Opto-electronic assembly having an encapsulant with at least two different functional zones

Abstract

A radiation emitting device of the present invention includes at least one radiation emitter, first and second electrical leads electrically coupled to the radiation emitter, and an integral encapsulant configured to encapsulate the radiation emitter and a portion of the first and second electrical leads. The encapsulant has at least a first zone and a second zone, where the second zone exhibits at least one different characteristic from the first zone. Such different characteristics may be a physical, structural, and / or compositional characteristic. Preferably, the at least one different characteristic includes at least one of the following: mechanical strength, thermal conductivity, thermal capacity, coefficient of thermal expansion, specific heat, oxygen and moisture impermeability, adhesion, and transmittance with respect to radiation emitted from the radiation emitter. The radiation emitter may be in a form of an emitter, and is preferably an LED.

Description

CROSS-REFERENCE TO RELATED APPLICATIONS [0001] This application is a divisional application of U.S. patent application Ser. No. 10 / 338,540, entitled “METHOD OF MAKING RADIATION EMITTER DEVICES,” filed on Jan. 8, 2003, by John K. Roberts et al., which is a divisional application of U.S. patent application Ser. No. 09 / 835,278, entitled “RADIATION EMITTER DEVICE HAVING AN ENCAPSULANT WITH DIFFERENT ZONES OF THERMAL CONDUCTIVITY,” filed on Apr. 13, 2001, by John K. Roberts et al., which claims priority under 35 U.S.C. §119(e) on U.S. Provisional Patent Application No. 60 / 265,489, entitled “RADIATION EMITTER DEVICES AND METHOD OF MAKING THE SAME,” filed on Jan. 31, 2001, by John K. Roberts et al., and is a continuation-in-part of U.S. patent application Ser. No. 09 / 426,795, filed on Oct. 22, 1999, entitled “SEMICONDUCTOR RADIATION EMITTER PACKAGE,” by John K. Roberts et al., now U.S. Pat. No. 6,335,548, which claims priority under 35 U.S.C. §119(e) on U.S. Provisional Patent Application ...

AI Technical Summary

Benefits of technology

"The present invention provides a radiation emitter device that has improved performance and is less vulnerable to damage. The device includes a radiation emitter, electrical leads, and an encapsulant that has different characteristics in different zones. The method of making the device involves attaching the emitter to a lead frame, inserting it into a mold cavity, and partially and fully filling the cavity with encapsulant materials. The invention increases the reliable package power capacity, reduces thermal resistance, and decreases damage susceptibility of conventional discrete opto-electronic emitters."

Problems solved by technology

Unfortunately, most discrete opto-electronic emitters exhibit a characteristic contravening to the goal of reduced internal operating temperature.

However, increased power in devices with finite (positive, non-zero) thermal resistance results in elevated internal operating temperatures.

Without these attributes, the use of LEDs becomes cost prohibitive or otherwise unattractive from a quality standpoint for most high-volume applications.

Hand soldering suffers from inconsistency and high cost.

Mechanical connection schemes are expensive, bulky, and generally ill-suited for large numbers of electrical connections in many circuits.

Conductive adhesives such as silver-laden epoxies may be used to establish electrical connections on some circuit assemblies, but these materials are more costly and expensive to apply than solder.

Spot soldering with lasers and other selective-solder techniques are highly specialized for specific configurations and applications and may disrupt flexible manufacturing procedures preferred in automated electronic circuit assembly operations.

The impact of this property is far reaching, because these solder operations can introduce large thermal stresses into an electronic component sufficient to degrade or destroy the component.

The problem instead is the ability of these leads to conduct heat along their length into the encapsulated body of the device.

This can harm the somewhat delicate encapsulation, encapsulated wire bonds, die attach and chip.

This phenomenon represents one of the fundamental limitations of low-cost, opto-electronic semiconductor devices today.

This deformation from polymer phase transition and thermal expansion in encapsulants can generate mechanical stress and cumulative fatigue severe enough to damage a discrete semiconductor device, resulting in poor performance of the device and a latent predisposition toward premature field failure.

Such damage typically consists of: 1) fatigue or fracture of electrical wire bonds (at the chip bond pads or at the lead frame); 2) partial delamination or decomposition of die-attach adhesive; 3) micro-fracture of the chip itself; and 4) degradation of the device encapsulant, especially near the entry points of the leads into the encapsulant, and a compromised ability to seal out environmental water vapor, oxygen, or other damaging agents.

However, the need for optical transparency in encapsulants for opto-electronic emitters and receivers obviates use of most high-performance polymer-filler blends, ceramics and composites that are suitable for non-optical semiconductors.

As such, they are not suitable for exposure to transient temperature extremes greater than about 130 degrees C., such as occurs during normal soldering.

While useful, this has only partially alleviated the problems noted—the newest materials in use still fall 50 degrees C. or more short of parity with conventional non-optical semiconductor encapsulation materials.

While effective at protecting prior art devices from thermal transients associated with soldering, there are limits to this approach, particularly in the application of high power semiconductor opto-electronic emitters.

Increased lead thermal resistance results in elevated internal operating temperatures in prior art devices, severely compromising operational performance and reliability of these devices.

Radiation and natural convection from prior art devices play only a minor role in transferring internal heat to ambient, and thermal conduction through their encapsulating media is severely impeded by the low thermal conductivity of the optical materials used.

Greater thermal resistance in the solderable pins of these devices, necessary to protect the device from the transient thermal effects of soldering operations, therefore causes a higher internal temperature rise within the encapsulated device body during operation.

