Inspection of Conductive Adhesive Surface Mount Joints

Transcription

1 Inspection of Conductive Adhesive Surface Mount Joints Bob Willis Electronics Academy Webinar Presenter Your Delegate Webinar Control Panel Open and close your panel Full screen view Submit text questions during or at the end

2 FREE Electronics Academy Webinar Series The Electronics Academy Webinar Series takes an in depth look at the issues affecting PCB/SMT assembly and the challenges of achieving Zero Defect Manufacture. Understand the common causes of solder joint failure and learn how to identify and rectify process defects improving quality and reducing costs Learn expert tips to identify quality issues Understand the common causes of process failures A convenient and quick way to update your skills Have access to a video library of online training sessions Electronics Academy Webinar Series Future Vision Engineering webinars may cover Solder paste and stencil inspection Microsection inspection Inspection of underfill and staked components Destructive solder joint assessment and inspection To download slide presentations and watch a video of previous webinars Solder Joint Inspection & Process Defects Counterfeit Component Inspection & Detection Conformal Coating Inspection & Coating Faults Crimp Connector Inspection & Quality Control Printed Circuit Board Inspection & Quality Control Go to academy webinar series

3 Bob Willis Bob Willis has been involved with the introduction and implementation of lead-free process technology for over ten years. He received A SOLDERTEC/Tin Technology Global Lead-Free Award for his contribution to the industry, helping implementation of the technology. Bob has been a contributor to Global SMT magazine since the launch of the magazine. He was responsible for co-ordination and introduction of the First series of hands-on lead-free training workshops in Europe for Cookson Electronics during These events were run in France, Italy and the UK and involved lead-free theory, hands-on paste printing, reflow, wave and hand soldering exercises. Each non commercial event provided the first opportunity for engineers to get first hand experience in the use of lead-free production processes and money raised from the events was presented to local charity. He co-ordinated the SMART Group Lead-Free Hands On Experience at Nepcon Electronics This gave the opportunity for over 150 engineers to process four different PCB solder finishes, with two different lead-free pastes through convectionandvapour phase reflow. He also organised Lead-FreeExperience2, in He has also run training workshops with research groups like ITTF, SINTEF, NPL & IVF in Europe. Bob has organised and run three leadfree production lines at international exhibitions Productronica, Hanover Fair and Nepcon Electronics in Germany and England to provide an insight to the practical use of lead-free soldering on BGA Ball Grid Array, CSP Chip Scale Package, 0210 chip and through hole intrusive reflow connectors. This resulted in many technical papers being published in Germany, USA and the United Kingdom. Bob also defined the process and assisted with the set-up and running of the first Simultaneous Double Sided Lead-Free Reflow process using tin/silver/copper for reflow of through hole and surface mount products. Bob also had the pleasure of contributing a small section to the first Lead-Free Soldering text book Environment - Friendly Electronics: Lead-Free Technology written by Jennie Hwang in The section provided examples of the type of lead-free defects companies may experience in production. Further illustrations of lead-free joints have been featured in here most recent publication Implementing Lead-Free Electronics He has helped produce booklets on x-ray inspection and lead-free defects with DAGE Industries, Balver Zinn and SMART Group. Most recently creating defect guides on conformal coating and cleaning for SMART Find out more at: Bobwillis.co.uk Mr Willis led the SMART Group Lead-Free Mission to Japan and with this team produced a report and organised several conference presentations on their findings. The mission was supported by the DTI and visited many companies in Japan as well as presenting a seminar in Tokyo at the British Embassy to over 60 technologists and senior managers of many of Japans leading producers. Bob was responsible for the Lead-Free Assembly & Soldering "CookBook" CD-ROM concept in 1999, the world s first interactive training resource. He implemented the concept and produced the interactive CD in partnership with the National Physical Laboratory (NPL), drawing on the many resources available in the industry including valuable work fromnpl and the DTI. This incorporated many interviews with leading engineers involved with lead-free research and process introduction; the CD-ROMis now in its 3rd edition Bob helped to create the NPL Defect Database of soldering and assembly defects available 24/7 in addition he has created over thirty Defect of the Month videos featured on the IPC and Circuits Assembly websites Typical Soldered Joints Conductive Adhesive Joints Chip resistor SOT23 SOT89

4 Benefits of Conductive Adhesives Compatibility with different surfaces Low temperature curing Reduced pad sizes Low VOC No flux residues Do not leach other metals No lead Disadvantages of Conductive Adhesives Lower mechanical strength No component realignment Selected surface finish compatibility Higher electrical resistance Changes in resistance over time with humidity More difficult to rework components Higher costs

