Reprinted from "Electronic Packaging & Production", October 1993
CLEANING AND HANDLING OF FLEX CIRCUITS
Proper handling and cleaning of flex circuits will result in
higher yield and quality, and in customer satisfaction
By Jack Lexin
As the demands for miniaturization, electromechanical performance, environmental operation and reliability increase, the processing of flexible circuits becomes more difficult and process control more critical. Fine lines and spacing, and minimal thickness of dielectric and conductors demand a narrow margin of error. Furthermore, as the thickness of the flexible circuit decreases, the instability of the plastic dielectric increases.
Materials of construction
The majority of flex circuits are constructed from a combination of dielectric materials with varying physical, electrical, mechanical, chemical and thermal properties.
These differences are illustrated through a comparison of flexible circuits (polyimide with a modified acrylic adhesive system) and rigid boards (glass/epoxy FR-4 or G-10).
Considering the electrical properties, flex circuits have a 20 to 30 percent lower dielectric constant than rigid boards, which allows for thinner construction and better electrical isolation. The dielectric strength is as much as two times greater than that of epoxy boards.
Flex circuits can bend both statically (one time bend for fit or assembly) and dynamically (disk drive, etc.). Multilayer rigid/flex greater than 20 layers can be designed and fabricated to bend around obstructions or to conform to an area. Thinner dynamic applications of shielded flex (emulating coaxial cable) can oscillate and flex in the tens of millions of cycles.
Polyimide has excellent chemical resistivity; however, the adhesive system does not. The adhesiveless systems have poor bond strength after processing, making them functionally less desirable. The flex is more sensitive to alkaline environments, unlike the epoxies that can easily be cleaned, stripped and etched back in solutions that would either swell or completely destroy the flex adhesive and etch the polyimide. The flex is also prone to chemical drag out, and degradation after long thermal exposure.
Polyimide has a high affinity for moisture and can absorb up to 600 percent more than glass/epoxy. This characteristic creates the problem of chemical drag out, and can contaminate non-compatible chemical baths or break down the adhesive systems.
Mechanically, flex circuits can be drilled, sheared, cut, routed, chemical milled or laser cut.
Thermally, polyimide is more stable than epoxy, with a coefficient of thermal expansion of 45:56 x 10-6 in./in./ oC vs 11:50 x 10-6 in./in./oC of glass/epoxy. Its thermal conductivity is about 50 percent that of a rigid board.
Dimensionally, flex is far less stable than glass-reinforced rigid boards. Flex circuits are typically non-reinforced. Also, rigid PCBs are very thick in comparison to flex circuits. For example, the minimum thickness for a thin core epoxy board is 0.003 in.; the flex counterpart is 0.001 in. standard down to 0.0005 in. minimum.
Dimensional strength becomes an important factor in the fabrication process. Under heat and pressure, copper expands and contracts in the X-Y directions. The non-reinforced flex circuits are elastic enough to be distorted from the pressure of the expanding or contracting copper. This distortion creates an elastic tension that is stored in the substrate (Fig. 1). Depending on the ratio of thickness between the copper and the substrate, this elastic tension can result in shrinkage when the copper is etched from the substrate. This movement varies from layer to layer depending on various factors, and adjustments must be made in the manufacturing cycle, especially with high-density multilayer flex.
During the manufacturing cycle, cleaning and handling of flex circuits is more critical than when processing rigid boards (Table 1). Defects that are caused by mishandling or improper cleaning can affect manufacturing yields, and may not appear until after assembly or fail in application.
The sensitivity of the materials used for flex circuits plays the most important role in the processing of the panels. The substrate is affected by mechanical force (scrubbing, lamination, fabrication) and thermal changes (baking, lamination, plating). The copper foil must be kept free from dings, dents and elongation to ensure maximum flexibility. Mechanical damage to the copper foil, or work hardening, will decrease flexibility life of the circuit. By improper handling or processing prior to lamination, the details can be damaged or dimensionally altered, compromising the quality of the circuit.
