QFN Design Considerations to Improve Cleaning A Follow on Study

Transcription

1 QFN Design Considerations to Improve Cleaning A Follow on Study Mike Bixenman, Kyzen Corporation Dale Lee, Plexus Corporation Bill Vuono, TriQuint Semiconductor Steve Stach, AAT Corporation The Science of Cleaning Green

2 Outline Introduction Previous Research Review Unanswered Questions New Testing Results Conclusions Recommendations

3 Introduction - Why Clean QFN s One of today s toughest parts to clean Floods the perimeter/streets with flux residue Un-cleaned flux residues trapped under the QFN Potential to be hydroscopic and conductive Can lower dielectric strength and result in dendrite growth May change the RF properties Dielectric strength Surface resistance Q-resonance

4 Problem Statement BGA 15-25μm MBGA μm CSP QFN 5-10 μm 2-5 μ m Flip Chip 1-2 μ m Decreasing Gap Height, Increases Cleaning Difficulty

5 QFN s Packages are Becoming Very Popular MELF QFN Introduced around

6 Many New Package Designs are Similar to QFN s Package Name Manufacturer DFN dual flat no-lead package Atmel cdfn TDFN UTDFN XDFN QFN QFN-TEP TQFN thin dual flat no-lead package ultra-thin dual flat no-lead package extremely thin dual flat no-lead package quad flat no-lead package quad flat no-lead package with top-exposed pad thin quad flat no-lead package ichaus LLP leadless leadframe package National Semiconductor LPCC leadless plastic chip carrier ASAT Holdings MLF micro-leadframe Amkor Technology and Atmel MLPD MLPM MLPQ DRMLF micro-leadframe package dual micro-leadframe package micro micro-leadframe package quad dual-row micro-leadframe package VQFN/WQFN very thin quad flat no-lead TI and others (such as Atmel)

7 Low Standoff Gap As the distance between conductors narrow Z-axis of the bottom termination reduces BTC gap heights less than 2 mils Create a vacuum effect during the soldering process Capillary action of the flux residue flows and bridges conductors Flux residue fills open gaps in QFN structures

8 Flux Residues Bridging Conductors

9 Partially Cleaned Flux Residues

10 Previous Research Review

11 Prior Studies Show Good Cleaning of QFN s Requires Long wash time Slow cycle times Slow production May create a bottle neck High Energy Many batch and inline cleaning machines do not deliver flow and pressure needed to clean under the QFN bottom termination

12 In 2014 s research, we: Research Purpose Studied QFN cleaning improvements as a function of Ground pad designs Solder mask definition strategies Via Holes within the ground pad Gap height Capillary forces

13 We Evaluated Various Ground Pad Designs to Look for Improvement in Cleaning Channels

14 We Evaluated Via Hole Designs Provide an avenue for: Flux residue to drainage under QFN Clearing air channels under package Increasing stand-off height Via Hole diameter mils

15 Evaluated the Solder Mask Designs for Cleaning Improvement? Solder Mask Defined? Non Solder Mask Defined No Solder Mask

16 Methodology

17 2013 s Test Vehicle

18 Test Vehicles Assembled at Raytheon Circuit Card Assembly Center of Excellence (CCA-COE) McKinney, TX Components delivered in Feeder tubes Transferred to a tape and reel format Compatibility with the placement equipment

19 Four solder stencils Design of Experiment 4 mil thick Nano-Coated Stencil for Square, Standard and Slot designs 3 mil thick Nano-Coated for Hexagon Prevent break-out of Hexagon pattern design

20 Standard Stencil Design

21 Advanced Square Design

22 Hexagon Stencil Design

23 Slot Stencil Pattern

24 Type 5 Solder Paste Multiple Solder Paste No-Clean Lead-Free Solder Paste Water Soluble Lead-Free Solder Paste

25 Test matrix Fully populated Cleaned Not Cleaned Test Vehicles Unpopulated boards without components View residues remaining from each stencil design Overall print quality

26 Test Board Processing Reflow Ramp-to-spike profile Peak temperature target of 235 C Identical for both pastes Inspect X-ray to determine Voiding from the manufacturing process Stencil/board combinations Clean Cleaning in inline cleaner High energy jets Inspect Visual per IPC J-STD-001 Part removal for under chip inspection

27 Response Variables 1) 5DX X-Ray Laminography 2) 2-D-X X-Ray Laminography 3) Microscopy of the Via Pattern on the Back Side of the Board 4) Side View Microscopy of QFNs Before and After Cleaning 5) Removal of the QFNs on Uncleaned and Cleaned Boards

28 Data Findings

29 5D X-ray Laminography

30 Agilent 5D X-Ray Laminography Inspect voiding of the ground pad A comparison of component placements Solder Mask Definition NSMD NoSM Via Holes

31 5D X-Ray No-Clean Data Findings Void formations reduced when Via holes were placed within the ground plain Voids were smaller in diameter when placing via holes into the ground plane Solid 9 Via Holes 25 Via Holes

