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A Study of Electrically Conductive Adhesives as a Manufacturing Solder Alternative F M COUGHLAN1and H J LEWIS1 2 1 Department of Manufacturing and Operations Engineering University of Limerick Castletroy Limerick Ireland 2 E mail huw lewis ul ie Soldering processes using tin lead solder are standard interconnection technologies for electronic manufacturing These processes are currently under threat from the Waste Electrical and Electronic Equipment and the Restriction of Hazardous Substances RoHS environmental directives issued by the European Union in 2000 These directives explain that solder is to be free from lead by 2006 as lead has been recognized by the European Union as an environmentally harmful material One solder alternative that has been investigated by the electronics industry is the area of electrically conductive adhesives ECAs This paper outlines the electrical and mechanical analysis of two isotropic conductive adhesives where the main properties of joint resist ance and adhesive strength were examined before and after different environ mental treatments Joint resistance was measured with a four probe tester and adhesive strength was examined with the use of shear testing Cross sectional and scanning electron microscopy analyses were used to determine problems such as oxidation and moisture absorption that may have an affect on the adhesive properties of the connection An experimental design was carried out to examine the adhesives on a standard production line Taguchi analysis was used to determine the build parameters required to produce an optimal adhesive joint The results show that although ECAs can pass a func tionality test the electrical properties of ECAs are inferior to that of solder with ECAs having a high joint resistance They also exhibit poor mechanical strength when shear or drop tested and illustrate that current visual inspec tion techniques used to examine solder joints are not applicable to conductive adhesives Key words Electrically conductive adhesives ECAs lead free solder printed circuit board INTRODUCTION On June 13 2000 the European Union adopted two proposals the Waste Electrical and Electronic Equipment and the Restriction of Hazardous Sub stances RoHS 1to deal with the problem of envi ronmental pollution in Europe This led to the development of Directive 2002 95 EC of the Euro pean Parliament and the Council on RoHS which explains that from July 1 2006 member states shall ensure that electrical and electronic equipment does not contain materials such as lead cadmium and mercury This has resulted in the elimination of lead based solder from electronics and has driven the industry to explore alternatives such as electri cally conductive adhesives ECAs Electrically conductive adhesives consist of a poly mer binder and conductive fi ller and can be cate gorized as either isotropic or anisotropic Isotropic adhesives conduct in all directions and have been used in industry primarily as die attach adhesives Anisotropic adhesives provide unidirectional electri cal conductivity in the vertical or Z axis and require simultaneous heat and pressure to cure 2 Themostcommerciallyavailableconductive adhesives suitable for screen printing are isotropic adhesives consisting of epoxy binder and silver fi ller Epoxy is a thermosetting material which is very diffi cult to rework since it does not melt when reheated However conductive adhesives developed from this type of polymer exhibit higher shear Received July 12 2005 accepted October 24 2005 Journal of ELECTRONIC MATERIALS Vol 35 No 5 2006Regular Issue Paper 912 JOBNAME jem 35 5 2006 PAGE 1 OUTPUT Wednesday May 3 12 28 27 2006 tms jem 117584 1593 R4 strength than those from thermoplastics 3In addi tion thermosetting materials take less time to cure which is essential to production Silver tends to be the most common fi ller metal Although it is an expensive material the most important property of silver is its ability to retain high conductivity as the silver particles oxidize 4 The two main electrical characteristics of conduc tive adhesives are volume resistivity and joint resistance Volume resistivity is an adhesive prop erty and is measured in Vcm Joint resistance is the resistance measured through an adhesive between two metal adherents and is measured in mV s Conduction within the adhesives can occur in a variety of ways In ICAs it is generally by direct metallic contact between the fi ller