As previously discussed, severe consequences will result if the device internal temperature rises substantially above the encapsulant Tg value.

Above this temperature, the Cte of the encapsulant typically increases very rapidly, producing great thermo-mechanical stress and cumulative fatigue at the LED wire bond and die attach.

Since the emitted flux of semiconductor optical emitters are typically proportional to the electrical current passed through them, limitations upon maximum electrical current also create limitations on flux generated.

Other prior art devices have avoided these constraints, but have achieved high performance only by ignoring the needs of standardized, automated electronic assembly operations and adopting configurations incompatible with these processes.

Still other prior art devices have achieved high performance by employing unusually expensive materials, sub-components, or processes in their own construction.

While relevant for certain high-cost aerospace and telecommunications applications (where component cost is not a significant concern), such devices require expensive materials and unusual assembly processes.

This results in high cost and restricted manufacturing capacity—both of which effectively preclude the use of such components in mass-market applications.

However, KOVAR has relatively low thermal conductivity, necessitating the inclusion of a separate heat coupling means made of a material such as copper with a higher thermal conductivity.

These devices are complex and typically involve insulated pin and header construction and / or include specialty sub-components such as ceramic isolation sheets within them.

This configuration, however, may complicate integration of such components with electronic circuits having higher complexity—such circuits are conventionally made using printed circuit boards, automated insertion equipment, and wave or reflow solder operations.

However, there are several significant problems with this configuration that inhibit its broader use.

The package geometry of the SuperFlux package renders it incompatible with conventional high-speed THD radial or axial insertion machinery or by SMT chip shooters known to the present inventors.

Instead, it must be either hand-placed or placed by expensive, slow, robotic odd-form insertion equipment.

The SuperFlux package geometry is configured for use as an “end-on” source only—no readily apparent convenient lead-bend technique can convert this device into a 90-degree “side-looker” source.

The moderate thermal resistance of the solderable pins of this device and relatively low heat capacity may leave it vulnerable to damage from poorly controlled solder processes.

It may be inconvenient or costly for some electronic circuit manufacturers to control their soldering operations to the degree needed for this configuration.

Finally, there is no convenient mechanism known to the inventors to outfit a SuperFlux package with a conventional active or passive heat sink.

A principle factor impeding further application of these and other LED devices in signaling, illumination and display applications is that there is not currently available a device that has a high power capability with high emitted flux where the device is easily adaptable to automated insertion and / or mass-soldering processes.

These limitations have either impeded the practical use of LEDs in many applications requiring high flux emission, or they have mandated the use of arrays of many LED components to achieve desired flux emission.

Conventional “5 mm” or “T 1¾” devices have a high thermal resistance, typically in excess of 240 degrees C. per watt and usually are limited by clear encapsulation materials that lead to unreliability if the emitter in the device is operated continuously, routinely or cyclically above 130 degrees C.

This means that the device power must be limited to approximately 0.18 W. With a reasonable design tolerance of 33 percent to accommodate manufacturing variances, the practical reliable power limit of this device must be approximately 0.12 W. This is not a lot of power, and the emitted flux of these devices is thus limited.

As with “5 mm” or “T 1¾” devices, SuperFlux or Piranha devices usually are limited by clear encapsulation materials that lead to unreliability if the emitter in the device is operated continuously, routinely, or cyclically above 130 degrees C.

This means that the device power must be limited to approximately 0.3 W. Because these devices are attached subsequently with thermally stressful wave or other solder operations, and because their thermal resistance from lead to junction is reduced, they are more susceptible to damage during processing into circuits.

Thus, a higher design tolerance of 40 percent should be used to accommodate manufacturing variances and increased susceptibility, and the practical reliable power limit of this device must be approximately 0.18 W. This is a substantial increase (33 percent) compared to “5 mm” or “T 1¾” devices, it still is not a lot of power and the emitted flux of these devices is thus also limited.

As with “5 mm” or “T 1¾” or SuperFlux, Piranha, or SnapLED devices usually are limited by clear encapsulation materials that lead to unreliability if the emitter in the device is operated continuously, routinely, or cyclically above 130 degrees C.

This means that the device power must be limited to approximately 0.45 W. As noted above, because the thermal resistance of these devices from lead to junction is so low, they cannot be soldered by conventional means without being damaged.

This severely limits their utility, but they still are suitable for some applications.

Because these devices are attached subsequently with mechanically stressful clinching operations, they remain susceptible to damage during processing operations.

Thus, a higher design tolerance of 40 percent should be used to accommodate manufacturing variances and potentially increased processing damage susceptibility, and the practical reliable power limit of this device must be approximately 0.27 W. This is a significant increase compared to “5 mm” or “T 1¾” or SuperFlux or Piranha devices, but it still is not a lot of power (and is achieved at a sacrifice in conventional solderability).

Such a design and process are difficult and expensive to execute with high yield and high quality.

Accumulated variances would be excessive from the multistage molding scheme, interrupted by die and wire bonding.

An additional problem faced by designers of conventional LED devices is that the wire bond used to join one of the LED leads to the LED chip can break or lose contact with the lead or the chip.

Such failure can occur, for example, due to shear forces that are transferred to the wire bond through the encapsulant or thermal expansion / contraction of the encapsulant around the wire bond.

The other forms of radiation emitters mentioned above also experience performance degradation, damage, increased failure probability, or accelerated decay if exposed to excessive operating temperatures.

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