32 Analysis of Variance Main Effects Plot for 5D X-Ray Ground Pad Voids Data Means Component Ground Plane Pattern % Ground Pad Voids Mean MLF124 Double Row Solder Mask Definition MLF88 Singe Row HEXAGON SLOT Via Holes SQUARE STANDARD NoSM NSMD Solid No Clean Data

33 2D X-ray Laminography

34 2D X-Ray No-Clean Laminography Nikon Metrology 2-D-X X-ray Images the internal structure Detect and compare voiding Data findings Via holes reduced void patterns Solid 9 Via Holes 25 Via Holes

35 Analysis of Variance Main Effects Plot for 2D Xray Ground Pad Voids Data Means % Ground Pad Voids Mean Component MLF124 Double Row MLF88 Singe Row Solder Mask Definition Ground Plane Pattern HEXAGON SLOT SQUARE STANDARD Via Holes NoSM NSMD Solid No Clean Data

36 Via Holes on Backside of Board

37 Via Hole Patterns on Backside 11 to 12 mils in diameter Via holes copper plated The purpose of the via holes Flux residue to drain to the opposite side of the board Air supplied through the via holes during reflow Repels the capillary forces that can form under the component

38 Via Hole Patterns on Backside 25 Via Holes 9 Via Holes

39 Component Identification

40 Formed Solder Bumps

41 Flux Residue % Flux Residue on the Bottom Side of Board M LF 124 Dual Row Main Effects Plot for Flux Residue Present Data Means Component Placement MLF88 S inge Row HEXA GON NoSM Ground Plane Pattern S LO T SQ U A RE Solder Mask Definition NS MD STA NDA RD

42 Flux Residues as a Function of Via Holes

43 Flux Residues as a Function of Solder Mask Definition Interaction Plot for Flux Residue Present Data Means Flux Residue Mean Values Solder Mask Definition NoSM NSMD Placement

44 Flux Residues as a Function of Ground Pad Design Flux Residue Mean Values Interaction Plot for Flux Residue Present Data Means Ground Plane Pattern HEXAGON SLOT SQUARE STANDARD Placement

45 Solder Bumps as a Function of Ground Plane Interaction Plot for Formed Solder Bumps Data Means Solder Bumps Formed Under Component Ground Plane Pattern HEXAGON SLOT SQUARE STANDARD Placement

46 Solder Bumps as a Function of Solder Mask Definition Interaction Plot for Formed Solder Bumps Data Means # of Solder Bumps Formed on Via Holes Solder Mask Definition NoSM NSMD Placement

47 Solder Bumps as a Function of Via Holes Interaction Plot for Formed Solder Bumps Data Means Solder Bumps as a Function of Via Holes Placement Via Holes 9 25

48 Via Hole Data Finds 1. Flux residues in the streets was reduced 2. The solder on the ground plane tended to coalesce and fill many of the via holes 3. By draining the flux to the opposite side of the board, the levels of flux residues was reduced in the streets

49 QFN Gap Height

50 QFN Gap Height Main Effects Plot for Gap Height (µm) Data Means 110 Component Ground Plane Pattern 100 Gap Height (µm) Mean MLF124 Double Row MLF88 Singe Row Solder Mask Definition HEXAGON SLOT SQUARE Via Holes STANDARD NoSM NSMD

51 Standoff Gap as a Function of Solder Paste Thickness Interaction Plot for Gap Height (µm) Data Means Gap Height (µm) Mean Ground Plane Design 25 Via Holes 9 Via Holes Solid HEXAGON SLOT SQUARE Ground Plane Pattern STANDARD

52 Less than 2 Mils Standoff Gap

53 Greater Than 4 mil Standoff Gap

54 QFN Gap Height Data Findings Flux residue under the bottom termination is a function of Attractive and repulsive capillary forces When the Z-Axis is less than 2 mils Flux residue capillary forces attract during reflow Heavy flux residue deposits accumulate in the Streets Interconnecting pads Attractive force renders Significant level of flux residue Underfills component with flux residue Flow channels closed Repelling force render Low levels of flux residue Flow channels open

55 Removal of Components to Inspect for Residues

56 Flux Residue Under QFN Test Board 48 QFN patterns Strategically placed MLF 88 Component Placements MLF 124 Component Placements

57 Component Removal

58 Not Cleaned Boards MLF 88 dirty component example

59 Not Cleaned Boards MLF 124 Dual Row dirty component example

60 Flux Residues Bridging Pads Main Effects Plot for Flux Residue Bridging Pads Data Means Component Ground Plane Pattern Flux Residue Means MLF124 Dual Row MLF88 Singe Row Solder Mask Definition HEXAGON SLOT SQUARE Via Holes STANDARD NoSM NSMD

61 Flux Residues in Streets Main Effects Plot for Flux Residue in Streets Data Means Component Ground Plane Pattern Flux Residue Means MLF124 Dual Row MLF88 Singe Row Solder Mask Definition HEXAGON SLOT SQUARE Via Holes STANDARD NoSM NSMD