particles but conduction can occur by other methods such as thermionic emission 5 In this study an experimental design was used to examine the manufacture of two isotropic conduc tive adhesives Taguchi analysis was utilized to determine the optimal manufacturing parameters required to produce a functional joint with high shear strength The adhesives were then reliability tested with the use of different environmental treatments Cross sectional analysis with the use of scanning electron microscopy micrographs was used to examine joint degradation and component cracking EXPERIMENTATION Materials Two commercially available adhesives with the ability to be screen printed were selected for this study Table I The adhesives utilized a short cur ing schedule ideal for manufacturing and a low volume resistivity However one disadvantage of using these adhesives is the problems experienced when they are used with tinned surfaces It has been reported that the joint resistance between a conductive adhesive and a non noble metal in creases dramatically during elevated temperatures humidity aging 85 C 85 RH and temperature cycling 40 to 1125 C 6However although this increase particularly applies to adhesives with sil ver fi ller fl akes and tinned surface fi nishes the work by Li and Morris7claims that this can also be applied to bare copper terminations Thus due to the popularity of tinned surface fi nishes in elec tronics and the aim of examining commercially available adhesives in a typical manufacturing envi ronment the following adhesives were utilized for this research Manufacturing Parameters In the manufacture of adhesives the choice of stencil and squeegee used in the printing process is pertinent to a good adhesive joint Stencil thick ness should be smaller for ECAs than for solder as a thinner joint is favorable The typical stencil thick ness is 0 10 0 15 mm but is dependent on applica tion Contact printing is preferred over noncontact A metal squeegee is preferred as rubber squeegees are too soft and tend to scoop out the adhesive from the stencil apertures during printing Recommended squeegee speed varies from 45 mm s to 75 mm s as does the angle from 20 to 45 8 In component placement the effect of placement pressure on an adhesive is important since ECAs have a low viscosity which reduces further during curing and do not self align or contract Too high a pressure may cause the adhesive to fl ow out and create a short circuit after curing Too low a pres sure may cause a high joint resistance in the connection 8 In curing extension of the schedule times has been recommended to ensure a complete cure Research carried out by Rorgren and Liu9sug gested that a schedule chosen for a particular ECA should greatly exceed the minimum times recom mended by the supplier This will ensure that lag effects caused by thermal mass of the assembled board will be reduced and the risk of undercuring minimized Using these parameters the following manufac turing test was designed Manufacturing Test The test was carried out on a standard production line in an automotive electronics plant The test vehicle was a single sided multilayer standard PCB with tinned tin solder layered pads Figs 1 and 2 The components were tinned resistor 0805 chips The tool used to determine the different factors that affect the use of an ECA as a connection medium was an L18 Taguchi design Table II Although component placement is an important factor in adhesive joint formation this factor could not be examined as part of the experiment This was due to the inability of the placement pressure to be changed for the experiment Each machine used however was examined prior to the experiment to ensure process stability Initial test runs carried out on the component placement machine indicated that the mounting force was suffi cient to produce an acceptable adhesive joint Table I ECAs Examined Name Emerson and Cuming Ablebond 8175A Emerson and Cuming Ablebond 8175 TypeIsotropicIsotropic FillerSilverSilver BinderEpoxyEpoxy Viscosity55 000 cps70 000 cps Volume resistivity 0 5 mVcm0 5 mVcm Cure condition0 5 h at 175 C3 min at 150 C JOBNAME jem 35 5 2006 PAGE 2 OUTPUT Wednesday May 3 12 28 29 2006 tms jem 117584 1593 R4 A Study of Electrically Conductive Adhesives as a Manufacturing Solder Alternative913 This was determined by visual inspection and with reference to IPC standards 10Tables III and IV outline the machine parameters and experimental factors used Evaluation Procedures For the