-
- News
- Books
Featured Books
- design007 Magazine
Latest Issues
Current IssueLevel Up Your Design Skills
This month, our contributors discuss the PCB design classes available at IPC APEX EXPO 2024. As they explain, these courses cover everything from the basics of design through avoiding over-constraining high-speed boards, and so much more!
Opportunities and Challenges
In this issue, our expert contributors discuss the many opportunities and challenges in the PCB design community, and what can be done to grow the numbers of PCB designers—and design instructors.
Embedded Design Techniques
Our expert contributors provide the knowledge this month that designers need to be aware of to make intelligent, educated decisions about embedded design. Many design and manufacturing hurdles can trip up designers who are new to this technology.
- Articles
- Columns
Search Console
- Links
- Events
||| MENU - design007 Magazine
Pitfalls to Avoid When Designing with Flex
April 5, 2007 |Estimated reading time: 7 minutes
Medical electronic designers continually struggle with shrinking real estate-- how to make devices smaller, thinner and lighter-- while simultaneously adding advanced capabilities and new features. The marketplace requires an accelerated pace of innovation while being uncompromising on reliability. Devices may operate for years on a battery, driving a need to minimize power consumption and maximize the capacity of the power source. And the designer is tasked with creating devices producible at lower cost.
When you consider the design objectives of modern medical electronics, there is tremendous synergy with the capabilities and advantages of flex circuits. Flex technology offers solutions for many critical needs of the medical device market, gaining adoption in a broad spectrum of applications. Products include cardiac rhythm management, neurostimulators, infusion pumps, hearing aids and implants, battery systems, surgical tools, imaging and diagnostic systems.
Flex circuits satisfy an incredible range of complexity and functionality in their applications. Simple single and double-sided flex circuits make reliable interconnection between electro-mechanical devices. Thin dielectrics provide high electrical isolation, low mass and a very flexible package. Medical batteries are an example of simple flex circuits used as space saving interconnects.
The savvy circuit designer discovers a broader potential for flex circuits constructed as a rigid flex. The rigid flex provides seamless, connectorless integration of mother and daughter circuits with various electro-mechanical devices. Elimination of connector transitions reduces cost, saves space and simplifies assembly. Boards and devices join seamlessly to increase product reliability. Many devices incorporate two rigid circuit sections populated front and back, then folded back upon themselves to increase the vertical component density of their electronic packaging. Hearing, neuro and cardiac devices take advantage of rigid flex. Small connecting arms often emerge from circuit edges to attach to power sources, communication coils and external headers.
The compliant nature of flex circuit substrates works to the designer's advantage when populating high density interconnect (HDI) circuits with small surface mount components. Compared to FR4, flex circuits reduce solder joint stress, resulting in thermal cycling improvements. Flex circuits also routinely withstand global shock and vibration better than FR4.
With all of these advantages, flex circuits are a natural first choice for many medical device designers. But, what are the pitfalls to avoid when designing with flex?
Vendor involvement
Medical device manufacturers are often fiercely protective of their intellectual property. Even with non-disclosure agreements, they may work independently in the earliest design phases. The mechanical layout of the package is often complete before the circuit manufacturer gets involved. Redesigning mechanical structures in the device package is generally more difficult than modifying a flex circuit; so many suggestions to improve circuit manufacturability are never implemented. Minco has been asked to reduce circuit thickness another mil or two just to make laser welding of a package easier.
A flex manufacturer can educate designers on good engineering practices, manufacturer core competencies, limits of material conditions and what is possible at lower yield. Regular exchange of technology roadmaps to communicate feature sizes, materials and electrical performance needs and capabilities is recommended along with involving flex manufacturers in the conceptual phases to brainstorm ideas.
Bending and forming
Most circuits in medical applications are categorized as "bend to install," bent at assembly and only straightened to disassemble the package for rework. The IPC provides guidelines for minimum bending radius based on circuit thickness, but circuits optimized for bending can often be formed at tighter radius. Numerous tips can improve circuit's bending reliability.
Rolled, annealed (RA) copper is more ductile than electrodeposited (ED) copper for a forming operation, particularly when the grain of the copper is oriented parallel to the tightest bend.
Minimize circuit thickness. A double-sided circuit can be reduced to a single layer in the bend area by etching the copper from one side of an adhesiveless substrate and applying the cover to only the opposite side in the bend area. A 4-layer circuit can similarly have only 2-layers in the bend area using cutback covers.
