What Are Flex Circuits, and How Do We Use Them?

Flex circuits are a minority sector of the North American PCB fab market — rigid PCBs represent an estimated 90% of the current PCB industry in North America, according to IPC's World PCB Production and Laminate Market Report — but this 10% of the market can be found in every end-use sector from medical to consumer. Mark Finstad and Mark Verbrugge, from Minco, recently spoke to the IPC and SMTA chapters here in New England, providing design and manufacturing best practices for those that are, or would be, using flex circuits in an electronics assembly. Some of their advice may surprise you.

Design best practices are of utmost importance with flex, not least of all because most OEMs and ODMs have only a passing familiarity with the capabilities and limitations of flex circuits. The bend ratio is the fundamental principle of flex design. It describes the ratio of circuit thickness to radius of the circuit bend. Bad bend ratios lead to field failures from rippling and cracked copper. Some other helpful design tips? Design elements that are good for a circuit mechanically are bad for it electrically, so find a balance. Copper thickness and shielding materials can make the difference between a bouncy flex circuit, like that in a flip cell phone, and a flex-to-fit circuit, that will not bend once installed. Finstad and Verbrugge recommend placing small conductors on the inside of the bend; keeping plated thru-holes away from bend areas, and debonding substrates for more flexibility.

Once the flex circuit is fabricated, it must be shipped to the electronics assembler, who will handle and install/form the circuit. Every effort in design and materials specs can be wasted by improper shipping and installation procedures. Forming and handling are the two main causes of flex disasters, according to the Minco engineers. Switching from bags to formed hard plastic packs for shipping will protect circuits from scratches, ESD, and other damage, Finstad and Verbrugge explained, showing us the damage mounted components will do to neighboring flex pads in transit. This method applies to storage and cleaning as well. Have you ever taken a $400 thick, multilayer flex circuit, and bent it over a table edge to form it for assembly? Even if the circuit is $4, this is an undesirable technique. Forming tools, which range from simple to complex, apply uniform and repeatable force and create the specified bend radius every time. ROI is all but guaranteed.

Finstad and Verbrugge have many more tips for successful flex use, and if you’d like to hear them, email mark.finstad@minco.com and mark.verbrugge@minco.com. Since flex circuits are often misunderstood or foreign to electronics assemblers, I’d like to go over a few of the definitions Finstad and Verbrugge presented.

Does a flex circuit really flex? All flex circuits can be exercised, or bent, but the spring in a circuit varies from case to case.

IPC 6013 breaks flex circuits out into four types: single layer (Type 1), double layer (Type 2), multi layer (Type 3), and rigid flex (Type 4).
Type 1 single-layer circuit.






Type 3 multi-layer circuit.

Types 1 and 2 circuits are generally very dynamic, meaning they can be exercised many times without performance-affecting damage. However, layer count isn’t the only determinant for how dynamic a circuit is. Layer thickness differences and materials specs can make two double-layer circuits as different as a Type 1 and Type 3.

Rigid-flex
Rigid-flex PCBs are a combination circuit, and they are expensive. Some applications, like double-sided surface mount, require the rigid-flex construct; however, the Minco engineers see a lot of unnecessary specs for rigid-flex. If the application calls for single-sided surface mount with flex, mounting a stiffener will do the job. It also will give you a 25–30% cost reduction from rigid-flex.

Stiffener used instead of rigid-flex.
Materials for the laminates of flex circuits include polyimide/acrylic — which dominates the North American flex market — LCP, and FEP. The copper in a flex circuit can be rolled annealed or electrodeposited, each with pros and cons.

No matter how small flex circuits’ slice of the overall circuit board market is, they can make or break a design. Spending some time familiarizing yourself with the terminology and methodology of flex can lead to better assemblies and smarter budgets, as well as deeper PCB fab relationships. One trade association meeting, or one class at a tradeshow conference, can pay exponential dividends.

Meredith Courtemanche, managing editor


*All images courtesy of Minco.

APEX and Universities: The Collaboration is Academic

SMT has long considered our role to be a teaching magazine, educating electronics assemblers in new technologies or most efficient uses of existing ones. Affirming this commitment to education, we added S. Manian Ramkumar, Ph.D., faculty professor and director at the Center for Electronics Manufacturing and Assembly (CEMA), Rochester Institute of Technology (RIT), to the Editorial Advisory Board in 2008. Professor Ramkumar has written multiple articles on the intersection of industry and academia, including A Workforce to Support Electronics Industry Growth and Industry Academia Partnerships: Sustaining Growth and Competitiveness. Industry itself is much a product of the universities and technical colleges that fuel it through the production of new methodologies, technologies, and engineers/technicians. I’m looking forward to the Academic Poster Competition, March 31–April 2 at IPC APEX in Las Vegas. Fourteen poster papers from domestic and international learning institutions were selected by the industry association. APEX attendees will vote on the top three posters.