purpose of the experimental design two properties joint shear strength and board function ality were examined as the experimental responses Table V Shear strength was measured with an Instron Tensometer model 1011 at the University of Limerick The standard shear strength for sol der used in electronic applications is approximately 3 KgF11and was the standard against which each adhesive joint was measured Board functionality was measured at the system test station on the production line where each PCB was individually examined For experimental purposes the measure ment taken was the percentage pass rate per panel For example if six out of seven boards on the panel were functional the pass rate was 86 if only fi ve were functional the pass rate was 71 These pass rates were applied to the design of experiment which yielded the following results Experimental Results The boards built for the experiment indicated the ability of conductive adhesives to produce fully functionalcircuitboards However theyalso highlighted the poor mechanical strength of the adhesive joints Although tests by Li et al 7and Boyle et al 12have indicated typical shear strength values with chip resistors between 1 KgF and 4 KgF these joints indicate the inability of the adhe sives to pass the recommended industrial standard of 3 KgF Both the functionality and shear strength values were applied to the design of experiment Tables VI and VII1 The values in the table indicate the level at which each factor should be run to produce an optimal joint To decide on a level the highest value of each factor is selected These results indicate that for optimal function ality and shear strength adhesive 8175A is the most favorable For both properties a cure schedule similar to that recommended by the manufacturer is more desirable which is in contrast to research carried out by Rorgren and Liu In addition higher speeds with lower forces tend to produce better results Table VIII However an analysis of variance ANOVA car ried out on the above results Table IX indicated that the type of adhesive tends to be the only factor that has a signifi cant affect on either property especially shear strength The results fail to show the effect of the other factors The following table Fig 1 Test vehicle Table II Experimental Parameters Array L18 Taguchi Including Repeats LevelsThree FactorsAdhesive type Squeegee speed Squeegee force Cure profi le ResponseBoard functionality Shear strength of joint PCB amount252 Panel amount36 Fig 2 Test vehicle closeup Table III Machine Parameters StencilMetal Apertures size100 Stencil thickness0 15 mm PrintingContact Squeegee angle45 Curing ovenConvection static Table IV Experimental Factors Level 1Level 2Level 3 ECAs8175A8175 Speed15 mm s45 mm s75 mm s Force96 N147 N196 N Schedule 8175A20 min30 min40 min 81753 min6 min9 min JOBNAME jem 35 5 2006 PAGE 3 OUTPUT Wednesday May 3 12 28 30 2006 tms jem 117584 1593 R4 Coughlan and Lewis914 highlights the F and P values of a set of one way ANOVA tables The confi dence limit selected was 5 or a p value 0 005 This experiment has highlighted that although printing parameters such as speed and pressure play an important role in the fi nal structure of the adhesive joint neither has a signifi cant affect on the strength or functionality of the joint In addition the curing schedule has no signifi cant affect of either electrical or mechanical property For ECAs to be accepted as a solder replacement eachadhesivemustconformtothefollowing guidelines 11 d The volume resistivity of the ECA must be lower than 1 mV d A shift in joint resistance of less than 20 after 500 h humidity aging 85 C 85RH d Pass six drops from 154 2 cm onto a vinyl fl oor Both ECAs pass the fi rst specifi cation with each having a volume resistivity of 0 5 mmV To examine the other characteristics reliability testing was used Reliability Testing After manufacture the reliability of the adhesive joints was examined under different environmental conditions such as humidity aging temperature cycling and temperature storage The main param eters examined were adhesive joint structure joint resistance and shear strength Adhesive Joint Structure To examine the structure of the adhesive joints a selection of fi llets were cross sectioned and analyzed in accordance with IPC standards IPC A 610 typi cally used for the examination of the structural properties of solder joints These standards were used here as a general guideline due to the lack of structural standards