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1 Cleanness Assessment Using Solvent Extract Conductivity (SEC) to Improve Circuit Reliability Ling Zou 03 December Your Delegate Webinar Control Panel Open and close your panel Full screen view Raise hand for Q&A at the end Submit text questions during or at the end NPL Management Ltd -Commercial 2

2 Tues 4 February 2014: How to Test and Qualify Packages with Scanning Acoustic Microscopy (SAM) Tues 18 March 2014: Effectiveness of Baking to Remove Moisture from Between PCB Ground Planes Tues 10 June 2014: Solderability Assessment - Testing, Ageing andpractical Impact on Assembly Yield NPL Management Ltd -Commercial Cleanness Assessment Using Solvent Extract Conductivity (SEC) to Improve Circuit Reliability Ling Zou 03 December

3 Introduction Why electrochemical reliability is important. How ionic contamination affects circuit reliability. What is Solvent Extract Conductivity (SEC) measurement, and how to measure ionic contamination using SEC. IPC standard for SEC testing Examples on using the SEC technique to qualify materials and assembly process. When SEC measurement is necessary and how to get SEC measurements done. 5 Electrochemical reliability Achieving high reliability in service is the key issue in today s high-density electronics assemblies, particularly for the assemblies used in harsh environment. Assembly process involve lots chemicals, these leave contaminants on the board and can cause long term reliability problems. Electrical reliability failures are mainly caused by contaminants, with an electrical bias facilitating electrochemical corrosion processes, which result in a loss of continuity, or short circuit. 6

4 Three conditions for electrochemical failure Electrical charge carriers must be present: Ionic contaminants, H + and OH - from water. Water must be present to dissolve the ionic material and sustain them in their mobile ionic state: Moisture, humidity Electric field between two opposite charged conductors on the circuit is needed to establish a ionic current (leakage current) in the water layer: Bias 7 Electrochemical reactions for biased circuit - M HO - M H 2 HO - I M +n H + C 6 H 9 O - H + M +n + M Corrosion of metal _ + Dendrite short two conductors Two opposite charged conductors with high ionic contamination on the circuit with moisture and bias can form a electrochemical cell, and result in a leakage current, and degrade circuit performance. If the contamination is corrosive, metal at the anode can corroded, and the bias applied to the circuit will accelerate this corrosion process, and form dendrites. Dendrite formation can cause circuit failure. 8

5 Solvent Extract Conductivity (SEC) Control of ionic contamination level on the circuit is critical How to detect ionic contamination? Solvent Extract Conductivity (SEC) measurement uses a mixture of isopropanol (IPA) and DI water to remove soluble contaminants from a sample PCB or PCA. The change in the conductivity of the solution used to wash the assembly provides a measure of the average ionic contamination present on the test board as µg eg.nacl. 9 Solvent Extract Conductivity (SEC) tester Ionograph 500M Maximum board can be tested: 250 x 300 mm 75% IPA + 25% DI water Flow rate: 107 cc/second Test temperature: 45 o C Test time: 10 to 20 minutes 10

6 SEC testing- System verification 5 ml 750ppm NaCl solution for verification of the system. Total 3750µg NaCl. Out +/- 5% range. System need to be calibrated. 11 SEC testing- System calibration 5 ml 750ppm NaCl solution for calibration of the system. New calibration factor will be added to the system. Perform verification testing again to check. 12

7 SEC results The system measures total ionic contamination (µg) equivalent of NaCl. Sample area can be added to have ionic contamination (µg/cm 2 ) equivalent of NaCl Ionic contamination (eg. NaCl µg/cm 2 ) Time(min) Pass/Fail 13 Advantage and disadvantage of SEC testing Test can be performed on your production boards directly. Cost effective assessment technique for ranking ionic contamination levels from different: Cleaning processes. Materials: paste, flux, resist, PCBs. Assembly process. SEC testing does not show the distribution of contamination, or the effects of any non-ionic residue and the result can be dependant on the components density of board and extraction efficiency. 14

8 Test standard IPC - TM- 650 IPC -TM- 650 Standard Temperature 45 o C 80 o C (KAPAK bag) Time (minutes) Solution 75% IPA /25% DI water or 50% IPA /50% DI water Method can remove more contaminants from test boards, and will give higher contamination results. 15 Effect of ageing on the cleanability of solder flux residues Test board assembled with SM and through hole components (chip resistors, SOICs, QFPs, through hole resistors) Assembled board stored (aged) at ambient condition for different time (2 hours to 720 hours (30days) Different ageing boards were simply immersion cleaned in 99.5% IPA for 30 seconds at room temperature. The cleanness of the boards were tested using SEC testing 16