This is IPC’s first year doing the poster competition, according to Tony Hilvers, IPC VP of industry programs, and they received 29 entries. With economies around the world searching for new growth and new ideas, greater industry/academia cooperation seems logical, immediately necessary, and vital. Academic achievement during a period of threatened economic growth and severe unemployment is a matter of bravery and resilience for students and schools willing to invest in an industry’s future. "We looked for innovative original research in accepting the winning posters," said IPC representative Kim Sterling. She adds, "We did it to create more access for the industry to what is currently going on in academic research — to build stronger relationships with the universities. In the past, there was a lot of R&D being done inside OEMs. With outsourcing, that has declined. This is a way of highlighting the R&D that is being done at the university level." Winners of the top three posters will be announced at the IPC APEX EXPO closing keynote on Thursday, April 2.

IPC’s top 14 poster papers are:
“Whisker and Hillock Growth Observed on Pure Sn, Sn-Cu, and Sn-Cu-Pb Electroplated Films” by Aaron Pedigo, Pylin Sarobol, John Blendell, and Carol Handwerker, Purdue University; “Silver-Bismuth Alloys as a High Temperature Lead-Free Solder” by Anthony Muza, Purdue University; and a third Perdue submission, “Utilizing the Thermodynamic Nanoparticle Size Effects for Low Temperature Pb-Free Solder Applications” by John P. Koppes, Purdue University.

“Room Temperature Sintering of Ag Nanoparticle Paste by Chemical Dip Treatment” by Daisuke Wakuda, Osaka University is an international submission, as is “Control of Tin Whisker Growth” by schoolmate Keun-Soo Kim, Osaka University.

The eastern U.S. research belt is well represented, with “Selective Electroless Nickel and Gold Plating of Individual Integrated Circuits for Gold Stud Bump Flip-Chip Attachment” by David Lee, Johns Hopkins University; “Reliability of Standard and Flexible Termination Multilayer Ceramic Capacitors under Temperature-Humidity-Bias Conditions” by Garry Brock and Michael Azarian, University of Maryland; and “Printed Wiring Board Delamination — Some Unique Findings” by John Folkerts, Johns Hopkins University. Also from the East Coast, “Bio-Composites from Chicken Feathers and Plant Oils for Printed Circuit Boards” by Mingjiang Zhan and Richard P. Wool, University of Delaware.

Not to be outdone, West Coast submissions include “The Calculation of Liquidus Temperature for Various BGA/CSP Assemblies” by Jianbiao (John) Pan, California Polytechnic State University and “Fluxless Bonding of Si Chips to Cu Substrates Using Ag-In System” by Pin J. Wang, University of California — Irvine.

One Polish submission will be in the poster competition: “Reliability Tests of Ecological Soldered Joints” by Professor Zdzislaw Drozd, Warsaw University of Technology.

The penultimate poster in our list is also from a European school. “Ink-jet Printed Electronics” by Rostyslav Lesyuk, Werner Jillek, Ewald Schmitt, Yaroslav Bobitski, and Zhen-Guo Yang comes from the National University “Lvivska polytechnika,” Lviv, Ukraine and University of Applied Science in Nuremberg, Germany.

Finally, Taiwan’s academic program is represented by “A Suitability Study for Mechanical Shear and Bend Tests as Alternatives for Thermal Cycling Reliability Test of Electronic Components” by Yeong-Shu Chen, Chen-Tse Fan, and Yu-chun Huang, Yuan-Ze University.

Chicken feathers? Printed electronics? Nano materials? These ventures do not look like the timid offerings of an industry clamming up in hard economic times. The ultimate test will come once these projects and students graduate into the electronics assembly “real world.” I hope we continue to have a flow of information between research and industry reality, as Sterling said, one informing the other, to continue advancement in our field.

Opportunities for learning abound at APEX. Check out these expo and conference events:
IPC APEX Expo Preview: Innovative Technology Center, Task Groups, and More
APEX Product Preview
APEX Products
IPC APEX Expo Preview: Technical Courses and Tutorials
Get Ready for APEX: Preview of Technical Sessions and Noteworthy Events
APEX Update: Designers Summit and Management Meetings
and see the APEX Product Preview in our March/April issue.

Electronics Economics: a Look at the Whole Picture

If you only followed the semiconductor equipment bookings book-to-bill ratio, news in the electronics sector would be depressing. However, IPC — Association Connecting Electronics Industries announced less-than-dismal results from its North American Printed Circuit Board Statistical Program.

Last week, the Semiconductor Electronics Association of Japan (SEAJ) reported a book-to-bill ratio for January of 0.55, a new low in a steadily decreasing semiconductor market as compared to 0.70 in December. The ratio is down 80.1% year over year.

