WWW.DISSERTATION.XLIBX.INFO
FREE ELECTRONIC LIBRARY - Dissertations, online materials
 
<< HOME
CONTACTS



Pages:   || 2 |

«The Study, Measurement, and Prevention of Tarnish on Immersion Silver Board Finishes Lenora Toscano and Donald Cullen MacDermid, Inc. Waterbury, CT ...»

-- [ Page 1 ] --

The Study, Measurement, and Prevention of Tarnish on Immersion Silver Board

Finishes

Lenora Toscano and Donald Cullen

MacDermid, Inc.

Waterbury, CT USA

ABSTRACT

With increased environmental legislation against lead in the electronics industry1, circuit board manufacturers are expecting

chemical suppliers to formulate lead free alternatives with the same functionality as tin/lead. These alternatives are needed

for board finishes, in the solder, and on the components adhered to the circuit board. It is a benefit for the chemical supplier to educate the assembler and Original Equipment Manufacturer (OEM) on the differences experienced with these new alternatives. The appearance and characteristics of these new finishes vary somewhat from the incumbent lead-containing standards. Functionality remains the same if not better in some cases. The main focus of this study was the immersion silver surface finish. As interest moves away from HASL final finishes there has been a greater attraction to immersion silver in particular. This paper will explore the effects of operating and storage environments on immersion silver surface finishes and help to explain the functional capabilities the finish has after accelerated aging conditions.

INTRODUCTION

With efforts to reduce the use of Hot Air Solder Level (HASL) as a solderability preservative in the electronics industry, there has been considerable growth in the use of alternate final finishes. Government and industry experts are concerned about technical and environmental limitations of tin/lead on circuit boards1. Alternate finishes include electroless nickel/immersion gold, organic coatings, immersion tin, and immersion silver. According to industry estimates, the newest of the alternative circuit board finishes, immersion silver, is now used in 8% of the market2. Chemical suppliers have offered immersion silver since the early 1990’s, however some board fabricators and OEM’s have been hesitant to convert to this final finish. Concerns principally fall into two categories: fear of dendritic growth with applied current and fear of surface corrosion. These concerns have resulted in a demand to quantify the occurrence of the phenomena and to measure their affect on performance.

Advances in technology translate into a greater demand on the performance of a board’s surface finish. Flatness, wettability, shelf-life, physical durability, contact functionality, and cosmetic criteria all contribute to the production success of a board finish. Each of these criteria may be affected by a perceived weakness of immersion silver, the tendency to tarnish. Present day finishes such as immersion silver must maintain reliability for long-term storage and operation in the final product without functional loss from tarnish.

This paper will explore the effects of tarnish on solderability, electrochemical migration and surface conductivity. By understanding the changes occurring on the deposited metal, appropriate experimental data will quantify the functional changes of the surface finish. Experimentation will include electrochemical migration, contact reliability and solderability after various environmental conditioning. Most importantly, data will quantify the depth of corrosion as silver is exposed to various environments. This will allow users to predict the functionality of the resulting product.

Immersion Silver Deposition Chemically, immersion deposits are advantageous to the board fabricator because there is no use of chemical reducers to drive the reactions. In systems such as electroless nickel, chemical reducers and their supporting chemicals can breakdown, requiring a great deal of chemical maintenance3. Immersion silver deposition occurs as a result of the oxidation of the base metal, in this case copper. The entire reaction is driven by the electromotive series. The presence of the underlying metal drives the reaction; once the silver has deposited over the copper surface, the reaction slows down4. This type of reaction also eliminates the use of activation steps and therefore reduces other problems such as extraneous plating. The simplicity of the immersion (galvanic displacement) mechanism is a significant benefit to PCB manufacturers.

Equation (1) 2 Ag+ + Cu0 → 2 Ag0 + Cu++

Physically, immersion deposits are more planar than traditional HASL surfaces. While variation in thickness of HASL may be forty or more microns, typical variation in an immersion silver deposit is measured in nanometers. The kinetics of the reaction result in a silver deposit that mimics the copper surface. The nature of the reaction produces a thin layer of silver.