9 Immersion cleaning method Ionic contamination (µg/cm² eq NaCl) Storage time prior to cleaning (hour) Simple IPA immersion cleaning is only effective for up to 2 days after assembly. Contamination level increased with ageing time. 17 Different method with different cleaning agents The same assemblies were stored at ambient condition for 30 days, and cleaned using different method and cleaning solution. Cleaning method Immersion Immersion and brushing Immersion and scrubbing Cleaning solution 100% IPA 75% IPA + 25% DI water Water based cleaner Solvent based cleaner 18

10 Results on cleaning method and agents Ionic contamination (µg/cm² eq NaCl) Immersion Brush Scrubbing 100% IPA 75% IPA Water base Solvent base Solvent based cleaner gave the best cleaning results, even with simple immersion cleaning. Brush and scrubbing more effectively remove flux residues from the assembly. 19 Publication Ling Zou & Chris Hunt An evaluation of effect of ageing on the cleanability of solder flux residues Soldering & Surface Mount technology, Vol. 11 Issue1 evaluation of n evaluation of the effect of ageing on the of solder flux residues 20

11 Comparison of five company s final products using SEC test Company Assembly method Cleaning process A Hand solder After assembly B A comparison of five companies product of identical assemblies using two PCB finishes (HASL AuNi). Finished assemblies were evaluated using SEC measurements Reflow, wave & hand solder C Reflow and hand solder No-clean 21 Before assembly, after reflow and wave solder D Reflow and wave solder After reflow and wave solder E Reflow and wave solder No-clean Test board Double side assembly. Top: Reflow or hand solder Bottom: Wave Through hole: Wave or hand solder 22

12 SEC measurement Method A: 75% IPA + 25% DI water At 45 o C for 25 minutes. Method B: Test board was in a KAPAK bag with 400ml extract solution (50% IPA + 50% DI water ) at 80 o C for 60 minutes. Measure the extract solution according to Method A 23 SEC results Company Method A µg/cm 2 eg.nacl Method B AuNi HASL AuNi HASL Bare PCB A B C D E

13 Ionic Contamination (µg/cm² eg.nacl) Ionic Contamination (µg/cm² eg.nacl) SEC results Method A AuNi PCB B D E C Company Method B AuNi PCB B D C E Company Ionic Contamination (µg/cm² eg.nacl) Ionic Contamination (µg/cm² eg.nacl) Method A HASL B D A PCB E C Company Method B HASL B A D PCB E C Company SEC method SEC method B is shown to be more efficient at removing ionic contamination. Both method give similar trend for all company. HASL finish PCB gave much high ionic contamination levels than AuNi PCB. The final assemblies from companies using cleaning processes (A, B & D) gave much lower ionic contamination than companies with no-clean production processes (C & E). 26

14 SEC method The contamination level on final assembly from HASL PCB is higher than that from AuNi PCB, this implies that the original contamination from the PCB process contribute the contamination level on final assembly after all production and cleaning processes. Contamination level for bare PCB need to be controlled. 27 Publication Ling Zou and Chris Hunt Cleanliness and Reliability Evaluation of Test Methods and the Impact of Contamination from Production Processes on the Reliability of Printed Circuit Assemblies NPL Report CMMT(A)

15 When SEC measurement is necessary When new materials and process are going to be introduced. SEC measurement is a good tool to evaluate the ionic contamination level from the materials and assembly processes. Customer specification. There may be some ionic contamination specifications from your customer for your PCBs or assemblies to ensue longer service life & higher reliability. There is certain ionic contamination requirement to ensue conformal coating adhesion. 29 How to prepare your samples for SEC test SEC test can be performed on your product samples directly. The test shouldn t cause any damage on you products. The test samples from your production line shouldn t be touched by bare hand. Normally 10 test samples are recommended for the test 30

16 Tues 4 February 2014: How to Test and Qualify Packages with Scanning Acoustic Microscopy (SAM) Tues 18 March 2014: Effectiveness of Baking to Remove Moisture from Between PCB Ground Planes Tues 10 June 2014: Solderability Assessment - Testing, Ageing and Practical Impact on Assembly Yield NPL Management Ltd -Commercial SEC test service in NPL NPL has SEC measurement service Ionograph 500M Please contact us For advise on SEC technique. SEC measurement. Contact detail Ling Zou (ling.zou@npl.co.uk) Tel: