inemi Lead-Free Rework Optimization Project: Phase 3-Assembly and Rework Evaluation

Transcription

1 inemi Lead-Free Rework Optimization Project: Phase 3-Assembly and Rework Evaluation Project Co-Chairs: Jasbir Bath (INEMI) Craig Hamilton (Celestica) Holly Rubin (Alcatel-Lucent)

2 Background Reliability tests in the previous inemi lead-free assembly and rework project indicated a need to initiate a follow on inemi Rework Optimization Project. Thermal fatigue life of certain components on the inemi Payette board such as the CBGA937 was reduced after rework. The overall project would evaluate and recommend best practices, rework equipment requirements, impact of adjacent component temperatures and procedures for best practice lead-free rework processing. 1 1

3 Lead-Free Rework Optimization Project Phase 3 Phase 1 and 2 of the project have: 1. Determined rework repeatability on a range of rework equipment 2. Developed initial rework processes for mini-pot rework 3. Optimized BGA socket rework processes 4. Developed rework processes for ubgas to prevent adjacent component reflow Phase 3 project is to: Assemble and Rework Boards with the developed rework processes and perform reliability testing to validate the rework processes developed 2 2

4 inemi Rework Optimization Project Participants Celestica, Sanmina-SCI, Plexus, Flextronics, Creation Cisco, Alcatel-Lucent Intel Senju, Nihon Superior 3 3

5 inemi Rework Optimization Project (Phase 3) Board redesign for DIMM daughter card: (Plexus) Board build for inemi Payette board and DIMM daughter card: (Sanmina-SCI) SMT board and Wave assembly: (Creation) BGA socket rework: (Intel) ubga rework: (Flextronics) DIMM and DIP rework (Celestica) ATC thermal cycling (Alcatel-Lucent) ATC thermal cycled board cross-sections (Plexus, Intel, Nihon Superior, Alcatel-Lucent) 4 4

6 inemi Payette 2 Board (125mil [3.2mm] thick, OSP, 7 x 17 inch) 5

7 inemi Payette Board Assembly (Phase 3) (Creation) 125mil (3.2mm) thick OSP board, 7 x 17 inch SMT & Wave with Sn3Ag0.5Cu: Lead-free Assembly of 56 inemi Payette 2 boards Board # 1 to # 35: For 1 st pass assembly then Rework and ATC testing Board # 36 to # 56: For 1 st pass assembly and ATC testing Planned 32 boards to be assembled and then reworked to go into ATC testing (Rework boards) Planned 16 boards to be assembled and then go into ATC testing (1 st pass boards) Other boards are for time zero cross-sectioning 6

8 SMT Reflow Profile (Topside) Solder Joint peak temp.: 234 to 249 C. Time over 217 C= 66 to 99 sec. 7 7

9 inemi Payette Board Assembly (Phase 3) (Creation) Lead-free Wave Soldering: Issues in assembling DIMM and DIP components, especially DIMM components. Wave pot temperature: 270 C, Topside board preheat temperature 120 C, Conveyor speed 2ft/min (0.7m/min) Issues in holefill due to thickness of board, copper ground connections to TH component barrels and issue with wave fluxer on wave machine Wave fluxer was not spraying flux on the bottom side of the board properly: alleviated somewhat by brushing some of the boards with additional flux on the bottomside of the board using a flux brush Sample size for ATC for DIMM connector 1 st pass and reworked boards was reduced because of this due to low holefill on some boards. 8

10 inemi Payette Board Assembly (Phase 3) (Creation) Wave holefill issue on J7 DIMM278 I/O connector (Board #1, J7) 9

11 inemi Payette Board Assembly (Phase 3) (Creation) Wave holefill issue on J8 DIMM278 I/O connector (Board #5, J8) Pin 1 (Square Pin) 10

12 Adjacent Component Rework 11 11

13 Adjacent Component Group inemi Payette board: Spacing between components is 0.5 inch (13mm) Previous Findings Based on previous J-STD-020 reflow parameters To maintain peak package temperature <250 C required increased bottom heater temperatures Reworking the μbgas with increased bottom heater temperatures caused secondary reflow of the CBGA joints, and adversely impacted thermal fatigue life 12 12