- Eliminate plating of conductors at or near the bend. Plating finish is not as ductile as RA copper. Reserving the external layers for pads only will eliminate the plating on conductors. Alternatively, plating may be applied selectively to only pad areas by masking the rest of the circuit during the plating operation.
Tight bends over 90* place the greatest stress on the formed area. The outside radius stretches severely while the inside radius compresses and wrinkles. Any attempt to straighten circuits formed in this manner induces further stress by compressing already stretched copper and pulling at wrinkled copper and cover materials. Either action may tear the copper or the cover, propagating the tear to result in an intermittent open. If a circuit is over-formed past 90*, restrain it to that shape during all subsequent processes.
Bring all bend areas to the circuit manufacturer's attention, usually by identifying bend lines on the drawing. Minco experienced a device manufacturer reporting failures of flex circuit prototypes. Failure analysis detected cracking in conductors plated with electrolytic nickel and gold where they were sharply deformed. The drawing did not show a bend area, and it was not communicated to Minco.
Etching aspect ratio
With packages shrinking, designers look at placing more traces into the same space. Many applications need low resistance traces. In order to have the same cross-sectional area of copper, designers are specifying thicker copper with narrower traces. Manufacturers of flex circuits using RA copper for its superior ductility, also use subtractive (etching) processes to form traces. Etching processes have some degree of lateral etching as copper etches vertically. The impact of lateral etching is to shape conductor profiles as a trapezoid. The effect grows more pronounced as the etched copper is thicker or the trace widths are smaller. The ratio of the width of the trace to its thickness is referred to as the etching aspect ratio. As the etching aspect ratio gets smaller, control of the cross-section is diminished. Small ratios also mean less copper is bonded to the circuit. Copper resists forming and small etching aspect ratio circuits tend to have the conductor separate from the substrate when bent. Minco sees customers specifying circuits with uncovered fingers using 0.003" conductor width/ 0.003" space on 2 oz copper, which will break off the substrate during forming. As a general rule of thumb, the etching aspect ratio is preferred at 5:1. This means a 2 oz copper conductor (0.0028") should be designed for a minimum etch width of 0.014" for best reliability. Smaller ratios are acceptable with thinner copper and Minco has had good performance at 4:1 ratio for 1 oz. copper (0.0014") and 3:1 ratio on oz. copper (0.0007").
Hole aspect ratio
Hole sizes keep shrinking and pushing the limits of circuit manufacturer capabilities. Small diameter holes present two challenges-- how to get the holes into the circuit economically and how to make layer interconnections through small holes. The aspect ratio related to holes is defined as the depth of the hole divided by the diameter of the hole. A thicker circuit or a smaller hole will both make the aspect ratio larger. Most flex circuit manufacturers prefer to use mechanical drills to make holes. The practical limit for mechanically drilled holes is on the order of 0.006" to 0.008" depending on the circuit thickness. Beyond mechanical drills, the most common method of hole creation is a microvia laser system. Holes can be created down to about 0.001" diameter. Lasers do have issues with beam focus through thicker circuits.
Once vias are formed, layer interconnections are needed, usually relying on copper plating equipment to create the conductive path. The challenge of high aspect ratio holes is to ensure that fresh plating chemistry is continually circulating in the hole, for consistent deposition of copper. If fresh solution doesn't circulate, copper will plate out near the hole entry point, leaving the center of the circuit hole with voids or thin copper prone to fracturing during thermal cycling. Either condition results in an electrical failure. Check with flex circuit manufacturers for limits.
Other issues
Dimensional tolerances specified tighter than material conditions of a flexible substrate can provide is another concern. The advent of laser imaging provided scaling capabilities to flex circuit manufacturers, allowing individual optimization for each circuit image across the panel. Still, a flex circuit can't be toleranced like a rigid circuit. Dimensions can be held tightly in clusters, but dimensions across flexible regions should be looser.
Most designers use 3D modeling software based on sheet metal fabrication techniques. Flex circuits behave differently than sheet metal. Minco recommends generating outlines on a mylar sheet to simulate flex characteristics to test fit. Obtain a physical mockup using actual circuit stack-up materials to verify fit before procuring prototypes. Minco can provide this service to save the expense of prototypes, in the event dimensional adjustment is needed.
Design Rule Checking (DRC) picks up a number of design violations. DRC is not an industry standard but is customized software for check-in of data to a set of rules that enable manufacturer and customer requirements to be met on finished parts. Common errors are border issues and undersized pads.