5 Text Books & Reference Documents The books and IPC specification provide some useful information on the subject and its introduction Two types of electrically conductive adhesives Isotropic Conductive Adhesive (ICA) These materials are conductive in all directions, pressure is normally applied to compress conductive particles in the adhesive to make the electrical connection between two conductive points Anisotropic Conductive Adhesive (ACA) These materials conduct in one direction and do not normally require pressure. They are available as two component adhesives requiring mixing or pre-mixed single part adhesive which are frozen at low temperature to prevent the cure reaction There are many different conductive fillers which have been evaluated by suppliers but silver is still most common for component assembly. Copper would be cheaper but has not provided the same performance as silver

6 Conductive Adhesives Conductive adhesive application The materials can be applied by jetting, dispensing or stencil printing. Whichever process is used consideration must be given to the pot life after the material has reached application conditions defined by the supplier. This is true for single and two part adhesives As the material is expensive all waste should be considered like any other process Conductive Adhesives Curing/bonding process After application of adhesive and placement of components curing is conducted between o C with specific time/temperature profile defined by the supplier. Although we are not reflowing It's good practice to run a temperature profile to confirm the correct parameters are being used rather than having the components ping off your board. Basically the same good practice for reflow profiling and thermocouple mounting needs to be followed I have had this happen to me at a major exhibition

7 Assembly with Conductive Adhesive There are two general types of adhesives, isotropic which conducts in all directions and anisotropic which only conducts in one direction. The anisotropic adhesive can be used for connecting surface mount components to rigid or flexible circuit boards Assembly with Conductive Adhesive Component Termination Conductive Adhesive Copper Pad PCB

8 Assembly with Conductive Adhesive Conductive Adhesive Circuit Pattern Conductive Particles Isotropic which conducts in all directions and is used on interconnections like a PCB to flexible circuit or a flex to a screen display. These materials require pressure during the curing process Assembly with Conductive Adhesive Conductive Adhesive Circuit Pattern Conductive Particles When the adhesive is compressed there is an electrical connection between the pad connections on opposite parts. However there is no connection between the surface pads on the substrate

9 What substrate do you mostly use with conductive adhesives? Rigid fibre glass PCBs Ceramic Circuits Flexible circuits Don't use conductive adhesive for SMT Conductive Adhesives Thermoset When the material is heated and cured it does not soften if reheated Thermoplastic When heated the adhesive softens allowing joints to be rework but not with the same ease as with solder

10 Conductive Adhesives The conductive material is either a fine powder or a flake suspended in bulk insulating material. Filler materials are silver, copper, gold, nickel, palladium, carbon, however, silver is the most well research conductive filler used. Conductive adhesive has approximately 50% metal by volume and 80% by weight The adhesive is shipped dry packaged by the supplier in this case (Henkel) often with Gel Packs or Dry ice which provides protection for 4 5 days. When the shipment is received the material boxes inside the shipping box and place the material mediately into a freezer for storage at 18/25 o Cor 40 o C depending on specification Only thaw the adhesive when required for use, it must be thawed naturally at room temperature and will take Conductive Adhesives SEM image of conductive flakes which are approximately 2 3um in size Image curtesy of Henkel SEM image of the surface of a joint where the component has been separated and we are looking at the joint surface. Image curtesy of Henkel

11 Metal Filler & Conductivity Conductivity Filler Percentage Reference Henkel presentation Conductive Adhesives Characteristic SnPb Solder Adhesive Volume resistivity ohm cm ohm cm Typical junction R 10 15m <25m Thermal 30w/m deg.k 3 5w/m deg.k Shear strength 100% % of solder Shock Good Poor 85%/85degC No change Material specific Fine pitch 12 mil Down to 6 mil Minimum process degc degc temperature Environmental impact Negative Minor Thermal fatigue Yes Can be minimal Table from Dr Ken Gilleo originally with Cookson Electronics

12 Comparison of Material Properties Thermal Properties Material Thermal Coefficient of Expansion ppm/c Sn/Pb Solder 25 Sn/Ag/Cu Solder Ceramic Chip Component 5 7 Conductive Epoxy Epoxy Fiberglass PCB Flexible Circuit Connections Early flexible circuit assembled with surface mount components has a conductive adhesive to form the electrical connection. The encapsulation then provides mechanical support and environmental protection Some users have used non conductive adhesives under components to hold parts on the substrate to increase the total strength and place less mechanical load on the conductive adhesive during testing. The non conductive adhesive cures at the same time as the conductive material

