ASTM_F_3147_-_15.pdf

Designation:F314715Standard Test Method forEvaluating the Reliability of Surface Mounted Device(SMD)Joints on a Flexible Circuit by a Rolling Mandrel Bend1This standard is issued under the fixed designation F3147;the number immediately following the designation indicates the year oforiginal adoption or,in the case of revision,the year of last revision.A number in parentheses indicates the year of last reapproval.Asuperscript epsilon()indicates an editorial change since the last revision or reapproval.1.Scope1.1 This test method covers a means to test a completedSurface Mounted Device(SMD)joint for bond strength andinter-layer stress compatibility1.2 A completed SMD joint includes;SMD(LED,resistor,etc),PTF ink land(typically silver),conductive adhesive(typically silver),staking compound(non-conductive),andencapsulant(non-conductive).2.Referenced Documents2.1 ASTM Standards:F1996 Test Method for Silver Migration for MembraneSwitch CircuitryF2750 Test Method for Determining the Effects of Bendinga Membrane Switch or Assembly3.Terminology3.1 bend,vto force from a straight form into a differentand especially a curved one.3.1.1 DiscussionIn this case,no“hard”or angled creaseor fold is to occur.The substrate will only be formed into aradius.3.2 bend cycle,na bend of a sample around a specifiedmandrel which is“rolled”in one direction,followed by rollingin the opposite direction,returning the sample to its originalposition(see Fig.1).3.3 conductive adhesive,na material used for electrical ormechanical bonding,or both,of the SMD to the substrate andland-pad.3.4 encapsulant,na non-conductive adhesive that is ap-plied over or around,or both,the SMD for added bond strengthand prevention of silver migration(F1996).3.5 land-pad,nthe printed circuit pattern at the locationthat interfaces with conductive adhesive,in this case conduc-tive link circuitry(commonly silver)that will make electricalcontact to the SMD via conductive adhesive.3.6 mandrel,na cylindrically shaped metal rod,such as abrazing or drill rod.3.7 SMD,nabbreviation for surface mount device(that is,light emitting diode(LED),resistor,capacitor,etc.).3.8 SMD joint,nthe combined interface of silver land-pad,conductive adhesive,staking compound(if included),andencapsulant that holds the SMD in place.3.9 SMD-populated flexible printed circuit,nflexible sub-strate with conductive circuitry and electronic componentsonlynot to include other laminates.3.10 staking compound,na non-conductive adhesive thatis applied at a location directly under the SMD and betweenconductive adhesive deposits(commonly two or more dis-pensed dots of conductive adhesive)to provide added bondstrength and prevent shorting or silver migration.4.Significance and Use4.1 The existing Test Method F1995,while very useful,isdifficult to conduct if an encapsulating dome is applied,anddoes not reveal the possible failures caused by mechanicalstress incompatibility in the overall SMT joint.This mandrelbend test will reveal possible mechanical stress incompatibilitybetween the various adhesives which can result in latent fieldfailures during production handling or with thermal cycling innormal use.4.2 The existing Test Method F2750 does not includespecifics for SMD attachments and only addresses the conduc-tivity change of the conductive trace.4.3 The different combinations of SMD types,attachmentmedias,circuit substrates and process variation can account forsignificant variation in test outcome.4.4 Bending of printed flexible circuit or their componentscan affect their visual appearance,mechanical integrity orelectrical functionality.This test method simulates conditionsthat may be seen during manufacture,installation,or use.4.5 Bend testing may be destructive,therefore any samplestested should be considered unfit for future use.1This test method is under the jurisdiction of ASTM Committee F01 onElectronics and is the direct responsibility of Subcommittee F01.18 on PrintedElectronics.Current edition approved June 1,2015.Published July 2015.DOI:10.1520/F3147-15Copyright ASTM International,100 Barr Harbor Drive,PO Box C700,West Conshohocken,PA 19428-2959.United States1 5.Interferences5.1 The following parameters may affect the results of thistest:5.1.1 Temperature and humidity,5.1.2 Mandrel diameter,5.1.3 SMD type,size,orientation to mandrel,5.1.4 Weight applied,5.1.5 Land-pad design and proper application of adhesives,and5.1.6 Lack of 48-h conditioning at room temperature(UV-cure encapsulant continues to cross-link after cure cycle).6.Apparatus6.1 Mandrel,allowed to rotate smoothly around its longitu-dinal axis,rigid,low friction smooth surface.Diameter to bespecified.6.2 Electronic test fixture,to monitor function of SMDsbefore and after bending(and during bend if applicable).6.3 Weights(mass),to hold switch onto mandrel in con-trolled force.6.4 Fixture,to hold test sample securely in place in avertical manner(see Fig.1).FIG.1 Bend CycleF3147 152 6.5 Mechanism,capable of providing a consistent force o