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How Much Should Device Designers Really Know about Micro-Coaxial or Twin-Axial Cables?

How Much Should Device Designers Really Know about Micro-Coaxial or Twin-Axial Cables?
Image courtesy of Hitachi Cable America
Answer: The more device designers know, the better.

Educating designers is a crucial element in specifying the right cable for the right job. As cables become smaller, the complexity of manufacturing these micro-cable designs increases, as do the challenges of obtaining reliable products to meet or exceed the performance criteria. At a certain gauge (approximately 44 AWG {0.002-in. [0.06 mm] outer diameter} and smaller), the list of capable vendors quickly dwindles. Having a better understanding of micro-cable manufacturing considerations will allow designers to make better decisions when it comes to specifying such a product.

Medical devices such as ultrasound catheters, catheter vision systems, and ablation technologies often require extremely small electrical cables for delivering the signals and power consistently throughout a procedure. For some disposable applications, the wire or cable costs must be low enough to meet the cost targets. This is a significant challenge.

As minimally invasive surgical procedures grow in market demand, medical OEMs are looking to reduce both cost and risk. Their focus is shifting toward disposable devices and vendor consolidation to drive down product costs while creating a more-robust supply chain. Even though there are fewer suppliers, the suppliers that are being awarded the business are getting larger opportunity shares in order to incentivize them for further cost reductions and supply-chain improvements. This consolidation is happening and has already happened in many organizations.

Disposable applications support reducing the risk of patient infections and post-procedural complications. Many of the new techniques drastically reduce the time required for a procedure and the length of in-patient hospital stays. This is critical to reduce risk and cost for hospitals and insurance providers, which do not want to pay for costly or what might be deemed an unnecessary or non-reimbursable procedure. With medical costs still on the rise and many hospital procedures not receiving adequate coverage, the burden of lowering costs has been pushed to the device OEMs, where every aspect is scrutinized.

Discussing upfront the design objectives for a cable is critical. Having such “design for cost” conversations sooner rather than later can mean all the difference to a product’s success. Many designers believe that as a cable or wire grows smaller, the cost should naturally drop because less material is used, but this is simply not the case. With very small gauge sizes (46, 48, and 50 AWG), delicate equipment for line processing and conductor/insulating materials play a very significant role in the cost of the product.

For example, conventional non-alloy products simply do not work well at the fine-wire sizes. They are too weak and exhibit inadequate tensile and elongation properties for even the best processing equipment. Also, these machines are very expensive. They typically reside in highly specialized work cells with significant overhead cost and associated training requirements. This is especially true of fine-wire braiders. These braiders cannot run at the same rate (speed) as conventional larger-wire braiders, so this not only extends cable lead time, but it also increases product cost. Every single level of manufacturing requires time and precision, so the smaller the wire, the longer the process usually takes.

Designers want all the benefits of a smaller size, but they need to understand that size is the attribute they are paying for the most. The smaller the cables get, the more challenging they become. This is why volume and product/program longevity are important aspects for driving down design cost. Many things can be planned for upfront if the long-term goals are known. Contractual agreements can be made. New equipment can be ordered to expand capacity. New equipment can be designed to improve product throughput. Materials can be bulk procured at a lower cost. Products can be manufactured in lower-cost regions of the world. These are all aspects that come into play with volume. Sadly, lower-volume cables will simply not be as cost effective as their higher-volume counterparts. This is partly why R&D is so expensive. Low-volume development is timely and not always cost efficient. This is also why it is so crucial to know which technology to apply and when, in order to achieve the design intent, while lowering cost and reducing manufacturing time.

To better understand this, it is important to first review the most common cable types and what they’re good for:

  1. Hookup wire. An insulated, unshielded wire typically used for single-ended signaling. Enamel-coated magnet wire is one such example.
  2. Micro-coax. A single insulated central wire with a co-axial shield and outer jacket, typically used for single-ended signaling. Coaxes are good for video and applications requiring moderate- to high-frequency/high-data rates.
  3. Unshielded twisted pairs (UTP) and shielded twisted pairs (STP). UTP is essentially two insulated wires twisted helically around each other for the length of the cable. STP applies a shield over a twisted pair, and the shield can be either isolated or non-isolated from its neighboring pairs. Twisted pairs are generally used for differential signal applications.
  4. Twinaxials. These are very similar in construction to a shielded twisted pair except there is no twisting of the wires. Twinax designs have a shield and, in some ways, perform like two coaxials in parallel. With the wire lengths matched, this improves the impedance consistency for the differential signal.
  5. Quads. These are essentially two twisted pairs in a bundle. Opposing wires act similar to twisted pairs. Quads can be shielded or unshielded, depending upon the need.

Of these five styles, designers must mix and match between the needs of the system and the assembly factors. The construction and material options are not endless, but there is still a considerable amount of detail that should be taken into account. Below is how the cable types generally break down in terms of cost (from least expensive to most expensive):

In theory, a twisted pair will generally be cheaper than a micro-coax or micro-twinax design, but designs requiring a smaller size might want to lean toward coax for performance, durability, signal isolation, and ease of termination. A really good coax is pretty hard to beat.

Other factors such as wire gauge size (as it relates to material consumption, i.e., larger the wire, the more raw-material cost), jacket material selection, shield design (tape/foil, spiral, braid, or a combination of all), product durability, flex life, electrical performance, noise susceptibility, product termination, and overall diameter size are all critical concerns. Finding the right balance between performance and cost means tradeoffs are needed. Having a small cable that reliably performs in critical environments, such as an operating room, should always trump cost. Too many people set their sights on lower costs, and they lose focus on the real objective of improving patient care. Plus, if a product outperforms the competition, it’s very likely that as a result it will also outsell the competition.

Designers should work with their cable suppliers. They should be flexible in the design and be upfront about the most-critical concerns so that tradeoffs can be properly understood. They should share as many details as possible, such as system frequency; power and grounding needs; device length; termination criteria; product intended use and potential misuse situations; crosstalk concerns; size constraints; and bundling requirements, if more than one cable and type is required. Believe it or not, even bundling can have an impact on cost and manufacturing complexity.

Michael Levesque, a sales engineer in Hitachi’s Performance Cable Systems & Materials Division, recently explained how a simple color change can positively impact cable cost in a recent MD&DI article: “A Simple Cable Modification to Reduce Cost.” This article highlights the subtle impact that manufacturing steps have on an overall cable cost. He demonstrates how adding additional colors to the cable results in the non-obvious effect of lowering the overall assembly cost.

Understanding the manufacturing process is always valuable. Customers should visit their suppliers whenever possible. Sometimes seeing the equipment in operation makes all the difference in understanding how a product is made and the impact that a design change might have.

Hitachi Metals and Hitachi Cable America specialize in the field of very small cables and cable bundles. Hitachi Cable America and Hitachi Metals can provide design suggestions to help those designers feel more comfortable about choosing the right cable design to achieve their end goals. Feel free contact Hitachi via its Website or visit the company at MD&M West in Anaheim February 6th - 8th at Booth #4069.

TAGS: Automation
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