IPC-4554 Immersion Tin Plating for PCB

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  1.1 Scope
This specification sets the requirements for the use of Immersion Tin (ISn) as a surface finish for printed circuit boards. This specification is intended to set requirements for ISn deposit thickness based on performance criteria. It is intended for use by supplier, manufacturer, contract manufacturer (CM) and original equipment manufacturer (OEM).
1.2 Description
ISn is a metallic finish deposited by a chemical displacement reaction that is applied directly over the basis metal of the PWB, i.e copper. Primarily used as a solderable surface. It has been used in press fit connections and as the interface for Zero Insertion Force (ZIF) edge connectors.  
The immersion tin protects the underlying copper from oxidation over its intended shelf life. Copper and tin however have a strong affinity for one another. The diffusion of one species into the other will occur inevitably, directly impacting the shelf life of the deposit and the performance of the finish.
The latest generation of immersion tin deposits utilize various methods of retarding the diffusion process, including the use of co-deposition of organics, the use of another metal as a diffusion barrier or the use of grain structure refining. It is recommended that the user of the deposit clearly understand the different methods of copper migration retardation and that the supplier know the positive and negative impacts of the system chosen.  
1.3 Objective
This specification sets the requirements specific to ISn 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
1.4.1 Solderability
This primary function of ISn is to provide a solderable surface finish, suitable for all surface mount and through-hole assembly applications and with an appropriate shelf life. The deposit has demonstrated the ability to meet a shelf life of 12 months per J-STD-003 when produced per this specifications requirement.  
Due to the diffusion of the copper through the tin deposit and its impact on solderability, the ability to meet the 12 months shelf life is DIRECTLY related to the deposit thickness. Storage conditions for the I Sn PWB is of near equal importance with regard to meeting an acceptable shelf life. Exposure to high storage temperatures will accelerate the migration of copper tin intermetallics through the deposit. Storage in a high humidity environment with the PWB’s removed from their original packaging may increase the oxide thickness and species and have likewise a detrimental effect on solderability. This diffusion also has a negative impact on the correct thickness measurement of the deposit.
The ability to measure and differentiate the tin species in the deposit is imperative to ensure the manufacture and receipt of parts with this useable shelf life. The use of correct XRF standards is imperative. The use of foils over polyester (Mylar® is very common) prevents the impact of basis metal diffusion and should be the “standards of choice” – see Appendix 4 for detailed recommendations.  
1.4.2 Contact Surface
Immersion tin is not a suitable finish for soft membrane switch applications.
1.4.3 Electromagnetic Interference (EMI) Shielding
A key characteristic for this application is a consistent metal interface between the PWB and the shield material. Due to the dynamic nature of the ISn deposit and the basis metal (Cu) of the PWB, the interface between the EMI shield and the deposit is NOT consistent. The growth of Inter metallic compounds (IMC’s) will change the electrical characteristics of the interface between the EMI shield and the deposit. It has however been demonstrated to be a suitable interface for EMI shielding for certain specific applications. Testing for suitability is recommended.  
1.4.5 Connectors

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1.4.5.1 Press Fit
The use of ISn as a deposit suitable for press fit requirements shall meet Telcordia GR-1217-CORE. It should be noted that changing to a thin immersion deposit such as ISn from HASL may require a re-evaluation of the pre-fabrication hole sizes to ensure the correct interference fit required for press fit applications.  
The possibility of tin whisker formation, as a direct consequence of stresses induced on the deposit as a function of press fit insertion, exists. The end user shall determine the impact of whisker formation on the reliability of the module in its end use application.
1.4.5.2 Edge Tab
ISn as a surface finish for edge connectors utilizing a zero insertion force (ZIF) connector, i.e. for memory modules, has been successfully demonstrated as an acceptable finish.  Whisker formation is a concern particularly in fine pitch devices. The end user shall determine theimpact of whisker formation on the reliability of the module in its end use application.
1.4.6 Wire Bonding
ISn is not suitable as a surface finish for either aluminum or gold wire bonding due to the instability and metallurgical incompatibility of the deposit.
1.5 Definition of Terms
The definition of all terms used within this specification shall be as specified in IPC-T-50 and as defined below:
Solvent Extract Conductivity (SEC) Surface ionic contaminants may be extracted/dissolved by solvents. The bulk ionic cleanliness of an item is then quantified by measuring the solution of extracted contaminants with an ionograph instrument and reported in units of micrograms of sodium chloride equivalent per unit surface area of the tested item.
X-Ray Fluorescence (XRF) or Energy Dispersive X-Ray Fluorescence (EDS-XRF) A nondestructive chemical analysis techniques that uses and X-Ray beam aimed at an objects surface. The incident beam causes secondary (fluorescent) X-Rays to be generated. These fluorescent X-Rays are then measured to determine the identity of metal or other high density elements and their quantity (thickness) present.
2 APPLICABLE DOCUMENTS
2.1 IPC J-STD-003 Solderability Tests for Printed Boards IPC-2221 Generic Standard on Printed Board Design 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
2.3.25     Detection and Measurement of Ionizable Surface Contaminants by Resistivity of Solvent
   Extract (ROSE)
2.4.1     Adhesion, Tape Testing

