance plastics, two of which are known by the Ticona brand names Vectra and Zenite, offer a balance of properties unmatched by most other resins:

• UL 94 V-0 5VA flame resistance without additives.
• High temperature resistance (340 degrees Celsius, short-term) for lead-free soldering.
• Recyclable — regrind usage up to 50 percent with UL approval.
• Allows thin wall and complex designs.
• Extremely fast cycle times.
• Flash free molding.
• Very low coefficient of thermal expansion.
•  Excellent dimensional stability.

These high-flow polymers are easily molded into intricate parts, such as electrical connectors with high pin densities, and extremely tight dimensional tolerances. Moreover, thin-walled components made from LCPs are exceptionally strong and stiff because of the self-reinforcing nature of the polymer.

With the move to lead (Pb)-free soldering, LCP became the primary material for pre-molded air cavity packaging. This thermoplastic can withstand assembly temperatures well beyond the abilities of polyester, polyethersulfone, and other engineering polymers.

For instance, only LCP and polyetheretherketone (PEEK) — a more expensive and much harder to mold in thin cross-sections than LCP — can tolerate 260ºC or more without melting or distorting. In addition LCP has begun to displace more expensive ceramic packaging in high performance applications such as those that call for high power density and high frequencies.

Unique Physical Properties

The properties of LCP are influenced to a high degree by its liquid crystal structure. The rod shaped molecules are oriented in the flow direction during injection molding or extrusion and are rod shaped, even in the melt phase.

Due to the highly ordered nature of liquid crystal polymers, mechanical properties, shrinkage, and other part characteristics depend on the flow pattern in the part. During mold filling, the molecules are oriented in the flow direction.

In the finished part, these molecules are ultimately aligned on the surface where they form a skin that is highly oriented in the flow direction. The skin makes up 15 to 30 percent of the part's total thickness. This molecular orientation causes a self-reinforcement effect giving exceptional flexural and impact strength as well as good tensile performance. Relative strength values increase as a direct correlation with decrease in wall thickness.

As the wall, film or sheet thickness decreases, the highly oriented outer layer accounts for a higher proportion of the total wall thickness (skin/core effect).

This higher percentage of highly oriented surface layer, in general, results in greater strength and modulus in thinner sections.

The excellent flow properties of LCP make it possible to fill extremely fine structures and to produce very thin-walled parts.

LCP Provides Soldering Resistance

Parts molded from LCPs are suitable for applications that require vapor phase (wave) and infrared (reflow/through-hole) soldering. They are dimensionally stable and have extremely low thermal expansion coefficients at soldering temperatures. The LCPs exceptionally low moisture absorption has a positive effect on dimensional stability, minimizing dimensional changes, and warpage.

Another important consideration is heat deflection temperature (HDT) under load due to internal pressure in the component from trapped inert gas or other mechanical effects, such as clamps.

As a result of the increased use of lead-free soldering, temperature peaks in the soldering operation have risen to more than 270ºC. It should be noted that some LCP grades are suitable for such soldering operations, provided that they have been correctly processed.

LCP Packaging Process

Manufacturing with LCP differs from other packaging processes. LCP packages are injection molded around lead frame inserts, often at mold cycle times under 10 seconds. This method lends itself to high-efficiency approaches, such as a continuous process that molds strips of metal lead frames in multi-cavity molds.

Alternatively, conductor patterns can be placed on LCP after molding by such automated processes as laser direct structuring, in which a laser draws a circuit pattern on a platable LCP that is then metalized in an electroless plating bath.

Once a die is attached and wire bonded, the air-cavity package can be sealed with a metal, ceramic, glass, or plastic lid. Sealing often involves an adhesive such as epoxy, although heat, laser, or other means can be used. LCP works well in pre-molded packaging because many grades of this polymer withstand temperatures above 300ºC. It is inherently flame retardant and meets UL 94V-0 flammability criteria without the addition of halogen, phosphorous, or other flame retardants. And, as a thermoplastic, it is recyclable.

LCP has 10 times the water vapor barrier of epoxy and absorbs just 0.02 percent moisture, so it can create near-hermetic packaging. It has excellent dimensional stability in molding and creates precision parts having flat surfaces right out of the mold, without rework. It also has good electrical properties; for instance, it has a low dissipation factor and its dielectric constant is 3 to 4, compared to 9 to 13 for ceramic.

For advanced packaging designs, LCP offers greater functionality and flexibility than ceramic. While ceramic packages can be shaped in two dimensions, they have trouble with three-dimensional shapes or complex contours. Injection molded LCP does not, which lets many elements — connectors, inductors, and other structures normally placed on boards — be integrated into complex packages. This will reduce demand for board space and lower component, processing, and assembly costs.

As miniaturization and the convergence continue to shape every aspect of our electronic devices, component manufacturers will require materials such as LCPs that meet the strict quality and performance requirements without increasing costs.

Next month we’ll look at extreme powertrain performance and how Fortron PPS can help manufacturers of components reduce weight, drive down costs and perform in these challenging environments.

For Ticona technical papers and more information about Vectra LCP for E/E applications, visit http://www.ticona.com/home/homepage/green-electronics.htm.