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22、E PUBLISHED BY TIA WITHOUT SUCH LIMITATIONS. TSB-144iTSB-144Adhesive Bubbles and Voids in Fiber Optic Components:Guidance, Issues, and ChallengesContentsForeword iii1 Introduction .12 Scope13 Overview .14 Potential Issues.15 Bubbles versus Voids36 Size and Number 57 Bubble and Void Formation.68 Bubb
23、le and Void Minimization.79 Component Evaluation Techniques 910 Industry Challenges 10Annex A References 11Annex B Comparison to IEC or ITU Standards11TSB-144iiThis page left blank.TSB-144iiiTSB-144Adhesive Bubbles and Voids in Fiber Optic Components:Guidance, Issues, and ChallengesForewordFrom TIA
24、Project No. PN-3-0077, formulated under the cognizance of TIA FO-6.3, Subcommittee on Interconnecting Devices and Passive Products.There are two informative annexes.Key words:Fiber Optics, Fiber Optic Component Reliability, Fiber Optic ConnectorReliability, Fiber Optic Adhesive Reliability, Fiber Op
25、tic Epoxy Reliability, FiberOptic Polymer ReliabilityAdhesive, Fiber Optic Adhesive, Fiber Optic Epoxy, Fiber Optic PolymerAdhesive Bubble, Adhesive Void, Adhesive Bubble and Void, Epoxy, EpoxyBubble, Epoxy Void, Epoxy Bubble and VoidPolymer, Fiber Optic Polymer, Polymer Bubble, Polymer Void, Polyme
26、r Bubbleand VoidTSB-144ivThis page left blank.TSB-14411 IntroductionThis bulletin provides general information and guidance on adhesive bubblesand voids within fiber optic components and devices. Previously undocumented,but widely accepted subject knowledge is summarized herein. Potential issues,sol
27、utions, and challenges relevant to the telecommunications industry, military,and aerospace sectors are discussed.2 ScopeThe bulletin is applicable to fiber optic component reliability. The document iswritten for fiber optic component manufacturers, as well as end-users, and wasprepared by TIA Workin
28、g Group FO-6.3.1, Adhesives Reliability under thecognizance of FO-6.3 SC Subcommittee on Interconnecting Devices andPassive Products.3 OverviewLiquid adhesives are widely used in the manufacture of fiber optic componentassemblies. Two common assemblies that typically use adhesives in theirmanufactur
29、e are connectorized cables and fiber optic branching devices(couplers). During fabrication processes of such components, however, bubblesor voids can be introduced into the liquid adhesive. Once bubbles and voids arepresent in the adhesive fluid state, they may become part of the assemblyinternal st
30、ructure when the adhesive hardens or cures. Depending on size,number, and position within the component, undesirable or unexpectedconditions may then occur.4 Potential IssuesBubbles and voids in fiber optic assemblies may result in serious performanceand reliability problems due in large part to non
31、-uniform stress conditions.Optical performance can be adversely affected when a bubble or void allows afiber to flex or bow resulting in micro-bend losses. In polarization maintaining(PM) fiber applications, bubbles and voids can create non-uniform stresses thatTSB-1442result in a change of polariza
32、tion directions. Lastly, optical bit-error-rates canincrease due to refractive index changes when bubbles or voids are present inan adhesive optical transmission path.Assembly failure can occur over time when a bubble or a void creates acondition that allows lateral loading forces to act on a fiber
33、causing micro-crackpropagation and breakage.Finally, components inside an assembly can experience shifts in relative positionwhen bubbles and voids reduce the adhesive bond surface area, andsubsequent strength, resulting in undesirable fiber movement.In addition to the above problems, bubbles and vo
34、ids can alter adhesivecharacteristics. The mechanical properties of a void- and bubble-free adhesivecan differ from one containing a high percentage of bubbles and voids. Moreimportantly, adhesive suppliers generally assume void- and bubble-freeconditions in their specifications. Consequently, compo
35、nents having a largenumber or high percentage of bubbles or voids may have higher than expecteduncertainty in their design. Two adhesive characteristics that can be altered bybubbles and voids are: Coefficient of Thermal Expansion (CTE), andcompressive/tensile load handling capacity.Figure 1 graphic
