{"id":3869,"date":"2026-10-29T01:47:35","date_gmt":"2026-10-28T17:47:35","guid":{"rendered":"https:\/\/www.taichungbro.com\/?p=3869"},"modified":"2026-10-29T01:48:54","modified_gmt":"2026-10-28T17:48:54","slug":"foldable-screen-utg-hinge-failure","status":"publish","type":"post","link":"https:\/\/www.taichungbro.com\/en\/2026\/10\/29\/foldable-screen-utg-hinge-failure\/","title":{"rendered":"Why do foldable screens crack? Analysis of UTG bending stress and hinge tolerance failure."},"content":{"rendered":"<p>Foldable screens can shatter because their &quot;bendability&quot; is conditional: ultra-thin glass can deform elastically under specified curvature and support, but bending too sharply, being blocked by foreign objects, or the continued expansion of existing micro-cracks can all cause failure in a certain area. High-end materials and precision hinges can increase reliability, but they cannot eliminate the brittleness of glass.<\/p>\n<p>Recently, searching for &quot;iPhone Duo&quot; and &quot;crack&quot; has revealed numerous discussions about cracks near the hinge. This article first outlines verifiable disassembly observations and repair options, then uses the bending formula for Ultra Thin Glass (UTG), simplified calculations, and a failure comparison table to explain how materials, adhesive layers, and multi-link hinges collectively determine risk.<strong>The 30\u201350 \u00b5m mentioned below are examples of publicly available UTG specifications and do not represent the actual glass thickness or material of the iPhone Duo.<\/strong><\/p>\n<nav class=\"article-toc\" aria-labelledby=\"article-toc-heading\" style=\"margin-block: 1.5em;\">\n<h2 id=\"article-toc-heading\" style=\"scroll-margin-top: 110px;\">Table of Contents<\/h2>\n<ol>\n<li><a href=\"#iphone-duo-evidence\" style=\"color: #00c8d7; text-decoration: underline;\">Discussion on iPhone Duo cracks: Which are observations and which are still hypotheses?<\/a><\/li>\n<li><a href=\"#utg-bending-stress\" style=\"color: #00c8d7; text-decoration: underline;\">Why can 30-50 \u00b5m UTG bend but still break?<\/a><\/li>\n<li><a href=\"#neutral-plane-stack\" style=\"color: #00c8d7; text-decoration: underline;\">Where is the neutral layer? Why can&#039;t we just calculate the glass thickness?<\/a><\/li>\n<li><a href=\"#hinge-tolerance\" style=\"color: #00c8d7; text-decoration: underline;\">How do minute tolerances in multi-link hinges become localized stresses?<\/a><\/li>\n<li><a href=\"#particles-impact-cracks\" style=\"color: #00c8d7; text-decoration: underline;\">How do metal particles, drops, and moisture cause cracks to propagate?<\/a><\/li>\n<li><a href=\"#inspection-and-tests\" style=\"color: #00c8d7; text-decoration: underline;\">What should be done if an anomaly occurs? What should be considered in reliability testing?<\/a><\/li>\n<li><a href=\"#protect-local-curvature\" style=\"color: #00c8d7; text-decoration: underline;\">The reliability of a folding machine depends on maintaining a safe curvature at every point.<\/a><\/li>\n<\/ol>\n<\/nav>\n<h2 id=\"iphone-duo-evidence\" style=\"scroll-margin-top: 110px;\">Discussion on iPhone Duo cracks: Which are observations and which are still hypotheses?<\/h2>\n<p>First, separate the evidence.<a href=\"https:\/\/www.ifixit.com\/News\/119895\/iphone-duo-teardown\" style=\"color: #00c8d7; text-decoration: underline;\" target=\"_blank\" rel=\"noopener\">iFixit&#039;s iPhone Duo teardown<\/a>By combining physical objects, computed tomography (CT) scans, and models, we observed the hinge support and component interfaces, and proposed a simulated scenario where &quot;if the central support does not yield sufficiently, it may hit and bend the screen.&quot; This is a verifiable engineering hypothesis and cannot be directly taken as the common cause of all malfunctioning machines.