Examples of mismatched materials of varying thermal dynamics, cured at various temperatures, causing warpage, cracking and potential breakage. The results are based on the forces that are present during matching and the assembly processes. [Photo courtesy of Promex]
Understanding and managing thermally induced dynamics during assembly and use is essential as devices become increasingly integrated and miniaturized.
By Dave Fromm, Promex
Some complexities of integrating miniaturized components, such as microelectronics, into increasingly small medical devices are obvious. Examples include the precision required to position and align components with the requisite accuracy, or the identification of critical-to-function dimensions and methods that control and check assembly steps.
A more hidden and often overlooked complexity is the thermally induced dynamics that occur when producing assemblies at microscale.
Material choices and their related thermal processing requirements throughout the design, development, manufacturing and packaging of a medical device can make or break an integrated microelectronics assembly’s build, yield and function. Worse, if not understood or considered with appropriate significance, these decisions can ultimately lead to devasting results that affect the device’s launch date, performance and long-term reliability in the field.
Here are best practices for managing thermally promoted dynamics correctly the first time, ideally during the design stage, when there is more time and less risk to experiment, fail early, learn and make improvements to the device.
Materials and thermal expansion
Most integrated microelectronics devices are assembled with processes that use varying temperatures over a wide range. Solder reflows and thermal-cure adhesives are common adjoining methods.
It is imperative to understand a bit of the fundamental physics of constituent materials within your device, namely their coefficient of linear thermal expansion (CTE), a measure of the fractional change in size of an object per degree change in temperature at constant pressure. Knowing how varying temperature induces variable dynamics within different materials is critical.
With very few exceptions, all materials expand in three dimensions with increased temperature. CTE describes the magnitude of expansion with temperature, typically expressed in units of parts-per-million per degree C (ppm/°C). It is important to make choices that ensure compatibility within the assembly and in use over time.
Common material classes for small devices incorporating microelectronics include:
- Low CTE (range 3-15 ppm/°C): Ceramic, silicon and metals
- High CTE (range 40-100 ppm/°C): Plastics/epoxies
- Polymers/plastics beyond their glass transition temperatures (Tg): As you heat these polymers, they get more flexible when they move through a glass transition, a change in the morphology of the plastic when it transitions from a glassy state to a disordered flexible chain, further increasing CTEs.
- PC boards: Made from layers of materials that each have their own thermal properties, typically copper and organic materials laminated together (CTE in board plane approximates copper ~ 15 ppm/°C; CTE in z-axis approximates epoxy, CTE > 40 ppm/°C)
Understand how materials react over time
As temperatures change during medical device production, components get bigger, longer, wider and taller. Some materials are less stable than others. Imagine an epoxy that expands more than a ceramic it is bonding to. When stacked and dispensed at room temperature, then heated, the ceramic doesn’t expand as much as the glue. When everything cools, a warp has been created which will make the device less reliable over time. [Image courtesy of Promex]
Choosing adhesives and other core materials requires a technical understanding of how they will react not just during manufacturing, but in use over time as part of the whole medical device.
With greater CTE mismatch comes greater induced stresses within the part upon temperature cycling. A common example involves thermosetting resins used to protect components on a PC board substrate. Thermal expansion causes variable dimensional changes within the different parts (especially when adhesives exceed their Tg) and between components and metal mold tooling within an assembly during processing of the product. This can frequently exceed the error budget for positional tolerance within a device unless a manufacturing process is designed to accommodate this movement.
Proper material CTE matching — or using processing methods with smaller temperature changes — reduces stresses during production and within the device later. Device designers should have fundamental knowledge of these material dynamics at the design stage.
When choosing materials, consider:
- Temperature ranges required to build your device
- Compatibility with your requirements for function and assembly: Pay particular attention to glass transition temperatures (Tg) of epoxies and other encapsulants, remembering that CTE markedly increases for polymers above their Tg.
- Minimizing material CTE mismatches wherever possible
- Decreasing processing temperatures: Thermal expansion is proportional to the change in temperature (Δ°C). Lower temperature thermal cures are often necessary to reduce effects related to CTE mismatches.
- Keeping path lengths small: You may have to make panels smaller (assemble fewer devices within a single setup).
- Panel flatness and stiffness: The increased costs associated with ceramic substrates compared to traditional organic substrates are often easily offset by the ability to fabricate devices with larger panel size.
- Alternative techniques to join mismatched materials: Consider ultraviolet-cure or flexible, stress-relieving adhesives.
- Ship, store and use conditions for the device in the field: Large temperature excursions can occur during transport and handling.
- Biocompatibility for implantable device materials
- The total assembly cost (not just the part cost)
Proper material and process selection will reduce risks associated with component damage/failure during assembly, or latent field failure. CTE-induced stresses create massive forces acting upon small areas that can lead to issues such as intermittent electrical opens and mechanical fatigue within the device caused by:
- Warpage that misaligns components within the device, causing dimensional failures and breakage
- Stresses at the joints between the materials
- Delamination: Poor adhesion and voiding within the part causes peeling, leading to bubbles and air pockets. Air pockets are particularly troublesome in devices that heat up (such as laser devices); local hot spots due to air’s insulating characteristics often cause premature failure.
It’s essential that medical device designers understand and manage thermally induced dynamics during assembly and use, especially as devices become increasingly integrated and miniaturized.
A best practice is to involve the manufacturing team very early in the design so that they can ask the right questions about the development and manufacturing steps required to assemble per design intent. Choose a manufacturer that deeply understands the thermal environment, opportunities and challenges throughout production, and you will have a knowledgeable partner for getting these dynamics right.
Promex Industries Chief Operations Officer David Fromm [Photo courtesy of Promex]
Dave Fromm is chief operating officer and VP of engineering at Promex, a provider of microelectronic component assembly, process design, and packaging for the medtech and biotechnology markets. With cutting-edge equipment, and expertise in material properties and supply chain management, Promex partners with OEMs at all stages on early and small prototypes to R&D development and high-volume production.
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The opinions expressed in this blog post are the author’s only and do not necessarily reflect those of Medical Design & Outsourcing or its employees.