Creep Corrosion Test Using inemi FoS Chamber

Transcription

1 Creep Corrosion Test Using inemi FoS Chamber Listen to the webinar recording: This link will be active for up to six months following the webinar. Please contact Haley Fu if you would like to help us define further work on corrosion mitigation. Chair: Prabjit Singh (IBM) inemi Webinar September 26, 2018

2 Download whitepaper and full project report White Paper: A Cost-Effective & Convenient Approach to Creep Corrosion Testing (August 2018) Project Report: Qualification Test Development for Creep Corrosion (August 2018)

3 Authors n n n n n n Prabjit Singh and Larry Palmer IBM Corporation, Poughkeepsie, NY, USA Simon Lee and Geoffrey Tong The Dow Chemical Company, Taipei and Hong Kong Dem Lee and Jeffrey Lee ist-integrated Service Technology, Inc., Taiwan Karlos Guo and Julie Liu Lenovo (Beijing) Limited Corporation, Beijing, China Chen Xu Nokia, Murray Hill, NJ, USA Haley Fu inemi, Shanghai, China 3

4 Agenda Background Industry available corrosion test inemi Flowers of sulfur (FoS) chamber design Test chamber conditions and their control Test procedure Round robin tests Effect of PCB storage time Role of solder mask edge inemi FoS test effectiveness FoS chamber as a general purpose corrosion chamber 4

5 Background Poor creep corrosion resistance of the Pb-free PCBAs (flux, surface finish, etc.) Expanding electronics market in geographies such as Asia, which typically have high humidity as well as sulfur-bearing gaseous pollution, led to increased corrosion-based failures To eliminate creep corrosion, industry has initiated significant effort to better understand its cause and mitigation, including technical groups from ASHRAE, IPC, ISA and inemi. ASHRAE thermal guidelines for IT equipment 4 th edition Gaseous corrosivity levels ISA standard Severity ISA Reactivity level level Copper corrosion rate, Å/month Silver corrosion rate, Å/month G1 mild <300 <200 G2 moderate G3 harsh GX severe >2000 > Restricted 5

6 ASHRAE recommended corrosion monitoring locations in data centers n n Cu and Ag metal foils should be placed in front of the rack at ¼ and ¾ height from the floor. After one month exposure, the Cu and Ag foils are returned to the lab to measure the corrosion product thickness using coulometric reduction. 3/4 height 1/4 height X X

7 ASHRAE survey: Cu corrosion rate n n The open and solid data points are for data centers with and without reported corrosion-related hardware failures. Copper corrosion rate is NOT a good predictor of hardware corrosion because of the overlap of Cu corrosion rates for data centers with and without corrosion-related failures Copper corrosion rate, angstroms/month Aug Nov Feb-11 6-Jun Sep-11 Date

8 ASHRAE survey: Ag corrosion rate n n The open and solid data points are for data centers with and without reported corrosionrelated hardware failures. Silver corrosion rate is a good predictor of hardware corrosion because of no overlap of the silver corrosion rates for data centers with and without corrosion-related hardware failures Silver corrosion rate, angstroms/month Aug Nov Feb-11 6-Jun Sep-11 Date

9 Previous Study at inemi: Creep Corrosion Project Using Mixed Flowing Gas (MFG) Test ( ) Phase 1: Industrial survey Phase 2: Primary factors influencing creep corrosion Phase 3: Investigation of factors that influence creep corrosion on PCB (H 2 S = 1200 ppb; NO 2 = 200 ppb; Cl 2 = 20 ppb; SO 2 = 200 ppb; 40, RH 70-75%, 20 days)

10 Qualification test for creep corrosion using FOS ( ) The project explored a more cost-effective and convenient approach to creep corrosion testing that suppliers could use to satisfy customers that their products would survive moderately corrosive environments. Phase 1: Test chamber design & preliminary test ( ) Phase 2: 6 test runs to study test setup effectiveness ( ) Phase 3: The effect of relative humidity on creep corrosion and finalizing the creep corrosion qualification test ( )

11 MFG vs FoS: Key Difference MFG FoS Corrosive Agents Used H2S, NOx, SO2, Cl2 S (and Cl2 in inemi FoS) Test parameter H2S, NOx, SO2, Cl2, RH, T, monitoring air flow rate RH, T all parameter can be T, RH, S concentration Test parameter control independently controlled can be controlled and adjusted Corrosion products sulfide, sulfate, chloride, oxide, nitrate sulfide, oxide (and chloride in inemi FoS) Equipment and maintenance cost high low Equipment setup complex simple Test chamber size large moderate Air flow laminar and uniform non-laminar MFG: Most realistic and controlled simulation of real world environments. inemi FoS: Inexpensive and easy to maintain. MFG and inemi FoS are complementary.