<\/p>\n<figure class=\"wp-block-table\" style=\"overflow-x: auto;\">\n<table>\n<thead>\n<tr>\n<th>Information hierarchy<\/th>\n<th>This can be used to make a judgment<\/th>\n<th>Cannot be directly derived<\/th>\n<\/tr>\n<\/thead>\n<tbody>\n<tr>\n<td>Official product and service page<\/td>\n<td>Product design specifications, service access<\/td>\n<td>Stress and failure rate of each machine<\/td>\n<\/tr>\n<tr>\n<td>Disassembly, CT and Simulation<\/td>\n<td>Support location, possible contact path<\/td>\n<td>Confirmed causes of mass production defects<\/td>\n<\/tr>\n<tr>\n<td>Photos of individual cracks<\/td>\n<td>Damage appearance and location<\/td>\n<td>Rupture initiation point, batch range and liability<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n<\/figure>\n<p>When interpreting a chart, the key is to &quot;observe what&quot; and &quot;infer where it leads.&quot; A bright line may indicate localized electrical damage affecting an entire column or row of pixels, and its apparent length is not equal to the length of a glass crack; further testing is needed to confirm the initiation layer and the fracture point.<\/p>\n<p>If the device develops new cracks, bulges, noticeable jamming, or unusual sounds, stop repeatedly opening and closing the device and pressing the creases. Record the appearance and the events that occurred. iPhone users can access this information via...<a href=\"https:\/\/support.apple.com\/zh-tw\/iphone\/repair\" style=\"color: #00c8d7; text-decoration: underline;\" target=\"_blank\" rel=\"noopener\">Apple Taiwan iPhone Service and Repair Portal<\/a>Inspections will be arranged; the actual handling methods and warranty eligibility will be confirmed by the authorities on a case-by-case basis. This document was verified as of October 29, 2026, and the list of affected batches is not based on online photos.<\/p>\n<h2 id=\"utg-bending-stress\" style=\"scroll-margin-top: 110px;\">Why can 30-50 \u00b5m UTG bend but still break?<\/h2>\n<p>UTG is still glass; making it thinner reduces the surface elongation for the same bending radius.<a href=\"https:\/\/thejntc.com\/en\/product\/F0104.do\" style=\"color: #00c8d7; text-decoration: underline;\" target=\"_blank\" rel=\"noopener\">JNTC discloses UTG product thickness<\/a>It includes 30, 50, 70 and 100 \u00b5m, so not all foldable screens can be considered the same thickness.<\/p>\n<p>Under the approximation of homogeneous, linearly elastic, pure bending conditions where the thickness is much smaller than the radius of curvature, the axial bending strain and stress are:<\/p>\n<p><strong>\u03b5 = y \/ R; \u03c3 = E \u00d7 y \/ R<\/strong><\/p>\n<p>Where \u03c3 is the bending normal stress, E is Young&#039;s modulus, y is the distance from the calculation location to the neutral layer, and R is the local radius of curvature of the neutral layer. Under this model, the neutral layer does not elongate or shorten due to bending; it is under tension on the outer side and compression on the inner side. For a monolithic homogeneous glass pane, if the neutral layer is in the center of its thickness, the surface can be substituted with y = t \/ 2.<\/p>\n<p>The following adopts<a href=\"https:\/\/www.aimspress.com\/aimspress-data\/aimsmates\/2024\/2\/PDF\/matersci-11-02-019.pdf\" style=\"color: #00c8d7; text-decoration: underline;\" target=\"_blank\" rel=\"noopener\">2024 UTG Cyclic Load Study<\/a>The model uses SCHOTT AF32 glass with an E value of 74.8 GPa, assuming room temperature, single-piece free bending, and neglecting residual stress from reinforcement and point contact between the adhesive layer and the glass. This is a preliminary calculation and not the actual measured results for the following thicknesses in this study.