Engineered Solutions To Help Prevent LCD Failures

Transcription

1 Engineered Solutions To Help Prevent LCD Failures By Bruce Chew Senior Applications Engineer E-A-R Specialty Coposites Indianapolis, Indiana

2 ENGINEERED SOLUTIONS TO HELP PREVENT LCD FAILURES A liquid crystal display (LCD) utilizes two sheets of polarizing aterial with a thin layer of liquid crystal solution between the. The use of LCDs has increased substantially over the past decades, and today they can be found in devices such as notebook- and tabletsized personal coputers (PCs), personal digital assistants (PDAs), cell phones, digital clocks, calculators and watches, icrowave ovens, CD players and any other electronic devices. LCDs offer soe real advantages over other display technologies. They are thinner, lighter and draw less power than cathode ray tubes (CRTs), for exaple. Mobile, hand-carried equipent such as cellular telephones, PDAs and PCs are often susceptible to accidental drops. The part of the PC, PDA or cell phone ost likely to get daaged during an accidental drop is the protective front glass (touch panel in the case of a PDA) and the LCD behind it. Generally, the cost of replacing the front glass is noinal, though the LCD cost is substantial. In ost applications, the glass (touch panel) is peranently glued/attached to the LCD. This akes replacing the glass alone ipossible. There is hence a considerable need to protect the LCD assebly fro daage in obile equipent, like cell phones, PDAs, and notebook coputers. Shock and vibration solutions Space constraints in electronic equipent liit the aount of protection that elastoeric ounts or foa pads can provide against shock and vibration. Usually, the ost effective isolation is provided by highly daped aterials. Critical perforance requireents and tight space constraints in today s portable electronic devices are forcing product designers to re-evaluate traditional shock. Accordingly, a new generation of portable electronic products has evolved, destined for new applications that require new levels of ruggedness and reliability. Many of the new applications involve exposure to high levels of shock and vibration, either of which can induce preature fatigue or initiate failure in coponents. Shock often described as an ipact of short duration and large acceleration aplitude occurs when electronic packages are inadvertently dropped or buped against a hard surface. In contrast, vibration usually consists of continuous haronic or rando otion of relatively sall acceleration aplitude. Neither of these conditions will daage equipent, however, if the coponents are properly protected. And highly daped elastoeric aterials often provide the best choice for adequate protection in these applications. Conventional shock-ounting systes ust provide considerable freedo of oveent to protect electronic coponents effectively. This also eans that isolated electronic asseblies need to incorporate sway space, which often is not available or is severely liited because of today s saller package envelopes and high coponent density. These space constraints effectively establish a liit on the shock agnitude that can be accoodated. Liited space also requires that vibration aplification at resonance be closely controlled and that displaceent fro shock be iniized. Both requireents call for high daping in isolator aterials. Liiting syste response All coponents possess ass and stiffness and therefore are subject to resonance, a phenoenon that soeties results in daaging vibration aplification. Syste response at resonance usual- Page 2

3 ly can be controlled by specifying required daping properties for isolator aterials. Generally, aterials with a high loss factor η are preferred. Paraeter η is nuerically equal to twice the aterial s critical daping ratio c /c c (or ζ). As an exaple, loss factors can range fro 0.05 for natural rubber to greater than 1.0 for certain engineered aterials. Resonance probles in large, assive structures often can be iniized or eliinated by designing for a relatively low syste natural frequency f n, which falls out of the range of excitation frequencies. This approach is virtually ipossible for any electronic packages, especially portables, because the resulting required isolator deflection exceeds the available sway space. Shock transissibility The cobined daping of the isolation ounts and other syste coponents controls transissibility, which is the noralized agnitude of syste response to shock or vibration. High loss factor aterials effectively address shock Acceleration (g) Figure 1 Half-sine shock pulse 10 Natural loss factor η 0.1 Highly daped elastoer loss factor η 1.0 Syste natural frequency f s 50Hz Tie (s) Transissibility (db) Figure VersaDap 2000 loss factor η 0.75 Natural rubber loss factor η 0.05 Neoprene loss factor η 0.1 Butyl rubber loss factor η 0.4 Frequency ratio f/f s(log scale) ISODAMP C-1002 η 1.2 energy in a syste by liiting response to shock pulse input (Figure 1). With a lightly daped aterial, such as neoprene, shock transissibility is over 2.0 when a half-sine shock pulse is applied to a suspended rigid-body syste. Theoretical data analysis predicts that transissibility should be about 1.6, given a aterial loss factor of 0.1. In practice, transissibility is greater than this because the isolator bottos out due to its inherent deflection liitations, producing a rapid, nonlinear increase in ount stiffness. The point at which the ount stiffens rapidly shows up as a knee in the acceleration response curve for neoprene. In contrast, a highly daped aterial avoids significant shock aplification. Given a half-sine shock pulse, the suspended syste response is only about 1.1 ties the input acceleration of 25 g. Vibration transissibility Figure 2 illustrates the levels of daping that various aterials can introduce into a syste via their specific loss factors. When transissibility exceeds 0 db, the syste response is actually greater than Page 3

