Edited by Martha K. Raymond
Senior Research Associate
Avery Dennison Corp.
Specialty Tape Div.
Most engineers are not surprised there are tough standards for adhesives used in satellites and other spacecraft. But adhesives used in hard-disk drives must perform to even more stringent standards. The industry is so strict on adhesives suppliers that adhesives with zero, instead of low, outgassing requirements will probably become the norm.
Adhesives, particularly pressure-sensitive adhesives (PSAs) and labels, have been widely used in the hard-disk drive industry for many years. Because most of these adhesives are specifically developed for the application, the adhesive is proprietary for the manufacturer. However, proprietary agreements are not made with the disk drive manufacturers, for example Seagate or Western Digital. Therefore, the adhesive manufacturers can sell the adhesive in tape form to anyone. In brief, the adhesive is proprietary, but tapes made with adhesives are not.
Around five companies make pressure-sensitive tapes for the hard-disk drive industry, out of a group of 10 major and five intermediate pressure sensitive tape manufacturers.
Their use will continue to grow because they’re easy to apply and they bring performance advantages. With the advent of stricter standards and increasing demand, knowledge of PSA applications in the hard-disk drive industry is essential. An overview of the PSA manufacturing process alerts engineers to challenges of developing clean adhesives for high-tech industries.
Hard-disk drives made today often contain multiple rigid discs. Their multiple read/write surfaces coupled with the fact that they have a high areal density, around 4 Gbytes/sq in., makes a contaminant-free operating environment a requirement.
In hard-disk drives, PSAs bond together materials ranging from stainless steel to foam gaskets. Among their many applications are product ID labels, including nameplates and bar codes, which contain preprinted or variable information, or both. Data on the labels includes such information as serial number, firmware revision, part number, drive geometry specs, and jumper settings.
Usually, developers chose polyester facestock labels for durability and temperature resistance. Labels are typically patterned with conventional ink-jet, thermal-transfer, dot-matrix, and laser printers. When serving as warning labels, they generally must be UL and CSA recognized. Besides conveying information, label material also covers holes and screw heads on the outside of the drive.
Another requirement for hard-disk drives is that the inside of the drive must be sealed from outside air and possible contaminants.
One way to seal a drive is with tape seals, which are composite constructions that include adhesives as well as various combinations of films and aluminum foils. The tape seal goes around the drive to seal the gap between the cover and the housing. The tape seal keeps out contamination and acts as a tamper-evident seal for proof of warranty.
There are numerous ways to construct a tape seal depending on which of its properties are most critical in the application. A universal problem in tape seals is outgassing, because the seal’s adhesive is directly exposed to the inside of the drive.
Tape seals are one of the most complicated adhesive applications in a hard-disk drive. These applications demand properties that include repositionability, tight seal, RF protection, conformability, and printability. The many variables involved in tape seals are pushing the hard disk drive industry towards using more gaskets.
Gaskets are usually urethane or polyethylene foams laminated with an adhesive. These foam laminates are die cut to the right shape to seal the drive cover. The adhesive must provide an airtight, durable bond. Due to the flexible nature of the foam and the thin die-cut shapes, the adhesive must be repositionable so assemblers can move the gasket to the correct location.
Pressure-sensitive adhesive products ensure consistent and complete sealing, eliminating any gaps. Typical foams are about 1⁄32 in. thick and gasket adhesives often come in the form of tapes holding two adhesive layers that are separated by a film carrier. This film support helps the dimensional stability of the die-cut part and aids in assembly.
The environmental category of seals includes servo seals, clock seals, tape dots, and breather seals. These are generally made with blank polyester or polypropylene film-label stock material. In cases demanding a specialty film or adhesive, the seal is constructed with an adhesive laminated to a film.
Another application for PSA is vibration-control. The PSA bonds vibration damping plates to the stamped cover of the hard-disk drive. These types of adhesives are viscoelastic materials. While many adhesives may dampen, viscoelastic adhesives are developed specifically for their damping qualities. Besides absorbing vibrations, bonding together the plates and cover stiffens the structure, reducing vibrations.
The ever tighter microscopic tolerances between the read/write head and the disk surface have made adhesive compounds, such as residual solvents like toluene, more of a contaminant issue in manufacturing hard drives than in the past. As motors heat up the drives the adhesive warms up as well, emitting gases or chemicals. These contaminates can also be a negative factor in the assembly process. Though a label or damper may not be inside the drive, they are assembled in the same clean room. Airborne contaminants can damage the drive when emitted through outgassing.
The development of contaminant-free adhesives can be divided into two parts. One is from a process perspective. Engineers run process test-trials to determine the conditions, such as temperature and process speed, that drive off the maximum amount of contaminants during PSA adhesive manufacturing. Then, in hard-disk drive operation there’s minimal outgassing. Because these adhesives are solvent-based acrylic-polymers they’re coated onto a webstock in liquid form and then dried, making the drying process critical.
The second part of the challenge is to develop adhesives that contain materials that volatilize and burn off very easily. The qualification test for outgassing is a 3 to 4-hr test. The drying time in the production environment is only 5 to 10 min, which limits the time to drive off the materials.
Eliminating all contaminants during development may be possible, but high cost is associated with high heat and slow drying. The challenge is to manufacture adhesives with an acceptable level of contaminants at a realistic cost. Hard-disk drive adhesives cost slightly higher than ordinary adhesives because of a longer drying time. Adhesives are sold to equipment manufacturers, for example, a gasket company who produces their product. The gasket is most likely sent to Singapore where the drives would be assembled.
Taking a look at the basic manufacturing steps in preparing a PSA reveals the significance of drying time and temperatures. Adhesives are applied through a continuous web process, where blank paper unwinds and runs through a coater that applies a coating that later produces a protective release liner on one side of the tape. The paper runs through an oven and then through another coater that applies adhesive. The coated paper then proceeds through a second oven for drying. To speed the process, the drying heat must be as high as practical.
In addition, the drying stage is a diffusion or evaporation process. So higher heat forces faster evaporation and faster elimination of residual contaminants.
As speed decreases, the amount of residual contaminants remain steady, but drop off sharply as coating speed approaches zero. Basic research into the process is to determine that drop point, which is effected by a number of parameters such as speeds, contaminants, and coating thicknesses.