12 Industry Standard Using FoS Method There are several industry standards using FoS method as below: Standard ASTM B809 EIA-977 inemi FoS Temperature 50 o C 60 o C/105 o C 50 o C Humidity 85% 11%-90% Corrosive Gas S 8 S 8 S 8 + Cl 2 Air Flowing Factor X X O Major Mode Cu 2 S Ag 2 S Cu 2 S Corrosion Mode Pore/Pitting Corrosion Sulfur Corrosion Creep Corrosion Feature Industry Metal Porosity Golden Finger, Connector Passive, Switch PCB/PCBA 12

13 Test board used for developing the creep corrosion test Top Side (Reflow) Bottom Side (Wave Soldering) L1 L8 L4 L10 L6 L5 L2 L11 L15 L14 L12 L3 L7 L9 L13 Test boards are 140x110-mm by 1-mm thick. The FR4 epoxy used was compatible with Pb-free soldering conditions. 15 areas on the board with various traces and vias with various pitches designed to monitor the creep corrosion during test. 13

14 inemi FoS chamber conditions An acrylic cube with a paddle wheel can hold 8 boards rotating at 20 rpm 50 o C chamber temperature 200 g sulfur %RH controlled by salt solution in the 11 to 90% range 100 ml 8.25% sodium hypochlorite Clorox is used to introduce chlorine gas that would work in synergy with sulfur Cu and Ag foils are mounted for Cu and Ag corrosion rate measurement The corrosion rate of Cu and Ag should be and Å/day, respectively. The rate of formation of AgCl should be Å/day. 14

15 inemi FoS chamber The challenge is to have the Clorox release chlorine gas into the chamber but not upset the relative humidity. The saturated salt controls the relative humidity to its deliquescence value. The moisture coming off the Clorox is throttled and forced to flow over the saturated salt. 15

16 Influence of sulfur content We settled on 200 g sulfur Corrosion rate, Å/day AgCl growth rate, Å/day Sulfur content, g Sulfur content, g Red dots are for Cu corrosion Blue dots are for Ag corrosion Open dots are from mass gain Solid dots are from coulometric reduction NH 4 NO 3 saturated salt solution, 100ml Clorox, 50 o C, 20 rpm. 16

17 Influence of humidity FOS with 200g sulfur, 100ml Clorox, 50 o C, 20 rpm. The circular data points are from IBM; the triangular data points from ist; and the square data points are from Lenovo. The solid data points are corrosion rates obtained using coulometric reduction technique and the open data points are from mass-gain technique. 17

18 AgCl formation rate Corrosion rates in FoS chamber loaded with 6 test PCBs and one set of 25x50-mm copper and silver foils. 18

19 Chlorine gas content Chlorine gas that would work in synergy with sulfur Chlorine is very soluble in water. Chloride concentration in chamber is influenced by the saturated salt solution and by adsorbed moisture on surfaces % RH 74-80% RH 19

20 Silver corrosion rates in the FoS chamber as a function of time and Clorox dilution Daily AgCl and Ag 2 S formation rates during 5-day runs. 100% Clorox plots are shown with solid dots. 50% Clorox plots are shown with open dots. FOS with 200g sulfur, NaNO 2 saturated salt solution, 100ml Clorox in various dilutions, 50 o C, 20 rpm. 20

21 Effect of %RH on Creep Corrosion After 10 Days In FoS Chamber Saturated salt solutions and RH% ZnCl % MgCl % NaNO % NH4NO % NaCl 69-77% KCl 74-80% % of creep sites with creep corrosion HC LC HC LC HC LC HC LC HC LC HC LC IBM Lenovo OR ist ImAg IBM Lenovo RO ist IBM Lenovo OR ist ENIG IBM Lenovo RO ist IBM Lenovo OR ist OSP IBM Lenovo RO ist Ø 50 o C Ø 100ml household bleach Ø Rotating at 20 rpm Ø IBM used 1-mm Clorox setup gap; Lenovo used 0.5mm/1mm; ist used 1mm/3mm gap. Ø Sulfur content variation among three sites: 180g 800g 21