<\/p>\n<figure class=\"wp-block-table\" style=\"overflow-x: auto;\">\n<table>\n<thead>\n<tr>\n<th>Glass thickness t<\/th>\n<th>Neutral layer radius R<\/th>\n<th>Surface tensile strain<\/th>\n<th>Calculate bending stress<\/th>\n<\/tr>\n<\/thead>\n<tbody>\n<tr>\n<td>30 \u00b5m<\/td>\n<td>3 mm<\/td>\n<td>0.50%<\/td>\n<td>374 MPa<\/td>\n<\/tr>\n<tr>\n<td>50 \u00b5m<\/td>\n<td>3 mm<\/td>\n<td>Approximately 0.831 TP3T<\/td>\n<td>Approximately 623 MPa<\/td>\n<\/tr>\n<tr>\n<td>30 \u00b5m<\/td>\n<td>6 mm<\/td>\n<td>0.25%<\/td>\n<td>187 MPa<\/td>\n<\/tr>\n<tr>\n<td>50 \u00b5m<\/td>\n<td>6 mm<\/td>\n<td>Approximately 0.421 TP3T<\/td>\n<td>Approximately 312 MPa<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n<\/figure>\n<p>For example, in the first column: 74.8 \u00d7 10\u2079 Pa \u00d7 15 \u00d7 10\u207b\u2076 m \u00f7 0.003 m = 374 \u00d7 10\u2076 Pa.<strong>At the same thickness, halving the radius doubles the ideal bending stress; at the same radius, increasing the thickness also increases the surface stress.<\/strong>However, the table does not have a boundary for &quot;breaking if it exceeds a certain column&quot;. Breakage is also affected by defects, edge processing, reinforcement status and load time.<\/p>\n<p>Measurement methods cannot be mixed:<a href=\"https:\/\/www.neg.co.jp\/en\/assets\/DinorexUTG_PDF.pdf\" style=\"color: #00c8d7; text-decoration: underline;\" target=\"_blank\" rel=\"noopener\">Nippon Electric Glass Dinorex UTG Technical Data<\/a>The geometric relationship for the two-point bending test is listed separately. Half the distance between the two clamps cannot be taken as R for each position of the mobile phone screen without conversion.<\/p>\n<h2 id=\"neutral-plane-stack\" style=\"scroll-margin-top: 110px;\">Where is the neutral layer? Why can&#039;t we just calculate the glass thickness?<\/h2>\n<p>A true foldable screen has a multi-layered structure, and the neutral layer is not necessarily located in the center of the UTG. The protective film, optically clear adhesive (OCA), display and touch layers, and back support each have different thicknesses and rigidities; the actual layer order varies depending on the product.<\/p>\n<p>Imagine bending a stack of paper; the outermost sheets travel a longer path. If all layers are completely glued together, they must share the burden of stretching and contraction; if the adhesive layers allow for moderate shear deformation, some of the deformation difference between the layers can be released.<a href=\"https:\/\/pmc.ncbi.nlm.nih.gov\/articles\/PMC4950202\/\" style=\"color: #00c8d7; text-decoration: underline;\" target=\"_blank\" rel=\"noopener\">Research on the neutral layer of multilayer flexible structures<\/a>It is pointed out that the soft interlayer may allow different hard layers to form their own local neutral layers, and the result is also affected by the ratio of structural length and thickness.<\/p>\n<p>Therefore, whole-machine analysis must consider interlayer slip, viscoelasticity, temperature, and contact conditions, and cannot simply divide the total thickness by two. Hardening, aging, or local delamination of the adhesive material can all alter the deformation distribution; the idea that &quot;the softer the adhesive, the better&quot; is also not true, because support, recovery, and adhesion must also be considered.<\/p>\n<p>This also explains why thin electronic devices must be designed along with their interfaces. For a deeper understanding of how components balance protection, connectivity, and reliability within a small space, please refer to...<a href=\"https:\/\/www.taichungbro.com\/en\/2025\/02\/20\/high-density-packaging\/\" style=\"color: #00c8d7; text-decoration: underline;\">Structure and function of high-density IC packaging<\/a>It addresses a different system integration issue, rather than being a direct alternative to foldable display stacking.