62 Flux Residue Next to Pads as Function of Solder Paste Thickness Interaction Plot for Flux Residue Bridging Pads Data Means Solder Paste Thickness (mils) 3 4 Flux Residue Mean MLF124 Dual Row MLF88 Singe Row Component

63 Flux Residue in Streets as a Function of Solder Paste Thickness Interaction Plot for Flux Residue in Streets Data Means 0.30 Solder Paste Thickness (mils) 3 4 Flux Residue Mean MLF124 Dual Row MLF88 Singe Row Component

64 Cleaning Parameters Cleaned Boards Inline Cleaning Machine Progressive Energy Dynamics using Coherent Jets 5 minute wash cycle Aqueous Engineered Cleaning Agent 20% Concentration 65 C Wash Temperature The data finds Minor levels of no-clean flux residues No wet water soluble flux residues Some white ghosting on solder

65 No-Clean Flux Residue Next to Pads Main Effects Plot for Flux Residue Next to Pads - Cleaned Boards Data Means Component Ground Plane Pattern Flux Residue Mean MLF124 Dual Row MLF88 Singe Row Solder Mask Definition HEXAGON SLOT SQUARE Via Holes STANDARD NoSM NSMD

66 No-Clean Flux Residue in Streets Main Effects Plot for Flux Residue in Streets - Cleaned Boards Data Means Component Ground Plane Pattern Flux Residue Mean MLF124 Dual Row MLF88 Singe Row Solder Mask Definition HEXAGON SLOT SQUARE Via Holes STANDARD NoSM NSMD

67 Water Soluble Example

68 Inferences From Data Findings

69 Voiding Voiding is reduced when Via holes are placed into ground plane Results in smaller and less voiding

70 Beneficial Properties Via Hole Inferences Lower levels of flux residue under bottom termination Air flow and exhaustion Drainage Crack resistance Drawbacks Via holes reduce gap height Formation of solder bumps on back side Rework challenges

71 Gap Height Inferences During reflow Molten solder Flux Capillary forces can Attract or repel Dependent on forces of Adhesion Cohesion Surface tension

72 Attractive Flux Capillary Forces Absence of air flow and exhaustion during reflow Creates an attractive flux capillary force Forms a vacuum Heavy flux deposits underfill and dam the bottom termination

73 Repelling Capillary Force High standoff gaps Air flow and exhausts during reflow Capillary forces are negative Residues burn off with residue forming around solder pads Air Flow and Exhaustion

74 Flux Residue Before and After Cleaning

75 Not Cleaned Boards Visual Inspection Residues lower than normal Factors that contribute to air flow and exhaustion reduce residue formation Cleaned Boards Gaps heights rendered an easier than normal cleaning condition Cleaning was good for both no-clean and water soluble Water soluble had presence of ghosting on solder mask

76 Conclusions

77 Summary of Experimental Findings QFNs are extremely challenging to clean Driven by low standoff gap Research presented in this study Uncovered key factors Via holes can displace flux to bottom side of board and vent the bottom termination during reflow Ground pad patterns help to achieve higher gap heights Stencil thickness can increase gap height Increased gap height repels capillary forces

78 Acknowledgements Several people worked behind the scenes to make this research possible. First, Dale Lee and his Plexus team helped to frame the cleaning problem and design options that may improve the ability to clean QFN components. Plexus personnel designed the board and provided micro-section analysis. Raytheon McKinney under the leadership of Bill Vuono worked with stencil designer, provided the stencils, assembled the boards and X-rayed the bottom terminations. The people behind the scenes at Kyzen Corporation worked with the board fabricator and component suppliers to obtain the test vehicles. The boards were cleaned at Kyzen s Application Testing lab. Removal of components, imaging and grading was completed by Kyzen technicians. Steve Stach of Austin American Technology provided both insight and images that helped explain the capillary forces or repulsion and attraction. This data finding is a critical factor that explains why heavy flux deposits form under bottom termination components with gap heights lower than 2 mils.

79 References 1 Wikipedia, Definition of a QFN (2013) 2 S. Stach, M Bixenman (2008, Sep) Optimizing Batch Cleaning Process Parameters for Removing Lead-Free Flux Residues on Populated Circuit Assemblies SMTAI, Rosemont, IL 3 M. Bixenman, D. Lee, S. Stach (2012, Sep) High Speed Cleaning in a Reduced Manufacturing Footprint SMTAI, Orlando, FL 4 U. Tosun, R. Ravindran, M. Mccutchen; (2013) Determining Critical Cleaning Process Parameters for QFN s part one, Printed Circuit Design and Fab; p EMPF Tech Tips, (2007, January). 6 Lee, D., Helvestine, R & Bixenman, M. (2011, Sep). Design for Cleaning Medical Electronic Circuit Devices. Meptec/SMTA Medical Conference. Phoenix AZ. 7 Bixenman, M. & Lee, D. (2012, Sep). Cleaning Medical Electronics. Meptec/SMTA Medical Conference. Pheonix, AZ. 8 Stach, S. & Bixenman, M. (2004, Sep.). Optimizing Cleaning Energy in Batch and Inline Spray Systems. SMTAI, Rosemont, IL