The fabrication steps for multilayer flex are similar to rigid. However, the handling and cleaning of the two products are the biggest differences. Improper handling and/or cleaning of flex panels creates major quality problems. Flex circuits take longer to process than the PCBs because of the handling precautions that must be exercised. (Referring to batch type of processing, not automated roll-to-roll processing.) If proper precautions are consistently followed, process time and cost will not be a factor, and quality will predominate.
General work habits should be exercised. Gloves should always be worn when handling panels; panels should be wrapped in sturdy packages with slip sheet to support the panels when storing or moving; and all work surfaces must be clean and unburred. These special precautions will increase product quality and yields, and decrease rework.
A typical flex detail or single-sided circuit is cleaned a minimum of three times during its manufacturing cycle. A multilayer would have all the details cleaned three to six times after detail lamination, depending on its complexity. Comparatively, a rigid multilayer may experience the same number of cleaning cycles, but the procedures are different. Much more caution is needed when cleaning flexible material.
Flexible circuit material is subject to dimensional instability with the slightest amount of stress. Stress introduced during the cleaning process will elongate panels in X and/or Y directions, depending on the stress bias. If the copper is elongated greater than its elastic limit, the dimensions of the panel will change again during thermal cycling in processing.
Flex circuit panels can be cleaned chemically through a process that is environmentally safe. The process includes an alkaline bath, a thorough rinse, a micro-etch and a final rinse. The most frequent damage to the thin clad materials occurs with racking the panels, breaking the surface tension in the cleaning tanks, agitating the panels in the tank, removing racks from the tanks and unracking.
Different adhesive systems are used to bond the dielectric to the copper foils, many of which are sensitive to different chemicals (especially low pH at temperature). Dwell times and variations of the chemical bath must be carefully considered. It is easy to carry chemical contamination from cleaning baths with the details. This decreases the bond strength after final lamination, causing delamination during thermal exposure to soldering.
Mechanical and hand cleaning
A variety of power scrubbers are said to be safe for cleaning flex panels. One concern with power cleaning is mechanical stress introduced on the panel, and the effect it will have on the dimensional stability of the panel. This may result in difficult registration of coverlays or multilayers.
Hand scrubbing sustains high yields with low-cost setup. The panel is placed on a clean, flat surface; any debris or imperfection on the surface will transfer into the panel while being scrubbed. A pumice slurry mixed with DI water, pH adjusted down, is used as a scrubbing agent. After scrubbing, it is rinsed with DI water and dried in a conveyorized dryer.
The disadvantage of hand cleaning is operator variation and an increase in processing time. (A caution to observe: the thinner the panel, the more likely it is to be damaged. If an operator scrubs a panel too hard, the panel will elongate in random directions.) The increase in processing time, however, is usually offset by the increase in yields and decrease in rework.
Specifying the copper's grain structure and hence, direction in circuit layout is critical for dynamic flex applications. Grain direction must run parallel to the flex direction.
If a panel is wrinkled, scratched, dinged or dented in the flex area, the grain structure of the copper will be compromised and the circuit rejected. These defects are normally caused by mishandling. If the circuits are cosmetically reworked and passed on to the customer, field failures will randomly occur during the product's life. Such failures are seen frequently and are normally preventable without any effect on the product's initial cost.
Chemical contamination (sometimes referred to as drag out) can also affect the life of the circuit while in the field. Adhesive degradation may not show up as delamination until it is in the final assembly stages. Contamination under the coverlay or between layers can continue to degrade. After thermocycling and aging, a break in lamination will occur. Fractured plating may pass electrical test at the circuit level, but when in application, may result in intermittent opens.
The best way to ensure manufacturing of a reliable product is to have documented procedures for all processes, especially the handling and cleaning of the panels. QA documentation pertaining to in-process inspection should consider the circuit's function as well as workmanship and overall dimensions. As stated before, the expensive extra time spent on the handling and cleaning will be outweighed by the payoffs of higher yields, quality products and satisfied customers.