<\/p>\n<h2 id=\"hinge-tolerance\" style=\"scroll-margin-top: 110px;\">How do minute tolerances in multi-link hinges become localized stresses?<\/h2>\n<p>The hinge not only allows the left and right sides of the device to rotate, but also controls the shape, support, and movement path of the screen. A teardrop-shaped bend leaves more room for bending inside, preventing the display from being pressed into a sharp fold. When assessing risks, look at the most abrupt local curvature, not just whether the overall shape is round or not.<\/p>\n<p><a href=\"https:\/\/patents.google.com\/patent\/US12164344B2\/en\" style=\"color: #00c8d7; text-decoration: underline;\" target=\"_blank\" rel=\"noopener\">Apple&#039;s patent for a folding display hinge is US12164344B2.<\/a>The patent describes control methods such as connectors, guides, and gears. While the patent presents design concepts, it does not mean that every embodiment has been used in mass-produced machines, nor can it be used as a fault diagnosis report.<\/p>\n<p>The following are potential failure chains for common mechanisms: deviations in shaft hole position, connecting rod length, tooth backlash, or support height can alter the relative positions of various parts; if friction or jamming is added, supports that should have yielded may move out of place too late. When a screen passes over local bumps, it will simultaneously experience overall bending and additional contact deformation, resulting in stress concentration.<\/p>\n<p>but,<strong>The screen stress cannot be calculated by simply adding up all dimensional errors.<\/strong>It also depends on the correlation between the mechanism&#039;s posture, error direction, and dimensions. In engineering, dimensional chain and motion analysis must be performed first, and then the most unfavorable posture must be incorporated into the structural analysis involving contact; whether a case is caused by tolerance still needs to be verified by actual measurement.<\/p>\n<h2 id=\"particles-impact-cracks\" style=\"scroll-margin-top: 110px;\">How do metal particles, drops, and moisture cause cracks to propagate?<\/h2>\n<p>Foreign objects can alter the contact boundaries, while drops can create new defects or alter hinge alignment; both can make the next bend, which was originally tolerable, dangerous.<a href=\"https:\/\/www.samsung.com\/hk_en\/support\/mobile-devices\/what-is-the-foldable-display-and-protective-film-on-my-smartphone\/\" style=\"color: #00c8d7; text-decoration: underline;\" target=\"_blank\" rel=\"noopener\">Samsung&#039;s foldable display user manual<\/a>The guidelines clearly state the risks of sand particles and metal powder getting trapped, and also explain that the protective film, protective layer, and UTG each have their own functions. This is the brand&#039;s usage guideline and cannot be used to infer that other mobile phones have metal shavings mixed in.<\/p>\n<p>If hard particles are trapped between the screen and the support, the stress may be concentrated in a very small contact area, causing indentations, localized bending, or scratches. Whether a particle is dangerous depends on its hardness, shape, location, and load; it cannot be determined solely by whether the particle size is larger than the glass thickness. Metal particles may also originate from external sources or wear; the source must be supported by evidence of residue and wear marks.<\/p>\n<p>Even if a drop doesn&#039;t immediately shatter the screen, it can still damage the glass edges, cause interlayer damage, or slightly deform the frame and hinges. When bent again later, the original tensile stress may act on the new defects; &quot;it still lights up immediately&quot; does not prove that the structure is completely undamaged.