36、ally illustrates possible consequences associated with bubblesand voids. Assembly failure occurred at a large, adhesive vacant location withina specialty multimode connector. The Figure shows the adhesive carcass andoptical fiber after the metallic connector housing was etched away during failureana
37、lysis investigations. The etching process was specially designed to preservethe adhesive structure and integrity. A precision reflectometer was used tolocate and verify the fracture site, at 12 mm from the polished ferrule end, beforeetching. Mechanical structural analysis indicated that the optical
38、 fiber wassubjected to relatively high lateral loading forces within the adhesive void.Investigations also indicated that coating-stripping operations might havedecreased fiber strength. The assembly was deployed and in use for severalmonths before the failure occurred.TSB-1443Figure 1. Adhesive car
39、cass and broken optical fiber from a failed connector afterthe metallic connector housing was etched away by a chemical removaltechnique.5 Bubbles versus VoidsThe term bubble, as discussed here, generally refers to a trapped gas within theadhesive, most commonly air.Bubbles in liquid fiber optic adh
40、esives oftentimes start out small in size,approximately the size of an optical fiber or smaller. Bubbles can grow in sizeover time if they move within the fluid and merge together.The term void, as used here, refers to incomplete or partial adhesive filling withinan assembly. Voids tend to be larger
41、 in size than bubbles although not always.Voids larger than 10 times the size of an optical fiber can occur.In the molding industry, an “undercut“ is a type of void. The term “undercut”refers to parts with geometric features that do not easily allow air to be displacedby a resin. During resin inject
42、ion, the mold design will cause air to be trappedand retained resulting in an “undercut” or a vacant pocket devoid of resin. Thedesigns of some fiber optic components can create conditions where adhesive“undercuts” or voids will occur.Sometimes it is not clear whether a “bubble” or a “void” exists w
43、ithin anassembly. An analysis of the root cause will shed light on whether the vacantfeature should be termed a bubble or void. From a general polymeric materialsstandpoint, however, any vacancy within a potting material, encapsulant, moldedTSB-1444part, or adhesive is undesirable. The vacancies cre
44、ate discontinuities in theadhesive structure, thereby potentially leading to loss of mechanical integrity andload bearing capacity.Figure 2 and Figure 3 show specialty multimode connector cross-sections with abubble and void depicted, respectively. The adhesive structure in the Figures isred-amber o
45、r yellowish in color. In Figure 3, the void is quite large and onlysmall remnants of adhesive may be seen on the left where the optical fiberenters the ferrule, and on the right along the fiber coating.Figure 2. A specialty multimode connector cross-section with an adhesivebubble.TSB-1445Figure 3. A
46、 specialty multimode connector cross-section with an adhesive void.6 Size and NumberThe exact size and number of bubbles or voids that may cause problems in fiberoptic components cannot be precisely determined because many variables exist.Important variables are: bubble or void location and distribu
47、tion, componentdesign, component material, adhesive type, environmental requirements,fabrication processes, and stripped optical fiber strength. Uniformly distributedsmall bubbles may not cause the same cracking or micro-bend issues as large orinconsistently spaced bubbles. Voids due to “undercuts”
48、may not have the sameconcerns as large bubbles at optical fiber interfaces. Failure investigations do,however, indicate that the presence of many large bubbles or voids significantlyincrease the likelihood of future assembly failure particularly when optical fibersare improperly stripped using metho
49、ds that nick or scratch the surface of thefiber.It is useful to estimate the collective bubble and void volume as a percentage ofthe total adhesive volume required in an assembly, rather than counting a largenumber of extremely small bubbles (tedious and impractical). Estimating thebubble and void volume is valuable because manufacturing processeffectiveness can be gauged and performance metrics can be defined. Inpractice, bubbles and voids can actually displace a surprisingly large volume ofthe adhesive (refer to Figure 3). Factors that contribute to such occurrences