4 the vibration input; the higher the peak, the ore uncontrolled the response. Daped, proprietary elastoers E-A-R Specialty Coposites has kept pace with the stringent energy control needs of increasingly downsized equipent, with the copany s long-established ISODAMP line of energy- absorbing theroplastics, ISOLOSS faily of highly daped urethane elastoers, and with the ore recent developent of the VersaDap faily of daped theroplastic rubber and CONFOR line of highly daped urethane foas. With their high aterial loss factors, these high perforance aterials are forulated to effectively absorb and dissipate shock and vibration energy and quickly restore syste equilibriu. Their high internal daping properties enable the aterials also to control syste response at resonance, by internally dissipating vibration energy at the olecular level. E-A-R offers its proprietary aterials in a broad range of forulations that provide the optiu cobinations of physical and perforance properties for virtually any application. A wide variety of standard ISODAMP, VersaDap and ISOLOSS olded isolators are available, including groets, bushings, and etal-bonded and non-etal bonded ounts. E-A-R has the capability to old custo configurations as well. CONFOR foas are available in die-cut parts, sheet forat or in buns. Applications Saller forat LCD PDA or handheld devices Most anufacturers of PDAs or handheld devices specify a axiu height fro which the units would survive daage if dropped accidentally. TheLCD is one of the ost sensitive coponents that need shock protection. In ost cases, there is little or no gap between the LCD and cover or PCB or battery. Breakage (%) Figure 3 Drop Shock Perforance PDA Touch Panel & LCD Breakage, with and without CONFOR Shock Pad Without CONFOR Pad Touch Panel Total drops = 35 4 feet onto carpet 3 feet onto concrete Orientation: flat With CONFOR Pad LCD Figure 3 describes experiental work done at E-A-R Specialty Coposites on protection of the LCD panel in a PDA. To siulate an accidental drop, a PDA was subjected to free-fall drop tests of 48 inches onto carpet and 36 inches onto concrete. The baseline testing involved dropping the PDA without any shock isolation treatent. When the ipact of the PDA was studied using a high-speed video caera, it was observed that after ipacting the hard surface, the PDA would typically rebound and bending waves were set up in the PDA. These bending waves were siilar to the resonant bending waves set up in a bea under free-free boundary conditions when excited by a broadband source. It was also noted that the LCD and touch panel ight not crack at the first ipact but on the rebound. The challenge was not only to reduce the G levels but to also prevent or at least reduce resonant flexure in the PDA assebly, which in turn will reduce the flexure in the LCD. During flexure, the tendency to crack would be in a zone that had axiu bending stress concentration. Page 4

5 E-A-R s approach in solving this issue was unconventional. It was proven, not only in this test but other applications in the field, that a CONFOR pad with the right stiffness and thickness behind the LCD as a snubber provides the ost shock protection. Energy-absorbent CONFOR foa is soft and flexible. When sandwiched between an LCD and PCB, it typically exerts little or no force statically. But under dynaic loading, such as a shock event, the aterial stiffens to liit the flexure of the LCD. The copression and shearing of highly daped CON- FOR aterial also work to dissipate echanical energy. Molded ISODAMP isolators can also be used to ount the LCD to the frae structure to provide copliance to the ounting points of the LCD so that there is not a direct path for energy transfer fro the frae or structure in shock events. Laptop Coputer and Cross Section Side View Larger forat LCD notebook or tablet PC E-A-R Specialty Coposites has also done considerable work in protecting larger forat LCDs in notebooks and tablet PCs fro shock and vibration. Except for ruggedized versions, such equipent usually has a lower drop height requireent and higher vibration input than the saller forat LCDs. In addition, the definition for LCD failures includes not only breakage but also scuff arks, scratches or paint residue transferred to the LCD during vibration testing. When the notebook is in its closed position, the gap between the LCD s front surface and the keyboard, palrest, or point stick is often inial (refer to Figure 4). During shock or vibration, the abrasion action of the keyboards, point stick or other area on the palrest will leave scuff arks on or crack the LCD. Hinge LCD assebly Front cover One preventive easure for this type of failure is to strategically place a CON- FOR pad between the LCD and top cover. The CONFOR pad acts like a daped snubber that easily confors to the often uneven gap between the LCD assebly and top cover. This liits the flexure of the LCD and protects it fro contacting adjacent surfaces such as the keyboard, palrest or point stick and therefore reduces scuffing or scratching. Figure 4 Keyboard/point stick Palrest Page 5

6 Design Guidelines Use CONFOR foas as a snubber pad behind LCD to liit resonant response to shock and vibration input and thereby reduce LCD cracking. Use CONFOR foa pads to help reduce LCD scuffing behavior typical in notebook coputers. Place sall isolation ounts at LCD attachent points to reduce transission of shock and vibration energy to the LCD. Optiize the LCD shock pad design (aterial grade and thickness), based on shock levels, LCD size and surrounding gap diensions. Typical copression is 25 percent to 75 percent. Contact an E-A-R Applications Engineer to deterine optiu shock pad configuration options for your application Zionsville Road Indianapolis, IN Phone (317) Fax (317) Website: Electronics Website: Aearo Copany Printed 1.04