Immersion Gold ....pdf
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3 1 SCOPE IPC-4552 Specification for Electroless Nickel/Immersion Gold (ENIG) Plating for Printed Circuit Boards 1.1 Scope This specification sets the requirements for the use of Electroless nickel/immersion Gold (ENIG) as a surface finish for printed circuit boards. This specification is intended to set requirements for ENIG deposit thicknesses based on performance criteria. It is intended for use by supplier, printed circuit manufacturer, electronics manufacturing services (EMS) and original equipment manufacturer (OEM). 1.2 Description ENIG is an electroless nickel layer capped with a thin layer of immersion gold. It is a multifunctional surface finish, applicable to soldering, aluminum wire bonding, press fit connections, and as a contact surface. The immersion gold protects the underlying nickel from oxidation/passivation over its intended life. However, this layer is not totally impervious and it will not pass the requirements of a classic porosity test Phosphorus/Boron Content Phosphorus or boron containing reducing agents are used for the reduction of the electroless nickel during the deposition process. Phosphorus or boron is thus incorporated in the nickel deposit. The level of these co-deposited elements should be controlled within the specified process limit. Variation of phosphorus or boron level, outside the specified process limits, may have adverse effects on the solderability of the finish. 1.3 Objective This specification sets the requirements specific to ENIG as a surface finish. As other finishes require specifications, they will be addressed by the IPC Plating Processes Subcommittee as part of the IPC-4550 specification family. As this and other applicable specifications are under continuous review, the subcommittee will add appropriate amendments and make necessary revisions to these documents. 1.4 Performance Functions Solderability This primary function of ENIG is to provide a solderable surface finish that has extensive shelf life and that is suitable for all surface mount and through-hole assembly applications. Due to the minimal thickness of the immersion gold layer, the possibility of gold embrittlement of the solder joint is negligible Contact Surface There is a substantial amount of experience using ENIG for the following applications: Membrane Switches ENIG is the de facto standard for membrane switches. The ENIG surface finish with as little as 0.025µm (0.984µin) of immersion gold is suitable for 1.5 million actuations with negligible resistance change Metallic Dome Contacts The Subcommittee attempted to complete this section, but was unable to obtain the necessary data to do so, at this time. Whenever possible, such data should be submitted to the IPC 4-14 Plating Processes Subcommittee to be considered for inclusion in upcoming revisions of this standard EMI Shielding ENIG is one of the surface finishes used as an interface between electromagnetic interference (EMI) shielding and the printed wiring board (PWB). Page 1 of 2525
4 1.4.4 Conductive (Replacement for Solder) and/or Anisotropic Adhesive Interface ENIG is ideally suited as an interface for anisotropic adhesive applications (the use of a conductive adhesive as an alternative to solder) Connectors Press Fit Press fit requirements shall meet Telcordia GR-1217-CORE. Excessive nickel thickness can lead to concerns with press fit insertion forces Edge Tab The ENIG surface finish is suitable for plug-to-install applications (five insertions/withdrawals) using low insertion force (LIF) or zero insertion force (ZIF) connectors. ENIG is not suitable as an edge connector for multiple insertion (more than five) withdrawals or high insertion force applications. This application generally requires an alternate metallic finish, such as electrolytic hard gold Aluminum Wire Bonding ENIG meets the requirements of MIL-STD-883, Method Variables that affect performance include cleanliness, substrate materials, wire thickness and surface topography. ENIG is not a surface leveler; the surface topography largely depends on the conditions of the underlying copper surface. 2 APPLICABLE DOCUMENTS J-STD-003 Solderability Tests for Printed Boards IPC-2221 Generic Standard on Printed Board Design IPC-6011 Generic Performance Specification for Printed Boards IPC-6012 Qualification and Performance Specification for Rigid Printed Boards IPC-6013 Qualification and Performance Specification for Flexible Printed Boards IPC-TM-650 Test Method Manual Detection and Measurement of Ionizable Surface Contaminants by Resistivity of Solvent Extract (ROSE) Adhesion, Tape Testing ASTM B (1998), Standard Specification for Autocatalytic Nickel Boron Coatings for Engineering Use ASTM B733-97, Standard Specification for Autocatalytic (Electroless) Nickel-Phosphorus Coatings on Metal MIL-STD-883, Method Test Method Standard, Microcircuits Bond Strength (Destructive Bond Pull Test) Telcordia GR-1217-CORE Generic Requirements For Separable Electrical Connectors Used In Telecommunications Hardware ISO-4527 Auto-Catalytic Nickel-Phosphorous Coatings; Specifications and Test Methods Annex A: Determination of Coating Thickness and Annex D: Determination of Coating Composition (Nickel and Phosphorous Content). Page 2 of 2525