13 Conductive Adhesive Joint Examples Conductive adhesive joints on chip component terminations. The microsection image comes from one of the NPL reports listed in this presentation and available to download free online Design & Layout Modification You can afford to reduce the pad size on the board design and with it the solder mask. The pads required for conductive adhesive do not have to allow solder to wet out. In the case of conductive adhesives the electrical performance and the mechanical strength of the joint are defined by the area of contact The most common PCB surface finish for conductive adhesives is gold over nickel as this has provided the lowest resistance. Other finishes are used in industry. Still the best performance for components is non tin finishes due to oxide formation during high temperature or humidity testing In the case of the stencil design the opening can be just slightly larger than the termination area to contact the pad. Having a larger surface area does not increase the bond strength, it only makes up for misplacement of component termination tolerances. Placement of parts should not be an issue with modern placement systems

14 Design & Layout Modification Excessive pad size Improved pad size Stencil Printing The adhesive volume printed does not change like solder paste which loses 45 50% of its volume so a thinner stencil may be used. It is dependent on the variation on component termination height or coplanarity. A stencil thickness of 100um and down to 75um may be used, in imperial measurements that is Prior to printing the boards must be clean. Care needs to be taken to avoid any problems of contamination before the adhesive is printed Take care during set up and run test prints before printing production boards. Cleaning the adhesive off as with a poor print is not recommended and not like cleaning solder paste

15 Stencil Printing Mesh screens have been used but standard thin metal stencil are used for printing conductive adhesive. Some years back a different approach was taken to use a electroform stencil with a mesh pattern to support the squeegee blade during printing. The video shows some example print strokes and the fillingofthe opening. This is all dependent on the materials beingprinted Component Placement Standard placement systems Need to maintain rigid and flexible PCBs flat Eliminate substrate vibration Monitor components for CA applications Good placement forced control Its important that the surface area of adhesive contact is made with the bottom of the component termination. This will impact resistance, jointstrength and reliability

16 Component Terminations Adhesive bond strength is based on the area of contact on the component termination and the number of termination so with the examples shown for just one of the component terminations which would probably have the greater bond strength? Component Placement

17 Component Placement With the SOT23 we can see the leads have been placed into the adhesive deposit successfully but the lead form does not have much surface area in contact with the base of the pin. In addition the adhesive is not separating cleanly from the stencil corners Dog ears at the end of the print deposit have no impact on the CA but indicate that the material is not separating. The size of the pads used in this example are excessive and do not provide better joint strength or electrical performance Stencil Printed Deposit The adhesive deposit should be even and flat and should cover the same area as the designed opening in the stencil or screen. The adhesive should not bleed out over the pads or cover the laminate surface unless by design. The adhesive should not be misplaced, misplacement and placement error

18 Component Placement Misplaced components do not realign during curing like reflow soldered joint do. The component termination after placement remain in the same position and the strength and the electrical resistance is based on the contact area Component Placement If the adhesive is around the complete lead then this may increase the joint strength. If the adhesive deposit is not in contact with all of the base of the lead the strength may be less and the resistance higher

19 Curing Conductive Adhesive Each supplier provides a temperature profile or set curing temperatures for their product. Just like reflow soldering a temperature profile should be defined and run in production There have been examples where adhesive has been used on a double sided assembly. First side paste reflow then second side low temperature. However with low temperature solder paste it s a cheaper option Curing Conductive Adhesive Making sure the cure is correct is important, too higher cure can result in low mechanical strength and changes in resistance. The profile shown was part of a NPL project on conductive adhesive use

20 In Process Testing Adhesive Bond CSP CSP + CSP + CSP BGA BGA BGA BGA CSP CSP + CSP + CSP BGA BGA BGA BGA Add test pads to the board or the waste strip and printed with other pads. If a process test is required components are placed before cure then shear tested for comparison with previous results In Process Testing Adhesive Bond Optical inspection Electrical test Resistance change Shear test Inspection of failure

21 Shear Force Measurement The results shown are based on testing one material after exposure to different aging of tin plated components. Shear force measurements are taken from one of the NPL Reports highlighted in this presentation Optical & X Ray Inspection To date there are no guidelines for the inspection of conductive joints. The basic criteria discussed is guide based on trials conducted with a range of products from the industry X ray inspection may be used to look at voids, gaps created during the placement process rather than curing. Gaps can change the resistance value of the joints formed Happy to hear from anyone who has created inspection criteria or using some specific guidelines??

