Product miniaturization and improved performance using Thick Film Hybrids
Where the size of a product or system is largely determined by the size of its electronics, the potential for miniaturizing that product by using Hybrid technology instead of PCB technology is significant. Circuits that are largely digital in nature present the greatest potential for miniaturization; possibly to 10% of the size of the equivalent PCB without any reduction in function. Largely analogue circuits are less easily reduced in size with reductions to between 30% to 50% of the equivalent PCB, however analogue circuits perform more efficiently as Hybrids providing additional benefits.
Thick film hybrids are statistically more reliable than PCB assemblies as they typically contain no solder joints. Components are interconnected by welded joints (wire bonding) or silver loaded conductive epoxy adhesive, an extremely reliable and long established joint technology.
Hybrids can sustain higher operating temperatures than PCBs, with componentry available now that can function in 200DegC indefinitely. The alumina or aluminium nitride substrates have a very high coefficient of thermal conductivity and junction-damaging “hot spots” beneath individual semiconductors are avoided by the swift transfer of heat away from that device.
Hybrids are hermetically sealed, either by way of sealed metal enclosure or conformal coating, making them the most suitable technology for electronics that operates in hostile environments.
Using unpackaged semiconductors in bare die format, with laser trimmed carbon ink pads instead of discrete resistors, Hybrids are very much lighter and typically <10% of the volume of the same circuit in PCB technology.
Where the size and weight of the electronics has been greatly reduced the infrastructure supporting the electronics can be miniaturized and lightened also, possibly allowing the whole product to be reduced in size. Miniaturized products inevitably bring market advantage and / or open up new market sectors hitherto un-addressable by the larger and heavier product.
Inherent thermal management
Substrates are commonly Alumina which is a highly thermally conductive material that transfers heat away from hot-spots quickly and efficiently, dissipating it over the whole surface. PCB material is commonly epoxy based with poor thermal conductivity, leading to hot-spots which reduce the life of most semiconductor junctions.
RoHS does not apply to parts internal to a device, only to external packaging and terminations. Hybrids contain palladium silver terminated devices attached with conductive adhesive and bare silicon die which are wire bonded with no RoHS implications. Hybrids do not need to be XRF tested unlike PCB assemblies used in some critical applications.
The tin whisker phenomena is real, tangible and well documented. Ultra Electronics Energy has seen examples of whiskers from <1mm long growing between components on PCBs to 15mm long whiskers on the exterior of a metal enclosure. Whiskers occur most often in the context of lead-free RoHS compliant solder terminations. However, hybrids predominantly use un-packaged bare die semiconductors which are connected using wire bonding and other discrete components. These in turn, are connected using conductive adhesive instead of solder. Tin whiskers do not grow within a hybrid.
Qualification to MIL spec as single part
In the context of MIL PRF, the thick film hybrid is regarded as a single component regardless of the number of devices held within it. Therefore, a single hybrid containing any number of parts can be qualified in a single process. This compares with a PCB assembly where multiple components may require individual qualification for a specified configuration.
For any end product needing a long production life or service life, storing packaged components can be both difficult (typically manufacturers only guarantee a shelf life of 2 years) and costly (a packaged part being more expensive than the bare die). Storing and manufacturing with bare die resolves both of these issues, especially coupled with our nitrogen storage facilities. Ultra Electronics Energy can continue to manufacture with these stored die for decades, as opposed to years, as typically found with packaged components.
Hybrids are microelectronic circuits comprising multiple devices, often unpackaged, within a hermetically sealed metal enclosure or an impervious conformal coating. All components are insulated and isolated to a high level, giving constituent parts protection from moisture ingress. Hybrid substrates have their conductive tracking and insulating layers fired at 850°C for one hour, with conductive adhesive device terminations being cured at 150°C for two hours. By comparison, FR4 PCB assemblies face a maximum temperature of approximately 80 – 100°C for short periods, beyond which damage to the PCB structure starts to occur. Comprising extremely small and light weight devices a Hybrid is very much less prone to the destructive effects of vibration than a PCB assembly.
Hybrids utilise components which are increasingly capable of long term operation at up to 200°C. The component industry is now moving toward 250°C operation. Ultra Electronics Energy is building on early experience of designing and fabricating high temperature hybrids. By using these emerging parts, that capability is further extended.
Active trimming of resistors
Resistor values can be adjusted by way of laser trimming during the functional operation of the circuit, allowing the dynamic tuning of circuits as an automated production process.
Reduced PCB complexity
Digital devices typically have a high number of interconnections and they can usually be obtained in bare die format suitable for hybridisation. The inclusion of analogue parts within the hybrid results in most interconnections being inside the hybrid. This reduces the number of external or PCB mounted interconnections and allows the PCB upon which the hybrid is mounted to be less complex: fewer layers, fewer parts and fewer fine-line tracks, gaps and via holes.
Bare die devices are extremely difficult to reverse engineer, as there is no possibility of incremental serialisation or unique ID, thus allowing software detection of non-uniquely identified clones and ensuring a high level of IP protection. Hybrids are commonly sealed with welded metal enclosures or high integrity conformal coatings, making it difficult to discover the identity of integral devices compared with the equivalent PCB assemblies.
Single tested component
A hybrid may contain a thousand devices but when it is received by the customer it is one device which has been tested thoroughly to the agreed specification.
By using bare die within a hybrid the total number of interconnections is reduced significantly as is the resultant number of solder joints on the host PCB, increasing the fundamental reliability of the circuit. Discrete devices within a hybrid are terminated with conductive adhesive, a highly reliable technology not considered to degrade reliability as in the case of solder joints on a PCB.
Metal hybrid enclosures are typically plated with nickel under gold and internal conductor layers formed with noble metals, removing the risk of corrosion in both cases.
As most interconnections are contained within the hybrid, the number of external connections is substantially reduced, resulting in less complex and lower cost host PCBs.
Lower cost and widely available instrument grade commercial die can be screened and released to MIL spec as part of the hybrid fabrication process.
Availability of die
Not all silicon is available in bare die form, which may effect the partitioning of the electronic circuit and how it is split between hybrid and host PCB. Parts assigned to the host PCB are necessarily packaged devices which reduce the overall miniaturization of the circuit. Some die may only be obtained subject to a minimum order quantity, the surplus die having to be stored in nitrogen storage in order to be usable in future.
Long term nitrogen storage
Ultra Electronics Energy offers dual redundant nitrogen storage facilities split between two sites.