For perspective, the thickness of immersion silver is less than one thousandth of the thickness of copper circuitry. The flat surface makes placement of small devices much easier at component assembly.

When exposed to contaminating environments, silver metal will react with anions from the atmosphere and convert them to silver tarnish on the surface. The tarnish is actually a reduction/oxidation corrosion product. Tarnish species include silver chloride, silver sulfide and silver sulfate. These principal corrosion products of silver have three different electromotive potentials5. This makes it possible to determine the type and amount of contamination on the surface of the silver using instrumentation, including Sequential Electrochemical Reduction Analysis (SERA), Auger, Secondary Ion Mass Spectroscopy (SIMS), and Electron Spectroscopy for Chemical Analysis (ESCA).





One disadvantage to using the SERA method is that silver sulfate and copper sulfate exhibit the same reduction potential.

Therefore, SERA cannot distinguish between these two important surface species. Low silver thicknesses normally result in copper sulfate contamination due to incomplete copper coverage. In failure mode analysis, silver thickness should be determined prior to running the SERA to determine if the silver thickness is too low. Below 0.15 microns, extremely heavy oxidation is usually due to the formation of copper sulfate as a result of under-protected copper.

Preliminary testing was conducted to help determine if visible tarnish resulted from the formation of copper sulfate or silver sulfate with respect to deposited silver thickness. Figure 1 is a Scanning Electron Microscope (SEM) image of a thin silver deposit over copper. The thickness of the deposit was 0.1 microns. The image shows dark areas scattered throughout the deposit. Electron Dispersive Spectroscopy (EDS) of the dark areas indicates that these areas are rich in copper (Figure 2).

Plating for an increased amount of time results in a thicker silver deposit. The increased plating time enables to silver to cover more area. In turn there is a decrease in copper pore sites as seen in Figure 3. An EDS of this area shows a deposit rich in silver (Figure 4). A small copper peak can be detected as a result of “shoot through” due to the accelerating energy of 20kv of the microscope.

Figure 1: Low silver thickness (0.1 microns) Figure 2: EDS of Pore in Low Silver Thickness Sample EDS Area Highlighted Figure 3: High Silver thickness(0.3 microns) Figure 4: EDS of High Silver Thickness Sample EDS Area Highlighted Early in the development of immersion silver processing, such thickness studies confirmed the preference for higher silver thicknesses. Results from the fundamental testing indicated that circuit boards should be plated to a silver thickness target centered at 0.25 – 0.3 microns. This thickness guideline was followed for the subsequent experimentation. The proper silver thickness would insure that the data are a result of silver corrosion and not copper corrosion.

EXPERIMENTAL

In several experiments dealing with the effect of tarnish on solderability, electromigration and contact resistance, a standard set of test vehicles was employed. Electroplated copper test coupons were processed through immersion silver cycles. The cycle included an acid cleaner, a modified persulfate microetch, a pre-dip for bath replenishment and finally an immersion silver bath with various proprietary organic additives6. Silver plating time was run at two different durations to achieve a deposit of 0.1and 0.3 microns.

Through Hole Solderability Test coupons (Figure 5) containing surface mount pads and eighty plated through holes (PTH) were processed through a standard immersion silver process to achieve an average silver thickness of 0.3 micrometers. Thickness was measured by Xray Fluorescence (XRF) and produced a range of 0.22 – 0.33 microns. The panels were then enclosed in a chamber containing a sulfur solution and a fan for circulating the air (Figure 6). The sulfur solution used was a mixture of 0.1g/L sodium bisulfide in de-ionized water with 1mL/100mL concentrated HCl. The solution was divided into three containers set on one side of the chamber. The fan directed airflow across the containers parallel to the hanging samples. A test coupon was removed every thirty minutes. Total tarnish was measured by Sequential Electrochemical Reduction Analysis (SERA) on surface pads. Finally, the through holes were floated on molten solder at 245ºC for three seconds and evaluated according to industry standard ANSI/IPC J-STD 003a7.

