HOMECURRENT ISSUEARCHIVEABOUT USSUBSCRIBEMEDIA KIT中文
Login


Special Feature: PCB and Assembly
Search :      Friday, November 21, 2008


U.S. Tech Part Search



Powered by


Terms Of Use

 

 

 

 

 

Flex Circuits: Meeting Today's Assembly Challenges
Flex circuit with multiple attachments.
By John Talbot, Director of Sales & Engineering, Flexible Circuit Technologies, Plymouth, MN
Challenges in the manufacture of electronic assemblies are common in today's "world is flat" environment. They include cultural differences in the workplace, communication, both time and language and quality. It seems that once the topic of manufacturing overseas is brought up, the discussion quickly propagates to these common points. We often neglect to discuss, or never get to the inherent challenges in electronic manufacturing in general.

We are constantly faced with new and cutting-edge applications that we must overcome in order to push our products to perform better, faster and fit them into smaller packages regardless of where they are built.

The luxury of having a manufacturing plant in China allows lower costs, which is appealing to the bottom line yet adds greater responsibilities to process, quality and delivery for all involved.

A recent case in point for Flexible Circuit Technologies was a retro fit power pack. There were many challenges to overcome quickly in order to meet the needs of the production schedule. A new, previously untested design, stringent test requirements including 10 days of high temperature with 100 percent Relative Humidity (RH), an existing package that limited the options available, usage of components in a different application than designed for and the ever present "lead time". Those challenges were compounded by new sources for components, new equipment necessary to assure that those sources provided high quality components consistently and reliably and issues such as logistics and accurate inventory tracking.

Simple Flex Circuit
The initial product calls for a relatively simple flex circuit assembly. Simple inasmuch as there were very few components and the construction of the bare circuit was standard. The assembly of this product, however, unearthed many challenges. The mixture of through-hole with Surface Mount, hand soldered components with components automatically placed as well as welded components. These components also needed protection from each other, the elements and a dramatic fall of four feet onto a hard surface. The basic United Laboratories (UL) so called "drop test".

One of the first issues revealed during the RH test was the need to seal a metal dome switch so that moisture and the elements would be kept out while providing enough room for the compressed air to move or disperse allowing the metal dome to remain operational. We initially added a Kapton tape seal over the dome, which provided adequate insulation against the elements and corrosion, but didn't allow air to move underneath it which, after only a few activations, created so much pressure under the switch that it could not be activated.

The solution required the design of a special head for a typical soldering iron, one that would seal only the edges of the Kapton tape and create a pocket for the compressed air. This continued to provide protection from moisture and allowed enough room for the air to move into and out of the area under the switch allowing it to work as designed consistently over time.

Another unusual problem arose with the heat activated fuses — Thermal CutOffs or TCOs. The TCOs, which resemble a standard leaded resistor, required a nickel tab be welded to one of the leads and a wire welded to the opposite lead. The exposed leads were necessarily protected by heat shrink material to prevent short circuits in the final assembly.

Incoming X-Ray Inspection
Several additional challenges surfaced. The fuses are designed with a wax ball as the activation trigger. Once the temperature rises high enough to melt the wax — during an over-current incident — the contacts open to safely shut down and protect the assembly. This necessitated a couple of processes to be included and designed. It was necessary to ensure that all fuses received included the wax ball. This required 100 percent incoming X-ray inspection to ensure that all fuses used in assemblies were operational. We used an RTX-113HV system from Glenbrook Technologies, Inc. (Randolph, NJ) to accomplish this. Also, the requirement of protecting the leads with heat shrink material dictated that the fuses were exposed to heat high enough to "shrink" the insulation material, which could in turn damage the fuses.

A fixture needed to be designed to protect the body of the fuse while the leads were insulated with the heat-shrink tubing. Once this challenge was overcome, we found that in the final assembly, the sharp corners of the nickel tabs could eventually protrude through the insulation material and cause short circuits. This problem was finally solved by designing the tabs to include rounded corners on only the side of the tab that was welded to the fuse leads.

Further refining the process, it was decided to inspect 100 percent of the fuses after the final assembly of the circuits, again via X-ray. Inspection after assembly confirmed that there was no damage introduced during the assembly processes.

Final Assembly Needs
Because of the unusual and demanding needs of the power pack's final assembly, it was decided that all solder joints would be protected by a flexible epoxy or conformal coating product for strength and ultimately covered with a protective oil to further protect the assemblies from moisture seepage. The conformal coating product that was initially chosen proved to create new problems: it migrated into connector contact areas, which degraded the performance of those connectors. A 3M product, 3M2216 B/A was decided on for the coating. It is a two part epoxy that cures either by air or when necessary with heat to speed up the cure time. The protective oil chosen was a polyphenyl ether (PPE) compound, OS-138, with UV tracer. The oil compound was mixed at a ratio of 95 percent Isopropyl alcohol with 5 percent PPE oil to allow it to flow easily into and under components as well as over the external areas. The alcohol evaporates very quickly leaving only the protective oil seal.

The epoxy would be dispensed with pneumatic equipment to resolve an issue with the depth of the epoxy required on either side of the SMT connector. One side needed to be lower than the other to eliminate interference with the mating PCB assembly, as well as to provide consistent looking assemblies for volumes that would reach hundreds of thousands annually.

A lesser issue faced, but tedious nevertheless, involved the logistics of the components. Some components came from a current U.S. supplier, while others of necessity came from overseas sources. A system to track lead times, inventory and reorder dates had to be perfected. In addition, there were several supplier audits to perform in order to approve the new local suppliers and their components.

While these concerns are inevitable with the introduction of a new product, they will typically generate unexpected delays and may spring some surprises. With continual tracking though, the reward is a larger, and more trustworthy supply chain.

As is always the case with new or redesigned products, we find ourselves faced with challenges that must be overcome in order to produce better, faster, smaller and more technical products for our customers. The challenges include previously untested assembly processes, previously unknown component suppliers and redundant test procedures to ensure that both perform as expected. Whether built in a factory that is around the corner from our offices or across an ocean, those challenges remain the same.

When we add to the equation the complexities of mismatched business hours, skills and experiences along with cultural differences, those inherent challenges become more difficult and test our skills, experience and dedication. We have had the advantage of creating and nurturing a culture that allows experience and flexibility grown in the U.S. to be easily transferred to our facilities overseas. This gives our customers a high comfort level along with worldwide competitiveness.

For more information, contact: Flexible Circuit Technologies, 9850 51st Avenue N, Plymouth, MN 55442 763-545-3333 fax: 763-545-4444 E-mail: jtalbot@flexctech.com Web: http://www.flexctech.com  

 
 
search login