22 Influence of pre-baking Unbaked card A Unbaked card B Pre-baked card C Prebaked card D Creep corrosion occurred only on pre-baked PCBs from a lot that suffered creep corrosion in the field. Pre-bake is necessary to avoid the VOCs coming off the PCBs from contaminating the limitedvolume FoS chamber. 22

23 inemi FoS Test procedure Prebake test PCBs at 100ºC in flowing nitrogen gas for 24 hours to eliminate the impact of volatile organic compounds (VOCs). Load paddle wheel with seven test PCBs plus one board on which Cu and Ag foils are mounted for Cu and Ag corrosion rate measurements. (Mounting board must be of a material that does not excessively absorb the gases and moisture in the chamber.) Expose PCBs to three five-day runs at 50ºC, each at different levels of relative humidity: Run 1: 31% RH using MgCl 2 saturated salt solution Run 2: 59% RH using NaNO 2 saturated salt solution Run 3: 81% RH using KCl saturated salt solution Copper and silver corrosion rates and rate of formation of AgCl measured using coulometric reduction and mass gain methods. Photograph and record the corrosion failures. 23

24 Pass/Fail Recommendation The acceptable extent of creep corrosion should be defined and accepted by the PCB manufacturer and procurer. Rank 0 Rank 1 Rank 2 Rank 3 Rank 4 No Corrosion Local Corrosion Light Creep Corrosion (<1/3 gap) Moderate Creep Corrosion (>1/3 gap) Heavy Creep Corrosion Oxidation or tarnishing is OK. Corrosion limited to pads or holes only, no spread. Corrosion spreading from pads or holes, but not bridging between features. Corrosion spreading from pads or holes, but not bridging between features. Severe corrosion, bridging between features. Pass Pass Pass Fail Fail 24

25 1 st round robin test Table 9.2c: Corrosion map after the 3rd 5 days with KCl saturated salt solution. The relative humidity range across 3 test sites was 61-78%. Location on board (Grey is for no corrosion, yellow for corrosion; pink for light creep corrosion and dark red Site Finish Flux for heavy creep corrosion. The resistance readings are in KΩ.) A Open Open Open Open Open Open Open Open 57 Open Open Open Open N/A N/A B OR Open Open Open 300 Open Open Open Open Open Open Open Open 520 N/A N/A C Open Open Open Open Open Open Open Open Open Open Open Open Open N/A N/A ImAg A Open Open Open Open Open Open Open Open Open Open Open Open Open N/A N/A B RO 0.2 Open 0.13 Open Open Open 58 Open Open Open Open Open 720 N/A N/A C Open Open Open Open Open Open Open Open Open Open Open Open Open N/A N/A A Open 0 Open Open Open N/A N/A B OR 240 Open Open 210 Open Open Open Open Open Open Open Open 800 N/A N/A C Open Open 0.04 Open Open Open Open Open Open Open Open N/A N/A ENIG A Open 0.01 Open Open N/A N/A B RO Open Open Open Open Open Open Open Open Open Open Open Open Open N/A N/A C Open Open Open 0.7 Open Open Open Open Open Open Open Open Open N/A N/A A Open Open Open Open Open Open Open Open Open Open Open Open Open N/A N/A B OR Open Open Open Open Open Open Open Open Open Open Open Open Open N/A N/A C Open Open Open Open Open Open Open Open Open Open Open Open Open N/A N/A OSP A Open Open Open Open Open Open Open Open Open Open Open Open Open N/A N/A B RO Open Open Open Open Open Open Open Open Open Open Open Open Open N/A N/A C Open Open Open Open Open Open Open Open Open Open Open Open Open N/A N/A