22 Optical & X Ray Inspection Optical & X Ray Inspection Optical and x ray example joints which show contact with adhesive to the side of the device. The x ray shows no gaps or voids under the termination of the device

23 Conductive adhesives, what is your biggest assembly challenge? Printing Placement Curing Rework Inspection Reworking Joints Many adhesive joints can be reworked by heating the joints and displacing the component terminations. After the component is removed the remaining adhesive on the pads may need to be scraped from the surface at elevated temperature to all component replacement, particularly on multi leaded components This allows a new adhesive deposit to be added to the pads for component replacement. Some materials are more difficult to completely remove with possible damage to the pad. In this case the component is placed in to fresh adhesive manuallydispensed on the existing surface and cured If components are being replaced low temperature solder could be used, quicker and easier to do provided the pads are cleaned and will wet

24 Reworking Joints Example video shows the rework of chip components. A soldering iron is used to heats up the part. The tweezers are then used to lift the component clear from the board while the component and adhesive are still hot Reworking Joints Example video shows the rework of chip components. Conductive/Hot tweezers are used to heats up and the part separates from the adhesive. The tweezers are the best way to rework selected conductive adhesives

25 Reworking Joints There are applications where non conductive adhesive or a coating have been placed on joints and components to provide more mechanical strength. This also provides some environmental protection to the joints and adhesive from moisture This combination of materials may also make rework difficult. Chemicals have been used on ceramic substrates but would not be suitable for most standard epoxy laminates of flexible circuits. It adds to the cost and complications. Abrasives could be used as we do for conformal coating materials Reworking Joints After removing the components you may have to remove residues on the pads before adding more adhesive, placing parts and curing the new adhesive. Alternatively the pads could ne cleaned and low temperature solder used

26 Joints Performance The following are some comparative results before and after temperature cycling of joints. The results will be different between materials used and the plated surfaces being bonded. The bond strength may be independent of the resistancevaluebetweentheconductiveadhesiveandthe component termination What is the biggest challenge in using conductive adhesive? Temperature/Humidity test performance Component termination compatibility Conductivity performance Storage/Use of conductive adhesive Cost of conductive adhesive

27 NPL Reports on Conductive Adhesives Selected reports from NPL on conductive adhesives to review all of the reports and download free go to NPL Reports on Conductive Adhesives Conductive Adhesives Versus Lead Based Solders: A Comparison of Performance 2000 Test Method for Recyclable Electronic Substrates Utilising Additive Technology 2002 Test Method for Recyclable Electronic Substrates Utilising Additive Technology 2002 Developing a Stress Screening Regime for Isotropic Electrically Conductive Adhesives 2004 Relative Reliability Measurements for Electrically Conductive Adhesive Joints on Subtractive Thermoplastic Substrates 2007 Test Method for Recyclable Electronic Substrates Utilising Additive Technology 2008 download free reports go to

28 NPL Joints Performance Conductive adhesive was printed with a 75um laser cut stencil NPL Performance Results download free reports go to

29 NPL Performance Results NPL Performance Results

30 Alternative Low Temperature Option In the last few years there has been growing interest in low temperature solder paste which can be reflowed and reworked. These products can be just like other solders but reflow at temperature below tin/lead alloys. Also they are a lower cost than conductive adhesives This has allowed reflow with components of a low soldering temperature specification. This is one of the major benefits of adhesives but at lower cost Low Temperature Lead-Free Solder Alloys Solder joint section left is reflowed on copper OSP, image on right on Sn63 HASL board leading to the large grain structure with the lead in the joint Reference Hewlett Packard Company

31 Low Temperature Lead-Free Solder Alloys Solder paste reflowed successfully with copper OSP surface finish, similar results have been found with nickel gold with the low temperature alloys Electronics Academy Webinar Series Future Vision Engineering webinars may cover Solder paste and stencil inspection Microsection inspection Inspection of underfill and staked components Destructive solder joint assessment and inspection To download slide presentations and watch a video of previous webinars Solder Joint Inspection & Process Defects Counterfeit Component Inspection & Detection Conformal Coating Inspection & Coating Faults Crimp Connector Inspection & Quality Control Printed Circuit Board Inspection & Quality Control Go to academy webinar series

32 Do you have any questions? Bob Willis bobwillis.co.uk