14 Adjacent Component Group Objectives Stay within J-STD-020D (260 C peak) parameters when reworking components Previous study work objective was to stay within 250 C peak temperature parameters when reworking components Ensure adjacent component joint temperatures remain below liquidus Solutions Explored Heat shields Rework profile reoptimization 13 13

15 New Lead-free UBGA Rework Profile Less bottom heat and more top nozzle heat used to develop this rework profile U40 Solder Joint peak temp.: 232 to 239 C. Time over 217 C= 60 to 74 sec. U40 Component top peak temp.: 255 C. Time over 217 C= 109 seconds

16 CBGA-uBGA Interaction Study Previous Study (Rework component top temp: 245 C) TC Location Peak Temp (C) Reworked ubga Joint Adjacent ubga Joint CBGA 1 Joint CBGA 1 Bottom PCB CBGA 2 Joint CBGA 2 Bottom PCB CBGA 3 Joint CBGA 3 Bottom PCB Current Study (Rework component top temp: 255 C) Joint and Package Temp Monitored Values Peak Temp TC Location ( C) Reworked ubga Joint 232 Adjacent ubga Joint 215 CBGA 1 Joint 204 U Reworked Row A, U Joint PCB Reworked CBGA CBGA Joint PCB Bottom side Heat Escape Hole U27 CBGA1 203 Reworked Bottom side Heat Escape Hole CBGA CBGA solder joints below & above liquidus temp. No CBGA solder joints above liquidus temperature 15

17 Conclusions A new lead-free rework profile was developed which kept the adjacent CBGA solder joints below liquidus (217 C) whilst keeping the reworked ubga component within J-STD-020D limits. This was done by reducing bottom heat and increasing top nozzle heat compared with previous study Found that heat shields (Al and Ceramic) helped to reduce the temperature of the adjacent CBGA but not to a large extent (2 C to 4 C) so not used in Phase 3 Cross-sectional and electrical analysis of adjacent ubga and CBGA: Electrical testing showed no change between as-assembled 1 st pass components (ubga and CBGA) and those components adjacent to the reworked ubga Cross-sections of the adjacent ubga at U41 next to reworked ubga and CBGA components showed well formed joints with no evidence of secondary reflow 16 16

18 BGA Socket Rework 17 17

19 Lead-free BGA604 Socket Rework Create a lead-free board rework profile for the BGA 604 socket on the inemi Payette 2 board Cross-section BGA604 sockets to verify that they have good solder joints 18

20 inemi Payette BGA604 Socket Profile Board Setup TC1-4 Socket Outer Corner Solder Joints TC5-6 Socket Center Solder Joints TC7-10 Socket Top Body TC11-12 Socket Body Top Pin Holes TC13 CAF1 Board Location Note: TC1-6 are drilled from the bottom of the board & only the top of the TC is at the pad surface 19

21 BGA 604 Socket Rework Temperature Profile 20

22 BGA 604 Socket Rework Temperature Profile Thermocouple Solder Joint Location Rising Ramp Rate ( C/sec) Flux Activation Time (150 C to 217 C) 100sec Critical Ramp Rate (205 C to 215 C) C/sec Socket Peak Temperature (230 C to 250 C) Socket Time Above Liquidus (40-200sec) Cooling Ramp Rate ( C/sec) TC1 (Pin 1) Outer Corner 0.71 C/sec 102sec 0.60 C/sec C 114sec C/sec TC2 Outer Corner 0.79 C/sec 98sec 0.58 C/sec C 101sec C/sec TC3 Outer Corner 0.64 C/sec 119sec 0.57 C/sec C 115sec C/sec TC4 Outer Corner 0.67 C/sec 108sec 0.58 C/sec C 118sec C/sec Flux activation TC5 Center time is 0.62 C/sec above our 100sec 115sec max target 0.56 C/sec on all thermocouples C except 156sec TC2 (highlighted C/sec in red) TC6 Center 0.64 C/sec 115sec 0.53 C/sec C 121sec C/sec The maximum time exceeded was only 19sec & is due to the bottom heating system in the rework machine The two center thermocouple locations (TC5 & TC6) had slower cooling ramp rates than what we would normally target Delta temperature between all TC s was 9.5 C (within our 15 C specification for sockets) Thermocouple Location Maximum Socket Body Temperature ( 260 C) Top Of Board Temperature At CAF1 Location ( 217 C) TC7 Socket Body C N/A TC8 Socket Body C N/A TC9 Socket Body C N/A TC10 Socket Body C N/A TC11 Socket Body C N/A TC12 Board (CAF1) N/A C Maximum socket body temperature was exceeded on TC9 by 0.1 C This TC measurement was much higher than all the other TC measurements (broken TC) The CAF1 board TC location is below 217 C but closer to the socket it is probably above 217 C 21