Figure 5: PTH Test Coupon

Figure 6: Sulfur Chamber Electromigration A set of IPC B-25 test coupons (Figure 7) were processed through a standard immersion silver process. The boards were exposed to Class II Mixed Flowing Gas8 (Method G) for nine days at Aspen Labs in a Batelle Chamber. Within the chamber, panels are subjected to chloride, sulfide and sulfate. The test panels included both immersion silver panels and a copper control. Mixed Flowing Gas exposure was measured on a scale calibrated by its effect on copper.

Figure 7: B-25 Test Vehicle Surface Conductivity/Contact Resistance The last functional test performed was a simulation of a cabinet insertion test. Electroplated copper laminate was processed through immersion silver and electroless nickel/ immersion gold, separately. The samples were exposed to the same Mixed Flowing Gas as described in the Electromigration section for 0, 5, and 9 days. The level of tarnish on the samples was measured after 24 hours, 5 and 9 days. A second set of these boards was placed in a Humidity Chamber at 85°C and 85% relative humidity (85C/85%RH) for up to five weeks. After the MFG and humidity exposures the boards were inserted to a cabinet (Figure 8) which causes friction to occur on the edges of the boards. Three pieces of copper laminate were placed on top of each bare board prior to insertion to simulate the weight of components on the board. The samples were inserted for 0, 1, 15, and 50 times and measured for surface contact resistance. Cabinet rails were cleaned with IPA and DI water between each insertion set.

Figure 8: Circuit Board in Cabinet Rail

Contact testing was conducted using a 50-microinch gold plated copper alloy burr free spherical radius probe. The probe had a diameter of 0.0625 inches. Resistance was measured by EIA 364 Test Procedure with a 100ma-test current and a 20mV open circuit voltage. A probe force of 25, 50, and 75 grams was used9. For an OEM involved in the study, maintenance of low-resistance electrical conductivity in a potentially contaminating environment is a key concern.

–  –  –

1018 1217 1069 1515 1586 274 408 581 586 823 918

–  –  –

Figure 10 shows images of varying tarnish. The first sample is a new silver sample that was not exposed to any environment.

Typically, a shiny silver deposit has a range of 0 – 100Å of total tarnish. The second sample shows a slight yellowing this is characteristic of 200 – 500Å of tarnish. The final sample shows a heavy amount of tarnish which indicates a level of 1500 – 2500Å of total tarnish.

Electromigration:

The nine-day exposure to Mixed Flowing Gas resulted in a total corrosion film thickness of approximately 3500Å as measured by coulometric reduction of copper. The 3500Å equaled 59-micrograms/cm2 weight gain on copper. The industry accepted age correlation is 12–16 micrograms/cm2 equals one year in the field. The levels of corrosion indicate that this test may represent 4-5 years of accelerated age. All silver samples including the copper control displayed relatively uniform levels of ‘heavy’ tarnish. The boards were then subjected to Electromigration testing at Trace Laboratories, according to Bellcore GR-78-CORE, Issue 1, Section 13.2.710. All boards passed the test for electrical and visual requirements for electromigration (Table 1). This test requires the maintenance of electrical resistance between adjacent conductors of within at least one decade of the original (96 hour) value.

–  –  –

Consistently, the contact resistance data shows that an increase in force load will result in a decrease of resistance as the test probe overcomes any surface corrosion. For the immersion silver deposits, resistance stayed below 50 miliohms up to 9 days in the Batelle chamber for each force load. The electroless nickel/immersion gold samples began to show large increases in resistance after 5 days. The humidity and cabinet insertions also appeared to have little effect on the contact resistance of immersion silver. Again the silver results maintained a small deviation through the testing even with increased insertion and extended humidity exposure. Similar trends were observed with surfaces that had been aged with 85ºC/85%RH.