for electrically conductive adhesives For experi mental purposes the joint structure was examined before and after environmental treatments Joint Resistance The joint resistance of an ECA is the resistance of theadhesivebetweentheleadandthepad Although a direct electrical property of the adhe sive its value may be affected by factors such as moisture absorption after humidity aging interfa cial cracking due to thermal stress and epoxy deg radation after storage at elevated temperatures Joint resistance is usually measured with a four probe tester Figs 3 and 4 and for this study 60 joints were measured before and after environ mental treatments Adhesive Strength The shear strength of 48 and drop test of 30 com ponents were carried out to determine the ability of the adhesives to hold the chip components onto the pads when subjected to mechanical shock and a shear force The shear strength test is outlined in the Evaluation Procedures section and the drop test explained in the Experimental Results section Environmental Treatments Each environmental treatment was carried out on 20 adhesive joints 10 of ECA1 and 10 of ECA2 Table V Sample Test Results of Shear Strength Test PCB Shear Strength Component 1 KgF Shear Strength Component 2 KgF 11 972 0 22 072 10 32 042 06 Table VI Functionality Results LevelECAScheduleSpeedForce 194 589 4292 9196 5 291 3397 6791 7592 91 3 91 6794 0389 33 Delta3 178 272 307 17 Rank3142 Table VII Shear Strength Results LevelECAScheduleSpeedForce 12 0311 9621 9591 963 21 9121 9831 9831 970 3 1 9701 9701 965 Delta0 1180 0200 0240 020 Rank1324 Table VIII Manufacturing Parameters for Optimal Results PropertyFunctionalityShear Strength ECA8175 A8175 A Cure time30 min20 min Speed75 mm s45 mm s Force98 N147 N Table IX Set of Anova Results Property FunctionalityShear Strength F ValueP ValueF ValueP Value ECA0 830 37662 230 001 Cure time2 340 1300 150 863 Speed0 140 8740 300 744 Force1 510 2530 140 874 JOBNAME jem 35 5 2006 PAGE 4 OUTPUT Wednesday May 3 12 28 33 2006 tms jem 117584 1593 R4 A Study of Electrically Conductive Adhesives as a Manufacturing Solder Alternative915 Humidity Aging Humidity aging was carried out to examine any change in joint resistance due to oxidation of the joints and to examine the possible reduction in adhesive strength due to the absorption of moisture The test vehicles were stored in a Gohn Godrich static environmental chamber at 85 C 85RH for 500 h These parameters were chosen since a shift in joint resistance of less than 20 at 85 C 85RH after 500 h is required if conductive adhesives are to be considered a solder alternative Temperature Cycling To examine the effect of temperature cycling on the adhesives each test vehicle was stored in a Design Environmental FS470 655 cycling chamber at the University of Limerick A conceptual presen tation of the temperature cycling profi le used is illustrated in Fig 5 This was selected in accordance with MIL STD 202F method 107G 13with a slight modifi cation The lower temperature was reduced to 40 C rather than 65 C with a uniform soak and a ramp time of 15 min This is the typical profi le sel ected by Jagt14for the examination of adhesive joints Temperature Storage The aim of this treatment was to establish any change in joint resistance or degradation of adhe sive strength under high temperature conditions The test vehicles were stored in a standard oven Carbolite 7 435 436 at 125 C for 500 h These stor age parameters were selected in accordance with MIL STD 883E 15 Fig 4 Placement of probes on a chip Fig 3 Probing an adhesive Fig 5 Temperature cycling profi le Fig 6 8175A before treatment Fig 7 8175 after treatment JOBNAME jem 35 5 2006 PAGE 5 OUTPUT Wednesday May 3 12 28 34 2006 tms jem 117584 1593 R4 Coughlan and Lewis916 Reliability Test Results Adhesive Joint Structure To ensure good joint resistance and adhesion strength a high quality joint is required Due to the lack of structural standards for electrically con ductive adhesives the IPC standards selected were those used for the examination of the structural properties of a solder joint Figures 6 and 7 are examples of typical joints formed These joints indi cate an acceptable fi llet height but a thin bond line between the termination and the pad which may have attributed to poor bond strength Internal voiding is also visible Voiding is a characteristic of conductive adhesives and can only be reduced by degassing where the samples are heated under vacuum for a period Degassing requires