23 BGA 604 Socket Board 25 & 26 Cross-sections Board 25 AD1 Board 25 A1 Board 26 AD1 Board 26 A1 Boards 25 & 26 show the lowest & highest stand off solder joints Board 25 AD1 location shows a slightly compressed solder joint but this does not affect reliability No issues found on new cross-sections with shorter TAL profile 22

24 BGA 604 Socket Stand-off Results Board # Cross-section Locations Stand-off Height Measurement Locations (mils) 22 A1-A31 & AE2-AE30 A A A AE AE AE A1-AD1 & A31-AD31 A M AD A M AD A1-AD1 & A31-AD31 A M AD A M AD A1-AD1 & A31-AD31 A M AD A M AD Board 22 cross-section shows that the socket warps down on A31 & AE2 corners A31 corner stand-off is approximately 2.5 mils lower AE2 corner stand-off is approximately 3.5 mils lower Boards 24, 25 & 26 cross-sections show only the AD1 corner to have a larger collapse than all other measurements AD1 corner stand-off is approximately 3 to 5mils lower This is fairly common in socket rework from previous internal data that was obtained 23

25 BGA Socket Rework Summary The rework profile developed successfully The BGA socket stand-off measurements had up to a 5 mil delta across one side This is within the normal internal data range examined This was similar to the range observed on a different suppliers BGA socket in previous studies IMC thickness measurements and elemental analysis results were as expected 24

26 Mini-pot Rework 25 25

27 Mini-pot Rework Sub-Group Problem Statement Challenge Simulate current production process Remove and replace a PDIP without board preheat Dissolved copper Pad / trace Rework Observations Achieving sufficient holefill resulted in the copper pad/trace dissolution on the bottom-side Cross-section View of Solder Joint (274 C) Total time in mini-pot = 60sec Enough time to dissolve nickel layer on 135mil thick NiAu board 26

28 Phase 1 Mini-pot Rework Status (Phase 1) Results on inemi Wave Test Board indicated that an alternative alloy from SnAgCu would be needed, in order to achieve up to a 2X PTH rework, without causing quality concerns Mixed Sn-Ag-Cu / Sn-Cu-Ni reworks included in Phase 2 and 3 builds Low Thermal Mass PCB (62mils) Sn-Ag-Cu Sn-Cu-Ni 1X Rework 2X Rework High Thermal Mass PCB (145mils) Sn-Ag-Cu Sn-Cu-Ni 1X Rework 2X Rework X Variation in Cu plating thickness on incoming boards were observed First article inspection critical before Phase 2 and Phase 3 builds Needed to further understand specifications and process controls used to monitor and control copper plating thickness for as-received boards 27

29 PTH Rework Locations (Phase 2): inemi Payette board, 125mil thick OSP 1X rework SAC305 DIP16 1X rework Sn-Cu-Ni DIP16 1X rework Sn-Cu-Ni DIMM278 2X rework Sn-Cu-Ni DIP16 2 DIMM278 Connectors 3 PDIP16 Components Note: 1 st pass wave using Sn3Ag0.5Cu 28

30 Phase 2 Mini-pot Rework Status (Phase 1 & 2) Further optimization of DIMM and PDIP rework process steps completed. Use of laminar flow well (center baffle rework nozzle) suggested and used to reduce Cu dissolution Convection(batch oven) vs. Infrared (In-line) preheat studied: In-line preheat helped preheat the board faster with less heat loss prior to rework Responses measured: dwell time, Cu dissolution, barrel fill & other defects Number of difficulties during DIMM connector rework (using Sn-Cu-Ni alloy) which required further attention. Establishing adequate barrel fill (50% for ground connections), while controlling level of Cu dissolution 29

