Investigation of Components Attachment onto Low Temperature Flex Circuit

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Roy Harvey
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1 Investigation of Components Attachment onto Low Temperature Flex Circuit July 2013 Q. Chu, N. Ghalib, H. Ly

2 Agenda Introduction to MIRA Initiative MIRA Manufacturing Platforms Areas of Development Multiphase Development Overview Phase 1 and Phase 2 Low Temp Flex Circuit Assembly Development Phase 3 Alternative Attachment Method Investigation Next Steps

Overview Phase 1 and Phase 2 Low Temp Flex Circuit Assembly

3 Introduction to MIRA Initiative Miniaturization Leveraging Manufacturing Platforms for Innovation Integration Ruggedization Automation 3

4 Miniaturization enabling improved performance in less volume. MIRA Manufacturing Platforms Integration providing greater diversity of enhanced functionality. Ruggedization delivering unparalleled in field reliability. Automation assembling complex components efficiently and safely. 4

5 Bumped Die Flexible Substrates Standard Polyimide Substrates Low Temp Flex (Polyester) Substrates Stretchable Substrate Components Standard Profile Components Thin Profile Components Thin Die (Passive Components) Thin Die (Active Components) Areas of Development Wire Bonded Std 305 SAC Attached Materials Adhesives Thermal Cure Moisture Cure UV/Thermal Cure Solder Alloys Low Temperatures Std LF Temperatures Low Cost Method of Attachments Convection Oven (Reflow/Batch) Alternative Method A Alternative Method B Alternative Method C Alternative Method D Rework Capability Reworkability Alternative Method E 5

Materials Adhesives Thermal Cure Moisture Cure UV/Thermal Cure Solder Alloys Low Temperatures Std LF Temperatures Low Cost Method of Attachments

6 Multiphase Development Overview Phase 1 Phase 2 Phase 3 Phase 4 Phase 5 Phase 6 Phase 7 Adhesive Materials Identification and Selection Material and Initial Process Characterization w/ Passive Components (i.e. Resistors) Investigate Alternative Attachment Process Adhesive Material Re assessment (Custom Blend?) Process Optimization w/ Passive Components Material and Process Characterization w/ Active Comp. (i.e. Thin Die CSP, Flip Chip, etc.) Reliability Testing Completed In Process 6

) Process Optimization w/ Passive Components Material and Process Characterization w/ Active Comp. (i.e. Thin Die CSP, Flip Chip, etc.

7 Phase 1 and Phase 2 - Low Temp Flex Circuit Assembly Development Proprietary and Confidential 7

8 Low Temp Flex Circuit Assembly Development Objective Characterize assembly process for attaching components onto low temp flex circuit

9 Development Process Flow Adhesive Selection Paper Qual of Electrically Conductive Adhesives (ECA) Down Select of ECAs Prescreen DOE Selection to 1 ECA Assembly Process Development (ECA) Application LED Placement ECA Cure (DOE) Post Cure Functional Test Glop Top Application Post Cure Functional Test

1 ECA Assembly Process Development (ECA) Application LED Placement ECA

10 Material Considerations Flex Circuit Test Board Dimensions: x 5.68 x Surface Finish: Ag ink Material: PET (Polyester) Components Top and Side Firing LEDs Flex Circuit mounted on SMT Pallet Electrically Conductive Adhesives ECA A ECA B ECA C ECA D

0105 Surface Finish: Ag ink Material: PET (Polyester) Components

11 Criteria Selection of Electrically Conductive Adhesive Low curing temperature Fast curing time Single part Screen printable Low resistivity Four adhesives were considered for prescreen evaluation One was selected based on adhesion and lowest cure properties for process characterization on flex circuit 11

Four adhesives were considered for prescreen evaluation One was selected

12 Rubber Squeegee as recommended by vendor resulted in unacceptable print deposits Squeegee scoops material from apertures during print Lessons Learned: Screen Printing Conductive Epoxy Metal squeegee blade resulted in more desired print volume Adhesive stencil life was found to be 4 hrs or less Higher frequency of stencil underwipe needed (after 2 pass)

Conductive Epoxy Metal squeegee blade resulted in more desired print volume Adhesive

13 Lessons Learned: Part Placement/Cure No self alignment properties Where you placed is where it will end up being after cure Placement force is critical Over driving the part down minimized lap joint (reduced mechanical strength) Two stage cure needed to not distort flex with selected adhesive Cure temp used was lower than vendor s recommendations to minimize distortion of flex circuit

lap joint (reduced mechanical strength) Two stage cure needed to not distort flex with selected

14 Lessons Learned: Handling Handling without the use of back support/pallets caused joints to crack at various interfaces Adhesive to lead terminal Within bulk adhesive Adhesive to pad Printed Ag ink pad to PET flex

various interfaces Adhesive to lead terminal Within

15 Lessons Learned: Reworking of Failed LED s Manual rework yielded positive results Achieved by dispensing adhesive over failed joints and then cure Draw back: Long cure time was needed

16 Summary of Observations Print Metal squeegee blade recommended Placement Misplaced parts remained misplaced after cure no self alignment Cure Current available off the shelf adhesive required longer cure time than stated due to Flex circuit deformation/degradation Handling Adhesive/Film assembly was sensitive to normal handling of flex circuit Rework Manual rework was possible, however, long cure time was required

cure time than stated due to Flex circuit deformation/degradation Handling Adhesive/Film assembly was

17 Phase 3 Alternative Attachment Method Investigation Proprietary and Confidential 17

18 Objective Investigate alternative solder attachment methods using a low temp substrate 18

19 Baseline Lesson learned from Phase 1 and 2 applied to plastic substrate test board using ECA material Assembly Results Electrically Conductive Adhesive (ECA) Good attachments observed Drawback: long cure time 19

Assembly Results Electrically Conductive Adhesive

20 Alternative Attachment Method A Inconsistent solder joints were observed post soldering process after multiple DOEs performed Only worked on NiAu surface finish Process not considered viable for mass production 20

multiple DOEs performed Only worked on NiAu surface

21 Alternative Attachment Method B Initial soldering of LF SAC 305 solder onto low temp plastic encouraging Surface temp of substrate during soldering reached 85C during solder attach Attachment of array packaging is still a challenge with method 21

22 Summary of Observations Repeatable results of components attached using ECA on Low temp plastic test board Alternative Method A was not successful and poses many challenges for mass assembly adoption Alternative Method B was found to be a via approach standard leadframe components but not for array packaging

23 Next Steps Phase 1 Phase 2 Phase 3 Phase 4 Phase 5 Phase 6 Phase 7 Adhesive Materials Identification and Selection Material and Initial Process Characterization w/ Passive Components (i.e. Resistors) Investigate Alternative Attachment Process Adhesive Material Re assessment (Custom Blend?) Process Optimization w/ Passive Components Material and Process Characterization w/ Active Comp. (i.e. Thin Die CSP, Flip Chip, etc.) Reliability Testing Completed Complete Eval of 3 Additional Methods Initial discussion with key vendor for special blend - <80C/<5min cure temp/time On hold Flex Test Board Designed/Fabbed 23

24 Thank You

Good Boards = Results

Section 2: Printed Circuit Board Fabrication & Solderability Good Boards = Results Board fabrication is one aspect of the electronics production industry that SMT assembly engineers often know little about.