Reliability of Glass and Coatings
Reliability of Glass and Coatings IEEE Reliability Stuart Douglas 03052015 Overview Reliability of Glass • Why is glass important? • • • • • • What is glass? • • Failure mechanisms What types of glass are used in consumer electronics? Types of failure modes Strength testing Fractography and failure analysis Reliability analysis Reliability of coatings • Types of coatings Reliability testing Google Confidential and Proprietary Why is Glass Important? Exploration Technology Google Confidential and Proprietary Optics Art Protection Vessel Glass in Consumer Electronics Google Confidential and Proprietary Glass Myths Google Confidential and Proprietary Myth: Glass is weak Optical fiber glass in axial tension can achieve >1 GPa stress in tension Myth: Glass flows over long periods of time Glass is a non-crystalline amorphous solid, with a melting point > 700oC, and viscosity of >1017 Pa∙s Myth: Glass can bend permanently Glass is a perfectly elastic material, relatively small amounts of plastic deformation may occur due to material densification Typical Glass Supply Chain Google Confidential and Proprietary mothersheet glass supplier singulation pattern Gen. 6: 150 cm x 185 cm painting glass finisher finishing Typical Display Module Glass display stack polarizers and diffuser sheets omitted cover glass ion exchanged (IOX) chemically strengthened aluminosilicate touch panel IOX chemically strengthened soda lime Google Confidential and Proprietary LCD display (CF+TFT) alkaline earth boroaluminosilicate General Material Properties 1 parameters cover glass touch panel display material aluminosilicate soda lime alkaline earth boro-aluminosilicate elastic modulus 71.5 GPa 72-74 GPa 80.7 GPa strength (modulus of rupture) ≥950 MPa @ 50 μm DOL1 ≥500 MPa @ @ 15 μm DOL ≥150 MPa2 typical thickness ≥ 0.4 mm ≥ 0.3 mm ≥0.2 mm density @ 20oC 2.42 g/cm3 2.52 2.57 g/cm3 Mirror Mist Factor 2.14 MPa∙m0.5 1.8 MPa∙m0.5 2.07 MPa∙m0.5 Vickers Hardness 200g load, 15 s dwell 596 kg/mm2 ~550 kgf/mm2 626 kgf/mm2 CTE ~87 x 10-7 /oC ~90 x 10-7 /oC 35 x 10-7 /oC separation method CO2laser, wheel CO2laser, wheel wheel Gorilla Glass, www.corning.com Lotus Glass, www.corning.com 2 Google Confidential and Proprietary IOX Chemical Strengthening Google Confidential and Proprietary Chemical strengthening is a diffusion process of Potassium ions (K+) into aluminosilicate glass containing Sodium ions (Na+) The process is done at high temperature in a Potassium salt bath and takes 2+ hrs Potassium ions are larger than Sodium ions, once diffusion is complete and the material cools the areas exposed become under compression, putting the interior of the glass into tension compression DOL surface compression center tension 1 tension thickness M. Y.M. Lee, Glass Part 3: New generation of specialty glass for LCDs and AMOLEDs, Gases and Instrumentation, www.gasesmag.com thickness Failure Modes glass fails in tension glass fails due to material defects and flaws glass is sensitive to moisture Google Confidential and Proprietary Tensile Failures Google Confidential and Proprietary Fracture patterns Q: Why is this important? A: The fracture pattern can show the type of tensile event Types of Tensile events Uniaxial flexure Example: LCD chip on glass COG TFT shelf Biaxial flexure Example: Ball impact Impact Example: Rock chip in windshield 1 Quinn, George D., NIST Recommended Practice Guide: Fractography of Ceramics and Glasses, NIST SP - 960-16 Crack Branching Angle 1 Quinn, George D., NIST Recommended Practice Guide: Fractography of Ceramics and Glasses, NIST SP - 960-16 Google Confidential and Proprietary Defects and Flaws Google Confidential and Proprietary glass is a brittle material and defect/flaw sensitive weakest link theory: the biggest flaw controls the strength of the material this determines the strength of the material Examples of Flaws 1 Google Confidential and Proprietary inclusion chip cutting damage scratch blunt impact surface damage sharp impact surface damage Quinn, George D., NIST Recommended Practice Guide: Fractography of Ceramics and Glasses, NIST SP - 960- blunt impact low speed < 20 m/s cone cracks Types of Impact mixed impact med. speed ~ 30 m/s Google Confidential and Proprietary sharp impact, high speed > 50 m/s radial cracks crushing median crack side view radial cracks side view lateral cracks side view radial cracks crushing crushing cone cracks top view 1 top view top view Quinn, George D., NIST Recommended Practice Guide: Fractography of Ceramics and Glasses, NIST SP - 960-16 2 S.M. Weiderhorn, B.R. Lawn,“Strength Degradation of Glass Resulting from Impact with Spheres,” J. Am. Ceram. Soc., 60 [9-10] (1997) Types of Contact 1 Google Confidential and Proprietary T.M. Gross, ACerS Fractography VI Ceramic Transaction Vol. 320 (2012) 113-112 T.M. Gross, J. Price, S. Glaesemann, Sharp Contact Damage in Ion-Exchanged Cover Glass, Functional Glasses: Properties and Applications for Energy Information, 2013 2 Flexural Strength Testing Google Confidential and Proprietary stress and strain inside of inner ring and inner load rods is equal - all flaws are tested equally ring on ring (biaxial) tests surface defects ASTM C1499 4 point bend (uniaxial) tests