31 Phase 3 PTH Rework Matrix (ATC testing) PTH Rework Objectives: Validate process and reliability of reworking a Sn3Ag0.5Cu joint with Sn-Cu- Ni alloy (mixed case) Validate process and reliability (ATC) of reworking Sn3Ag0.5Cu joint with Sn3Ag0.5Cu alloy Measure levels of Cu dissolution for both types of reworked joints 1X rework on DIMM278 1X and 2X rework on DIP16 30

32 Phase 3 PDIP16 Rework 31 31

33 Lead-free PDIP Rework 32

34 Lead-free PDIP Rework 33

35 Lead-free PDIP Rework Note 1: Copper barrel thicknesses after PDIP rework were > 0.5mils (12um) on test scrap boards during typical contact times used for SnCuNi rework Note 2: 0.5mils (12um) was used as the minimum copper barrel thickness based on reliability assessments published by different companies. 34

36 Lead-free PDIP Rework DIP Removal & Replacement Average Contact Times: SnCuNi (1x): 30 sec SnAgCu (1x): 40 sec SnCuNi (2x): 50 sec Note: Due to the narrow process window, some contact times were up to 79 seconds for 1X rework and up to 86 seconds for 2X rework which would cause board barrel copper dissolution 35

37 Phase 3 DIMM278 Rework 36 36

38 Effect of Board Preheat on Mini-pot Rework Experiments were conducted to understand the affect of board preheat on mini-pot rework using three different board preheat processes

39 DIMM Rework (3 Board Preheat Process Evaluated) Preheat Bake Process 1 1. Bake the board in the oven at 105 C for 20 min. 2. Take the board from oven to solder fountain. Preheat Bake Process 2 1. Bake the board in the oven at 150 C for 20 min. 2. Take the board from oven to solder fountain. Preheat Bake Process 3 1. Bake the board in the oven at 100 C for 15 min. 2. Take the board from oven to BGA Rework Machine in order to heat the board up to ~160 C at bottom side of board laminate. 3. Take the board from BGA Rework Machine to solder fountain. 38

40 Lead-free DIMM Rework Evaluation Stages + BGA Rework 160C 39

41 Reflow Profile for Process 2 (Batch Oven Bake, 150 C) Open door Open door Open door Open door Open door Time at (seconds) TCs Peak (B) J7_left joint (B) J7_center joint (B) J7_right joint (T) J7_left joint (T) J7_right joint (B) J8_center joint (B) Laminate (B) Laminate (T) Laminate

42 Reflow Profile for Process 3 (Batch oven bake 100 C followed by BGA rework heating of bottomside of board to 160 C) Time at (s) TCs Peak (B) J7_left joint (B) J7_center joint (B) J7_right joint (T) J7_left joint (T) J7_right joint (B) J8_center joint (B) Laminate (B) Laminate (T) Laminate

43 Lead-free DIMM Rework 42

44 Process Flow (DIMM 278 component) Bake the board in oven (Ts = 105 C) ~ min. Move to BGA Rework Machine, Heat the board up to 160 C at the bottom Move to Minipot set at 265 C Apply flux Remove and replace component 43

45 Lead-free DIMM Rework 44

46 Lead-free DIMM 1X Rework SnCuNi (J7 Location) Bare Board: 1.63mils (38um) First Pass Board: 1.3mils (30um) Evidence of low or zero copper barrel thickness (<0.5mils, 12um) at multiple pin locations after rework 45

47 Lead-free DIMM Rework (Contact times) Average contact time for DIMM Removal and Replacement (SnCuNi): 27 seconds Note: Due to the narrow process window, some contact times were up to 46 seconds which would cause increased board barrel copper dissolution after rework 46

48 Assembly and Rework Phase 3 Conclusions For the 1 st pass assembly build no real issues apart from lead-free wave holefill issues on some of the DIMM 278 connectors on the 125 mil thick OSP board. For rework, no real issues during the BGA socket and ubga rework. More issues during mini-pot wave rework for the DIP and DIMM component particularly for DIMM rework. Copper dissolution and lead-free wave rework holefill were areas of concern. 47

49 contacts: David Godlewski Grace O'Mallley 48