2.6.14.1 Electrochemical Migration Resistance Test
2.6.3.5 Bare Board Cleanliness by Surface Insulation Resistance.
2.2 Telcordia™
Telcordia GR-1217-CORE Generic Requirements For Separable Electrical Connectors Used In Telecommunications Hardware
GR78-CORE Surface Insulation Resistance Test section 13.1.4
3 REQUIREMENTS
3.1 Visual
ISn surfaces shall be inspected in accordance with the visual examination sections of the IPC-6011 series, and specifically IPC-6012 which specifies a nominal magnification of 1.75X (approximately 3 diopters). The coverage shall be complete and the finish shall be uniform on the surface to be plated (see Figures 3-1 and 3-2 for uniform coverage and Figure 3-3 for improper coverage, as visualIy identified with ISn-plated surfaces).
ISn appearance, like all thin immersion deposits, is directly influenced by the surface preparation of the underlying copper. Depending on the micro-etch chemistry used, the color of the deposit will vary from a white matte appearance to a shiny silver appearance. All colors are acceptable provided that the appearance is uniform through out the product supplied. The coverage shall be complete and the finish shall be uniform on the surface to be plated. There shall be no extraneous plating, skip plating, edge pull back, dark or discolored pads or mis-registered solder mask on the plated surface for all classes of product.

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ask on the plated surface for all classes of product.


Figure 3-2 Example of uniform plating
3.2 Finish Thickness
Thickness of the ISn shall be measured and verified following the Tin 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 ISn measurement. Due to the dynamic nature of the deposit and its interaction with the basis metal, a clear distinction of “useable” tin as compared to total deposit thickness needs to be identified. It is recommended that the use of coulometric stripping or similar techniques e.g. Auger / XPS be employed at a minimum during qualification and at a regular interval for confirmation “as agreed between user and supplier” (AABUS). See APPENDIX 5 of this document regarding the use of coulometric stripping and other accepted techniques.

3.2.1 Immersion Tin Thickness
The minimum immersion tin thickness shall be 1 micron (40 micro inches) at  -4 sigma from the process mean when measured on a pad of 1.5 mm X 1.5 mm [60 mil X 60mil] or equivalent area of
2.25 mm² [3600 mils²] and where the typical value ranges from 1.15µm  to. 1.3µm. While ISn deposit rates may not be as susceptible to pad geometry/feature size as compared to other Immersion metallic finishes, the use of a recommended pad size is to ensure accurate XRF measurement by forcing the 30% rule for collimator size relative to feature size.  Excessively thick deposits, while not impacting solderability, may impact other board fabrication process negatively, e.g. solder mask and legend adhesion. This minimum deposit thickness is to ensure 12 months of shelf life (Category 3) and to meet the acceptance criteria of IPC/EIA J-STD-003.
Coating durability categorys are defined as follows per the ANSI-JSTD-003:
Category 1- Minimum Coating Durability
Intended for boards which will be soldered within 30 days from the time of manufacture and are likely to experience minimum thermal exposures. Recommended  minimum ISn shall be  0.6 microns (24 micro inches) at  -4 sigma from the process mean when measured on a pad of 1.5 mm X 1.5 mm [60 mil X 60mil] or equivalent area of 2.25 mm² [3600 mils²]
Category 2- Average Coating Durability
Intended for boards likely to experience up to six months from the time of manufacture and moderate thermal or solder exposures. Recommended minimum ISn shall be  0.6 microns (24 micro inches) at -4 sigma from the process mean when measured on a pad of 1.5 mm X 1.5 mm