Then the North American semiconductor equipment book-to-bill announced a new low in January, dropping to a ratio of 0.48, down from December’s ratio of 0.81, and down year-over-year 54%. “Bookings are at the lowest levels since 1991,” said Stanley T. Myers, president and CEO of SEMI.

In the IPC book-to-bill results, rigid PCB shipments are down 14% and bookings are down 24.5% in December 2008 from December 2007. The book-to-bill ratio fell to 0.89. Flexible circuit shipments in December 2008 were down 17.1%, but bookings are up 23.8% compared to December 2007. The North American flexible circuit book-to-bill ratio in December jumped to 1.08. The combined ratio for rigid and flex in December dipped, but only to 0.90.

“Rigid PCB orders have trailed shipments for nine straight months now, and that is reflected in a declining book-to-bill rate,” said IPC President Denny McGuirk. “Strong orders have been keeping the flexible circuit segment growing, but the whole industry’s December sales were weaker than in prior years. However, the industry ended the year about 1% ahead of 2007,” he concluded.

Business news is primarily negative, but not in every company. Hewlett-Packard, world’s largest PC maker, reported that sales missed experts' estimates. Sales in the year ending in October will fall 2–5% from $118.4 B in 2008.

Mark Hurd, CEO at HP, is eliminating jobs and closing offices, working under the assumption that the economy won’t improve the rest of this year. Hurd’s pay will be reduced 20% as well, and other salaries will be reduced 2.5 to 15%.

On the flip side, at medium-sized EMS providers, business varies. IEC Electronics, a full-service Newark, N.Y.-based EMS provider offering a range of design, prototype, and volume PCB assembly services, announced its Q1 2009 financial results with positive results. Sales increased 25% in 2008. Operating income increased by more than 140%. Net income before tax increased more than 220%. Customers were added in target markets. And the company intends to invest $3.5 million over the next three years in manufacturing equipment to improve productivity, cut rework, and support quality.

In a webcast of the annual shareholders meeting, IEC Electronics revealed their positive numbers, but acknowledged that the world economy is in a deep recession. Part of IEC’s strength is in that the company attracts a diverse group of industries, and the military and medical sectors are holding strong.

There are hopeful signs in all areas that things will improve. PCs may be down; however, sales of mini/ultraportable devices and Midis, iPhones, and other consumer electronics show promise. The only question regarding the turnaround is: When? Patience is a virtue.

Gail Flower, editor-at-large

VISION Awards Recognize the Electronics Assembly Industry of Today and Tomorrow

Technology has a tempestuous relationship with the past. Many people in the electronics assembly industry have been with the technology from the beginning and can wax poetic about the good old days 10, 20, 30 years ago. It’s not that this past, the glory days, is irrelevant, it’s that their memory cannot overshadow the present. To paraphrase a recent visitor to SMT’s offices, outdated business models can be as “old” as 90 days in the past. Electronics were always advanced by manufacturing technology achievements, and under the shadow of global economic uncertainty, today’s innovations — not yesterday’s business models and volumes — deserve recognition. The 17^th annual SMT VISION Awards aim to do just that, looking forward to what the industry can be tomorrow.

Much like Girl Scout Cookies, the awards are given out only once a year, but much anticipated and valued throughout. The SMT VISION awards will be presented in more than 15 categories, ranging from underfills to contract services to test equipment, recognizing the best products to lead the SMT and microelectronics assembly industry into new-generation capabilities, business models, and end products. See the companies honored at the VISION Awards ceremony, March 30, 2009 at APEX Expo, Las Vegas. The party will be held in the Banyan Room D, Level 3 of the South Convention Center at the Mandalay Bay, from 6:30 to 8:00 pm, and all are welcome.

Products are qualified to receive VISION Awards by persnickety judges, thoroughly evaluating environmental impact, cost, innovation, and other important factors. Look for VISION awards on display with materials, equipment, software, and other offerings on the show floor at APEX. The award signifies a product’s relevance to the electronics assembly and manufacturing industry today and in the future.

IPC reports that APEX attendee registration is higher than expected, in the midst of the economic troubles. It seems that — at a time when “value” is being redefined for houses, banks, assets of all kinds — the educational and influential value of programs at APEX remains high, unshaken. If you’re attending the show, be sure to soak up as much useful information as possible for the here-and-now of our industry, and apply it to your business model, job, or research. To see the highlights of the technical conference, including SMT’s free forum on EMS trends, read IPC APEX Expo Preview: Technical Courses and Tutorials and Get Ready for APEX: Preview of Technical Sessions and Noteworthy Events.

To see the winning products from the 16^th annual SMT VISION Awards, see the VISION Award Winners.

Meredith Courtemanche, managing editor