<\/p>\n<p>In linear elastic failure mechanics, it is possible to use <strong>K_I = Y \u00d7 \u03c3 \u00d7 \u221a(\u03c0a)<\/strong> Understanding the influence of crack size: &#039;a&#039; is the characteristic size defined by the crack model, and &#039;Y&#039; is the geometric factor. All other things being equal, the larger the crack, the higher the stress intensity factor at the crack tip; it may propagate rapidly when the corresponding critical condition is reached.<a href=\"https:\/\/imechanica.egr.uh.edu\/sites\/default\/files\/Linear%20elastic%20fracture%20mechanics%202014%2003%2008.pdf\" style=\"color: #00c8d7; text-decoration: underline;\" target=\"_blank\" rel=\"noopener\">Lecture notes on linear elastic failure mechanics<\/a>This provides the basis for this judgment; if the actual machine has reinforced residual stress and complex contact, a single uniform tension model cannot be directly applied.<\/p>\n<p>Furthermore, the aforementioned UTG cyclic loading study discussed the effect of moisture reaching the crack tip on crack growth. Glass may slowly accumulate damage under loads that do not immediately fracture, which differs from the common plastic fatigue mechanism of metals, and the lifespan of the study specimens cannot be directly converted into the number of years a mobile phone can be used.<\/p>\n<h2 id=\"inspection-and-tests\" style=\"scroll-margin-top: 110px;\">What should be done if an anomaly occurs? What should be considered in reliability testing?<\/h2>\n<p>First, identify the changes, then have the repair team confirm the damaged layer. Stable creases do not necessarily mean cracks, and bubbling of the protective film does not necessarily mean that the UTG has broken; however, newly appearing lines, bumps, or changes in resistance warrant stopping operation and inspection.<\/p>\n<figure class=\"wp-block-table\" style=\"overflow-x: auto;\">\n<table>\n<thead>\n<tr>\n<th>Phenomenon or risk<\/th>\n<th>Possible mechanisms involved<\/th>\n<th>Reasonable next step<\/th>\n<\/tr>\n<\/thead>\n<tbody>\n<tr>\n<td>Existing creases, now appear normal.<\/td>\n<td>Stacked shapes and optical reflection<\/td>\n<td>Compare with the original manufacturer&#039;s instructions and observe the changes.<\/td>\n<\/tr>\n<tr>\n<td>New cracks, bright lines, or black spots<\/td>\n<td>Damage to glass, display layer, or circuitry<\/td>\n<td>Stop pressing, record, and send for inspection.<\/td>\n<\/tr>\n<tr>\n<td>The opening and closing suddenly become tight or bulge<\/td>\n<td>Support interference, deformation or foreign objects<\/td>\n<td>Stop repeated folding and hand it over for repair.<\/td>\n<\/tr>\n<tr>\n<td>Protective film peeling or bubbling<\/td>\n<td>Abnormal film adhesion<\/td>\n<td>Replace according to brand specifications; do not remove by yourself.<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n<\/figure>\n<p>This form is used to determine the next step; it cannot be used to determine responsibility for a malfunction remotely. Preserving purchase records, details of the drop or environmental exposure, and photos before and after the anomaly is more helpful for troubleshooting than simply bending it a few more times to &quot;check.&quot;<\/p>\n<p>For designers and verification personnel, the number of folds must be interpreted in conjunction with test conditions: sample size, opening and closing angles and speeds, temperature and humidity, failure criteria, and whether drop, contamination, or aging tests were performed beforehand will all affect the results. The results of repeated opening and closing at ideal room temperature cannot represent the reliability after drop or in foreign object environments.