7 3.2 Finish Thickness Thickness of the nickel and gold layers shall be measured and verified following the ENIG plating step in the PWB fabrication process. The use of X-ray fluorescence methodology for thickness determination shall be in accordance with APPENDIX 4 of this document and shall employ X-ray fluorescence (XRF) instrumentation equipped with software and hardware specific for ENIG measurement. The XRF test method is per ISO-4527, Auto-Catalytic Nickel-Phosphorous Coatings; Specifications and Test Methods Electroless Nickel Thickness The electroless nickel thickness shall be 3 to 6 µm [118.1 to µin] Immersion Gold Thickness The minimum immersion gold thickness shall be 0.05 µm [1.97 µin] at four sigma (standard deviation) below the mean; the typical range is to µm [2.955 to µin]. Higher gold thickness would normally require extended solution dwell time and /or increased solution temperature. This can increase the risk of compromising the integrity of the nickel undercoat due to excessive corrosion. Setup and measurement methodologies are crucial for accuracy (see APPENDIX 4 for recommended measurement techniques). 3.3 Porosity The ENIG immersion gold layer is not totally impervious and it will not pass the requirements of a classic porosity test (Nitric acid per ASTM B 735). 3.4 Adhesion The purpose of adhesion testing for ENIG finished boards is two fold. The first is for testing the plating adhesion of the immersion gold to the nickel as well as the nickel to the underlying metals. The second is for testing the adhesion of the solder mask to both traces and laminate. In both cases testing should be conducted on regions of high density on the board such as BGA sites, dams between fine pitch leaded devices or over areas of high trace density. The tape testing will be in accordance with IPC-TM-650, test method using a strip of pressure sensitive tape. There shall be no evidence of any portion of the surface finish or the soldermask being removed, as shown by particles of plating/pattern or soldermask adhering to the tape. If overhanging metal (slivers) or soldermask undercut breaks off and adheres to the tape, it is evidence of overhang or slivers, but not of adhesion failures. Because of the requirement to test in high density areas, the potential to leave tape residue that could interfere with soldering exists, especially if the wrong type of tape is used. Verification of zero impact to solderability must be demonstrated for the areas tested. 3.5 Solderability This thickness specification contained herein shall meet the coating durability requirements of Category 3 in J-STD- 003, i.e. greater than 6 months shelf life. Note: the use of steam aging (conditioning) is not an applicable accelerated aging (stress) test method for ENIG. 3.6 Cleanliness The ENIG finish typically will result in a surface that is cleaner than most other surface finishes. The process shall be installed and maintained using IPC-TM-650, Test Method Detection and Measurement of Ionizable Surface Contaminants by Resistivity of Solvent Extract, and shall meet the values given in IPC-6011 series of 1.56 µg/cm 2 NaCl equivalent. Failure to meet this cleanliness specification should trigger immediate corrective action. 3.7 Chemical Resistance Chemical resistance testing of the ENIG finish is not applicable. 3.8 High Frequency Signal Loss Higher frequency (>1 GHz) applications may experience signal loss due to increase electrical resistance (skin effect). Due to electrical differences such as this, special considerations should incorporated for any edge-coupled Page 5 of 2525
8 structures on the outer layers. This impact will be more prevalent when ENIG is applied over all copper features, such as application of ENIG before solder mask. This impact is minimized when only the pads (not the traces) are plated with the finish. 4 QUALITY ASSURANCE PROVISIONS General Quality Assurance Provisions are specified in IPC-6011 and each sectional specification. Additional requirements for PWB with electroless nickel/immersion gold (ENIG) plating are specified herein for qualification, acceptance and quality conformance. 4.1 Qualification Qualification of a PWB product is agreed upon by the user and supplier (see IPC-6011). The process capability of a supplier of PWB with ENIG finish shall be evaluated. APPENDIX 3 describes recommended qualification aspects of the ENIG process by a PWB supplier Sample Test Coupons Test specimens used for qualification of PWB with ENIG finish shall be as specified in IPC-2221 and each sectional specification. Specimen for additional testing requirements is listed in Table 4-1. Table 4-1: Qualification Test Coupons Test Type 1 Types 2,3,5 Types 4,6 Board Physical Requirements Plating Thickness M2, M5 M2, M5 M2, M5 X IPC-2221 Test Speciman M, surface mount solderability testing, mm [in] {EvaCombes: Please note, this drawing is designated IPC } 4.2 Acceptance Tests The sampling plan and frequency of acceptance testing except the solderability and plating thickness inspections shall be in accordance with IPC-6011 and each sectional specification. The sampling plan of the solderability and thickness of ENIG finish shall be as agreed upon by the user and the supplier (AABUS). 4.3 Quality Conformance Testing Quality conformance testing shall consist of inspections as specified in IPC-6011 and each sectional specification, with the addition of regular thickness inspections. The frequency of thickness inspection shall be 1 inspection (5 XRF measurements) per lot for Class 1 and 2, and as agreed upon by the user and the supplier (AABUS) for Class 3 products. Page 6 of 2525 153554b96e