Figure 13: Contact Resistance Testing

CONCLUSIONS

There are many advantages to using immersion silver as an alternative to lead-containing final finishes. The simplicity of the chemistry and the kinetics of the reactions enable easy processing during board fabrication and at the assembly house.

Though appearance of the deposited metal may change with aggressive environments, functionality remains reliable.

The above testing indicates that environment does have cosmetic effects on immersion silver finishes. Visual defects should not be linearly associated with functional deficiencies. The above experiments indicate that moderate visual tarnish does not effect the performance of the surface finish. Through hole solderability remained acceptable up to 800angstroms of tarnish.



Pages:   || 2 |


Similar works:

«Copyright by Jin Huh 2007 The Dissertation Committee for Jin Huh certifies that this is the approved version of the following dissertation: A Comparison Study on the Effects of Two Explicit Pronunciation Syllabi on Korean Adult EFL Learners’ Learning of English Sounds Committee: Zena T. Moore, Supervisor Elaine K. Horwitz Diane L. Schallert Daniel H. Robinson Orlando R. Kelm A Comparison Study on the Effects of Two Explicit Pronunciation Syllabi on Korean Adult EFL Learners’ Learning of...»

«Internet Journal of Criminology © 2011 ISSN 2045-6743 (Online) Broccoli or Broken Windows? The relationship between anti social behaviour and a nutrient deficient diet By Emma Gordon1 ABSTRACT The differential extent of research into nutrition and malnutrition and the impact this has on externalising behaviours is vast. It is generally accepted that nutrition and related factors such as food additives, hypoglycaemia and cholesterol plays an important role as a contributor of children and...»

«B-756 The Book of Romans (Exegesis) Spring 2012 Ronald Allen ron.allen@cts.edu Office: #211 Christian Theological Seminary 1000 West 42nd Street Indianapolis IN 46208 317-931-2339 Purpose The purposes of this course are fivefold. Firstly, it should familiarize the student thoroughly with the content of the letter to the Romans. Secondly, it should introduce the student to the major issues in the interpretation of Romans and help the students come their own judgments on those issues. Thirdly,...»

«CHEMTURA PUBLIC ADVISORY COMMITTEE (C.P.A.C.) DRAFT MEETING MINUTES Thursday, September 25, 2014 – 6:00 p.m. Council Chambers 24 Church Street West, Elmira Present: Dr. Dan Holt CPAC Voting Member, Chairperson Vivienne Delaney CPAC Voting Member Dr. Sebastian Siebel-Achenbach CPAC Voting Member Graham Chevreau CPAC Voting Member Ron Campbell CPAC Voting Member Mark Bauman CPAC Voting Member, Council Rep, Township of Woolwich Alan Marshall SWAT Richard Clausi SWAT Jeff Merriman Chemtura Canada...»

«MARYLAND TRANSPORTATION AUTHORITY BOARD MEETING THURSDAY, NOVEMBER 19, 2015 2310 BROENING HIGHWAY, BALTIMORE, MD 21224 OPEN SESSION Dennis Schrader, Acting Chairman MEMBERS ATTENDING: Rev. Dr. William C. Calhoun, Sr. P. Jack Basso William Ensor W. Lee Gaines, Jr. Michael Leahy, Esquire Randall Nixon, Esquire William K. Hellmann Michael Whitson (via Telephone) STAFF ATTENDING: Milt Chaffee David Chapin Percy Dangerfield Donna DiCerbo Trudy Edwards Gordon Garrettson Benjamin Gilmore Jim Harkness...»