26 2 nd round robin test Table 9.5c: Corrosion map after the 3 rd 5 days with KCl saturated salt solution. The relative humidity range across three test sites was 68-83% Site Finish Flux Location on board (Grey is for no corrosion, yellow for corrosion; pink for light creep corrosion and dark red for heavy creep corrosion. The resistance readings are in KΩ.) A 29.4 Open Open 6.67 Open Open Open 7.7 Open N/A N/A B Open 0.38 N/A N/A ImAg OR B* Open Open Open Open Open Open Open Open Open Open Open Open Open N/A N/A C Open Open Open Open Open Open Open Open N/A N/A A Open Open 5.6 Open Open Open 36.5 N/A N/A B Open 5.6 N/A N/A ImAg RO B* Open Open Open Open Open Open Open Open Open Open Open Open Open N/A N/A C Open Open Open Open Open Open Open N/A N/A A Open Open Open Open Open Open Open Open Open N/A N/A B ENIG OR 0.53 Open Open Open Open Open 1900 Open Open 900 Open Open Open N/A N/A C 0.02 Open Open Open Open Open Open Open Open Open Open Open N/A N/A A 96.7 Open Open Open Open Open Open Open Open Open Open Open N/A N/A B ENIG RO 0.24 Open Open 5.9 Open Open 0.1 Open Open Open Open Open Open N/A N/A C 0.04 Open Open Open Open Open Open Open Open Open Open Open N/A N/A A Open Open Open Open Open Open Open Open Open Open Open Open Open N/A N/A B OSP OR Open Open Open Open Open Open Open Open Open Open Open Open Open N/A N/A C Open Open Open Open Open Open Open Open Open Open Open Open Open N/A N/A A Open Open Open Open Open Open Open Open Open Open Open Open Open N/A N/A B OSP RO 15.4 Open Open Open Open Open Open Open Open Open Open Open Open N/A N/A C Open Open Open Open Open Open Open Open Open Open Open Open Open N/A N/A

27 Worst case creep corrosion at Company A 1st round robin test run MgCl2 ImAg OR ImAg RO ENIG OR ENIG RO OSP OR OSP RO NH4NO3 KCl 2nd round robin test run MgCl2 NaNO2 KCl

28 Effect of storage time Fresh PCBAs from two companies had suffered creep corrosion in the inemi FoS test. The PCBAs were from lots that had suffered creep corrosion in the field. After 2-year storage, the sister PCBAs from these lots no longer suffered creep corrosion in the inemi FoS test Fresh 2-year storage Fresh 2-year storage

29 Role of solder mask edge ImAg PCB soldered with NR330 flux from FOS run with 13-19% relative humidity. ENIG PCB soldered with EF8000 flux from FOS run with 74-80% relative humidity.

30 6th FOS test run: inemi creep corrosion examples All the locations on the ENIG finished PCBs, soldered with organic acid and with rosin flux, tested at the 4 companies, suffered severe creep corrosion in 5 days of exposure in FOS chamber at 50 o C with 40-ml Clorox in a 100-ml beaker at the 81% relative humidity afforded by saturated KCl solution.

31 inemi creep corrosion testing of various ENIG surface finish PCBs L2 SMD L2 NSMD L8 SMD L8 NSMD Control Mid-P, no flux, no treatment Example of HCC High-P, RO flux, PT3 treatment Example of LCC Mid-P, RO flux, PT3 treatment

32 Reproducing corrosion in lab Creep corrosion in inemi FoS chamber at 50 o C and 81%RH. 32

33 Experimental procedure for characterizing conformal coatings n n n n Ag serpentine thin films, 770-nm thick, were evaporated on 15x15 mm SiO 2 on Si dies. The Ag thin films were covered with the conformal coating under test. The corrosion rates of the conformally coated Ag thin films were measured using the electrical resistance approach. 4-point resistance measurements were made using wires connected to the thin film terminations with silver-laden epoxy.

34 Fluoropolymer coatings (1-3 µm thick) in dry FOS chamber at 60 o C 800 FP08 Ag film thickness, nm Bare Ag FP Time, days

35 Fluoropolymer coating FP04 in dry FOS chamber at 60 o C: 1X = 1-3 µm thick 800 Ag film thickness, nm Bare Ag 1X 2X and 4X hours

36 Summary The inemi FoS corrosion chamber is inexpensive, easy to maintain and can be used for creep corrosion qualification testing of PCBAs. The temperature, relative humidity, sulfur and chlorine content in the chamber air can be reasonably well controlled. The test has been very successful in reproducing creep corrosion on printed circuit boards (PCBs) from lots that suffered creep corrosion in the field. It s potential to be used as a general-purpose corrosion chamber for other applications/studies, e.g. characterization and ranking of conformal coatings, corrosion test for surface-mount multi-layer ceramic capacitors with soft terminations which may have corrosion-related failures in sulfur-bearing environments. Future work is considered to explore the usage of this FoS chamber for other purpose. 36

37 Q&A