surface and edge defects ASTM C158 General Guidelines to Strength Testing Google Confidential and Proprietary ASTM guidelines on glass and ceramics strength testing to evaluate (break) at least 32 parts Glass strength is a distribution, testing a larger area in 4 point bend or RoR, will help find more fractures Glass strength is controlled by the presence of moisture in the testing environment Typically, strength is indicated by a percentage of the stength population (ex. B10), but the tightness of the distribution is also an important parameter Strength distribution should be analyzed using MLE (maximum likelihood estimate) method with a 2P Weibull Distribution Strength Testing Statistics Google Confidential and Proprietary Glass strength is a distribution and is limited by the largest defect (i.e. surface, edge, inclusion) handling defects, random, wider distribution mixed mode distribution hidden in the data strength data β η (MPa) all data 7.9 202 handling defects 5.9 202 cutting defects 15 200 cutting defects, controlled, tighter distribution Stress-Strength analysis Google Confidential and Proprietary Strength Stress Reliability ~ 72.55% Source: 3DS Fracture Analysis Google Confidential and Proprietary Looking at the fracture surface can tell you: • Where the fracture started • • • What caused the crack to start (type of defect) Stress at rupture Information about the loading event (arrest, vibration, presence of moisture) fracture origin secondary wallner lines mist surface damage fracture origin 1 crack direction crack propagation Quinn, George D., NIST Recommended Practice Guide: Fractography of Ceramics and Glasses, NIST SP - 960-16 Google Confidential and Proprietary Testing Optical Coatings and Paint Types of Coatings Paints (primer, topcoat) Hardcoats (organic, inorganic) Monolayers (hydrophobic, hydrophilic) Optical (anti-reflective, light filters) Google Confidential and Proprietary Common Environmental Test Methods Deionized water boil Heat soak Dry heat Thermal cycling Dew testing Temperature and humidity cycling UV testing Salt fog Chemical testing Google Confidential and Proprietary Common Failure Mechanisms Google Confidential and Proprietary Buckling, Abrasion, Chalking, Delamination, Discoloration, Corrosion, Tensile cracking thin film buckling observed on anti-reflective in organic coating organic hardcoat delamination on dyed substrate environmental stress cracking in PC substrate extends to surface of organic hard coat Adhesion Testing Crosshatch ASTM D3599 Critical Scratch Force ASTM standards G171, C1624, D7187 scribe mark first defect 180° Peel Google Confidential and Proprietary spalling continuous delamination Types of Abrasion 1 • Abrasion testing is evaluates the wear interaction of two materials • Wear is the erosion or displacement of a material by another material • There are five different types of wear1: Google Confidential and Proprietary ∘ Adhesive - caused by relative motion, contact and plastic deformation which create wear debris and material transfer from one surface to another ∘ Abrasive - when a harder material contacts a softer material resulting in a deformation or loss of material in the softer material ∘ Surface fatigue - weakening of the surface by cyclic loading where material is lost through superficial or subsurface crack growth ∘ Fretting - cyclic loading resulting in material loss on one or both surfaces in contact ∘ Erosive - material loss through repeated contact by particles for a short period of time, usually associated with liquid systems http://en.wikipedia.org/wiki/Wear Abrasion Test Methods force Taber Linear Abraider Model 5750 stroke wearaser Military Standard MIL-E-12397B Consistent abrasion steel wool Origins unknown…. Google Confidential and Proprietary wearaser steel wool Other Abrasion Test Methods Google Confidential and Proprietary the abrasion method should be able to recreate failures in the field Taber Rotating Abraider example test area Falling sand RCA Tape Machine Cheesecloth Rancourt, James D., Optical Thin Films: User Handbook, SPIE Press, 1996 General Guidelines to Abrasion Testing • • • • Google Confidential and Proprietary Abrasion testing is only useful if it is able to recreate expected wear or failures observed in the field. It is very difficult to make an objective decision without understanding field wear. Wear is affected by environment, surface quality, the material properties of interacting materials. Steel wool: ∘ Each brand of steel wool is different, each brand of steel wool may have differences between lots/batches/shipments ∘ Steel wool may contain oil, volatiles, and moisture ∘ Orientation of steel wool fibers should be controlled Wearaser: ∘ Wearaser needs to be “charged” with sandpaper (new eraser) to get repeatable results ∘ Wearaser is a rubber, moisture may be absorbed Hardness Testing Google Confidential and Proprietary critical scratch force (ASTM standards G171, C1624, D7187) output: load at first defect, continuous defect scribe mark first defect pencil hardness (ASTM D3363 output: pencil hardness spalling continuous delamination indentation hardness (ASTM E2546-07) output: hardness, elastic modulus
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