«CUERPOS EXTRAÑOS ESOFÁGICOS Autores: Eva María Fernández Bonilla y Jose María Fraile Fraile Servicio de Aparato Digestivo. Hospital Clínico “Virgen de la Victoria”. Málaga.INDICE I. INTRODUCCIÓN. II. CLINICA. III. EXPLORACIONES PREVIAS A LA EXTRACCIÓN DE CUERPOS EXTRAÑOS. IV. INDICACIONES DE EXTRACCIÓN DE CUERPO EXTRAÑO. V. INSTRUMENTAL. VI. TÉCNICA ENDOSCÓPICA. VII. COMPLICACIONES. VIII. ACTUACIÓN EN EL ÁREA DE URGENCIAS. IX. REGLAS DE ORO. X. ALGORITMO DIAGNÓSTICO. I....»

«william hasker “THE END OF HUMAN LIFE”: BUDDHIST, PROCESS, AND OPEN THEIST PERSPECTIVES A human life has an ending, a terminus. Many of us believe that human life also has a goal, an inherent telos. The conflation of the two notions in the expression, “the end of human life,” is possible because, according to both Buddhism and Christianity, the telos of human existence is fully achieved, if at all, only in the ending of a person’s life. Needless to say, however, the conceptions of...»

«ANTHROPOLOGISTS AND MISSIONARIES: BROTHERS UNDER THE SKIN S]AAK VAN DER GEEST if Amsterdam University This article explores the ambiguous relationship between anthropolog~sts and rrussionaries, both in their work and in their writings. it describes, first, the well-known stereotypes by which anthropologists and missionanes are opposed, as conservers v. converters, doubters v. knowers, and listeners v. preachers, and then discusses some stnkmg similarities which have been largely ignored, ifnot...»

«UNITED STATES SECURITIES AND EXCHANGE COMMISSION Washington, DC 20549 FORM 10-Q x QUARTERLY REPORT PURSUANT TO SECTION 13 OR 15(D) OF THE SECURITIES EXCHANGE ACT OF 1934 For the quarterly period ended March 31, 2013.o TRANSITION REPORT PURSUANT TO SECTION 13 OR 15(D) OF THE SECURITIES EXCHANGE ACT OF 1934 for the transition period from _ to _. Commission File Number 0-50481 AEOLUS PHARMACEUTICALS, INC. (Exact Name of Registrant as Specified in Its Charter) Delaware 56-1953785 (State or Other...»

«IN THE FEDERAL COURT OF AUSTRALIA (FCA) NEW SOUTH WALES REGISTRY FEDERAL COURT OF AUSTRALIA FAIR WORK DIVISION No: NSD580/2012 NOTICE OF FILING AND HEARING This application was filed electronically in the FEDERAL COURT OF AUSTRALIA (FCA) on 20/04/2012. DETAILS OF FILING Originating Application under the Fair Work Act 2009 Alleging Document Lodged: Discrimination Form 81 Rule 34.05(1) NSD580/2012 File Number: File Title: James Hunter Ashby v Commonwealth of Australia & Anor NEW SOUTH WALES...»

«Enjoining Right & Forbidding Wrong by Ibn Taymiyya translated by Salim Abdallah ibn Morgan Enjoining Right & Forbidding Wrong 1 Enjoining Right & Forbidding Wrong By Ibn Taymiyya Introduction All praise is due to Allah. We praise Him, seek His aid, and seek His forgiveness. We seek refuge in Allah from the evil which is within ourselves, and from the evil in our actions. Whoever Allah guides, none can send astray; whoever Allah sends astray, none can guide. I bear witness that there is no deity...»

«White Paper Cisco ISE Plus SIEM and Threat Defense: Strengthen Security with Context What You Will Learn Network security threats are a fact of life. But the modern security arsenal has two highly effective tools: security information and event management (SIEM) and threat defense (TD) solutions. SIEM and TD platforms can provide a wealth of information to help you quickly and accurately assess security threats and take immediate remediation actions, all from a single console. Yet analytics are...»





 
<<  HOME   |    CONTACTS
2016 www.dissertation.xlibx.info - Dissertations, online materials

Materials of this site are available for review, all rights belong to their respective owners.
If you do not agree with the fact that your material is placed on this site, please, email us, we will within 1-2 business days delete him.