<\/p>\n<p>A feasible verification method is to track the changes in opening and closing torque with angle, support height and clearance path, interlayer damage and crack initiation, and then compare the differences before and after contamination and before and after drop testing. Temperature should also be included in the adhesive layer and mechanism testing; for further understanding of the contact and temperature rise issues at the other end of the electronic device, please refer to the following reading.<a href=\"https:\/\/www.taichungbro.com\/en\/2024\/08\/27\/tim\/\" style=\"color: #00c8d7; text-decoration: underline;\">How to reduce the thermal resistance of thermal interface materials<\/a>Thermal interface materials and display OCAs serve different purposes and cannot be directly interchanged.<\/p>\n<h2 id=\"protect-local-curvature\" style=\"scroll-margin-top: 110px;\">The reliability of a folding machine depends on maintaining a safe curvature at every point.<\/h2>\n<p>To understand foldable screen breakage, the three most practical questions are: How far are the glass and brittle functional layers from the neutral layer? Can the hinge maintain a sufficient local bending radius throughout the entire movement? After foreign objects, abrasion, or drops, are the support and contact conditions still within the design limits?<\/p>\n<p>For users, new jams, bulges, or cracks signal to stop repeated opening and closing and schedule an inspection; for engineers, it means verifying materials, adhesive layers, tolerances, and environmental loads. In the case of the iPhone Duo, the disassembly hypothesis should be subjected to testing of the faulty component to avoid prematurely attributing the general principle of brittle fracture to a confirmed product cause.<\/p>","protected":false},"excerpt":{"rendered":"<p>\u6298\u758a\u87a2\u5e55\u788e\u88c2\u8207 UTG \u7684\u5f4e\u66f2\u534a\u5f91\u3001\u4e2d\u6027\u5c64\u53ca\u5c40\u90e8\u63a5\u89f8\u61c9\u529b\u6709\u95dc\u3002\u672c\u6587\u4ee5 30\uff5e50 \u00b5m \u73bb\u7483\u8a08\u7b97\u5f4e\u66f2\u61c9\u529b\uff0c\u89e3\u6790\u591a\u9023\u687f\u9278\u93c8\u516c\u5dee\u3001\u91d1\u5c6c\u5fae\u7c92\u8207\u8dcc\u843d\u5982\u4f55\u4fc3\u6210\u88c2\u7d0b\uff0c\u4e26\u5340\u5206 iPhone Duo \u62c6\u89e3\u5047\u8aaa\u8207\u5df2\u77e5\u4e8b\u5be6\uff0c\u63d0\u4f9b\u7570\u5e38\u8655\u7f6e\u91cd\u9ede\u3002<\/p>","protected":false},"author":5,"featured_media":3868,"comment_status":"closed","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"_acf_changed":false,"footnotes":""},"categories":[45],"tags":[645,641,643,639,644,640,642],"class_list":["post-3869","post","type-post","status-publish","format-standard","has-post-thumbnail","hentry","category-industrial-knowledge","tag-iphone-duo","tag-utg","tag-643","tag-639","tag-644","tag-640","tag-642"],"acf":[],"_links":{"self":[{"href":"https:\/\/www.taichungbro.com\/en\/wp-json\/wp\/v2\/posts\/3869","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/www.taichungbro.com\/en\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/www.taichungbro.com\/en\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/www.taichungbro.com\/en\/wp-json\/wp\/v2\/users\/5"}],"replies":[{"embeddable":true,"href":"https:\/\/www.taichungbro.com\/en\/wp-json\/wp\/v2\/comments?post=3869"}],"version-history":[{"count":2,"href":"https:\/\/www.taichungbro.com\/en\/wp-json\/wp\/v2\/posts\/3869\/revisions"}],"predecessor-version":[{"id":3871,"href":"https:\/\/www.taichungbro.com\/en\/wp-json\/wp\/v2\/posts\/3869\/revisions\/3871"}],"wp:featuredmedia":[{"embeddable":true,"href":"https:\/\/www.taichungbro.com\/en\/wp-json\/wp\/v2\/media\/3868"}],"wp:attachment":[{"href":"https:\/\/www.taichungbro.com\/en\/wp-json\/wp\/v2\/media?parent=3869"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.taichungbro.com\/en\/wp-json\/wp\/v2\/categories?post=3869"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.taichungbro.com\/en\/wp-json\/wp\/v2\/tags?post=3869"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}