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Jan 2, 2021

PCBs—Applications in the Medical Field

With the medical field becoming more sophisticated and computerized, many diagnostic and treatment practices depend more on automated machines. These in turn, must depend on printed circuit boards or PCBs. These boards are an assembly of tiny, high-density circuits that pack a substantial number of components within a tiny area. The PCB assembly usually has several hightechnology features that make the product accurate and reliable.

The medical industry requires reliable, technologically advanced, and high-quality devices, and highly reliable PCBs make this possible. Some examples where the industry uses PCBs for medical applications are:

  • Hand-held wireless controllers
  • Drug-delivery systems
  • Disposable devices like pressure cuffs, wound management, diagnostic catheters etc.
  • Imaging and monitoring such as in-patient monitoring, X-ray apparatus, PET, MRI, CT, Ultrasound, etc.
  • Medical wearable devices
  • Blood glucose meters
  • Aesthetic care devices, hearing aids
  • Microelectronics for implantable devices
  • Laboratory diagnostic tools such as for dialysis, high pressure liquid chromatography, lab blood diagnostics
  • Medical display systems

PCB manufacturers provide design and engineering support for fabricating the full range of medical PCB prototypes to volume production, along with a complete support for medical PCB assembly, integration, and test solutions. This not only helps in quickly launching new medical products featuring the next-generation technology, but also to meet strict medical PCB standards through process control, quality, and traceability.

The above requires expertise in all aspects of medical-grade PCBs, an intimate knowledge of IPC Class specifications, and HALT/HASS testing. The robust PCB solution must also conform to the requirements of a medical device and ensure that the device manufacturer can get FDA, UL, IEC, and other necessary approvals.

What are Medical-Grade PCBs?

Medical-grade PCBs must be highly reliable as the medical devices they constitute depend on them, and human lives depend on both. With the advancement of medical technology, most medical-grade PCBs must conform to IPC Class III. The three major requirements of these PCBs are:

  • Pads must not have annular ring breakout
  • Through holes must have at least 1 mil of plating in the barrel
  • Copper track width and spacing must have tighter tolerance

Technology for Medical-Grade PCBs

The most suitable technology to meet the tight tolerances of copper tracks and spaces is the High Density Interconnect or HDI technology. With HDI technology, it is possible to reduce the PCB footprint with laser-drilled vias, via-in-pad, and 5 mil tracks and spaces.

However, manufacturing medical-grade HDI PCBs requires investing in equipment such as for laser direct imaging and modified semi-additive processes. Such technologies are necessary for achieving the tight tolerance requirements for track width and spacing.

Materials for Medical-Grade PCBs

Although FR-4 laminates are common for PCBs in medical instruments, more sophisticated equipment may require PCBs with special laminates like Polyimide, Teflon, and ceramic to meet their unique requirements.

Proximity of medical equipment to humans requires using PCBs that do not contain some elements harmful to life. Halogen free PCBs are common for medical equipment, and PCB assemblies conforming to RoHS specifications are basic requirements.

How are Medical-Grade PCBs made?

Design, fabrication, and assembly of medical-grade PCBs is not much different from those of regular boards, except for a few processes such as imaging and etching.

Fabrication of regular boards use a subtractive method of etching. The fabricator exposes a photoresist coating on the copper foil to UV light through the negative of a circuit pattern. Exposure to UV rays hardens the photoresist in the areas where the circuit pattern is necessary. An etching solution removes the unwanted photoresist and copper under it, leaving the copper in the form of the desired circuit pattern.

circuit_board_design_usa

Fig 1: Subtractive Process

The disadvantage of the above subtractive method of etching is that it forms non-uniform tracks. As etching removes copper from the top, the walls of the track are not vertical, but trapezoidal. That means, subtractive etching cannot form uniform track width and fine spacing.

Medical-grade PCBs with their tight tolerance on thin tracks and fine spacing require the modified semi-additive process or mSAP technology. Fabricators start by electrolytically removing most of the copper from a copper clad laminate, until only a very thin layer remains. They transfer an image of the desired circuit pattern on this thin copper layer using laser direct imaging. This leaves the required circuit pattern in the form of exposed copper.

The fabricator next deposits a thick layer of electrolytic copper on the exposed copper. Once they have reached the desired thickness, they chemically remove the unwanted resist, and etch the unwanted copper. As fabricators add copper rather than remove it during the fabrication, the process is an additive one.

pcb_board_design_usa

Fig 2: Additive Process

The advantage of the additive process is that it helps to achieve very thin tracks and fine spacing, as the walls of the tracks are vertical. Moreover, as the process electrically deposits copper on a laser image, the fabricator can maintain a much tighter tolerance on the track width.

Conclusion

Medical-grade PCBs can be challenging when designing and fabricating. They sometimes use very thin dielectric materials, very fine line and spacing, materials with dissimilar coefficient of thermal expansion or CTE, materials with dissimilar dimensional stability, and varying testing protocols.

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