Statement of Scope

This specification sets the requirements for the use of Immersion Silver (IAg) as a surface finish for printed circuit boards. This specification is intended to set requirements for IAg deposit thickness based on performance criteria. It is intended for use by supplier, printed circuit manufacturer, electronics manufacturing services (EMS) and original equipment manufacturer (OEM).

Description

IAg is a thin immersion deposit over copper. It is a multifunctional surface finish, applicable to soldering, press fit connections and as a contact surface. It has the potential to be suitable for aluminum wire bonding. The immersion silver protects the underlying copper from oxidation over its intended shelf life. Exposure to moisture and air contaminants, such as sulfur and chlorine, may negatively impact the useful life of the deposit. The impact can range from a slight discoloration of the deposit to the pads turning completely black. Proper packaging is a requirement.

Objective

This specification sets the requirements specific to IAg 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.

Performance Functions

Solderability

This primary function of IAg 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 and industry data, when handled per this specification’s requirements.

Similar to Gold, Silver forms a brittle intermetallic compound Ag₃Sn with Tin. For the recommended deposit thickness the possibility of silver embrittlement is negligible, especially when the assembly solder alloy is a Sn/Pb composition.

Excessive thickness of Immersion silver combined with lead free silver bearing solder, have the potential to create an embrittled solder joint. The committee is working to define the upper wt% silver content in a solder joint at which the long term reliability remains acceptable.

Contact Surface

There is a possibility for using IAg for the following applications. The use of Immersion Silver is acceptable for the IPC-6010 series Class 1 and Class 2 applications but is NOT currently recommended for the IPC-6010 series Class 3 applications which are for High Reliability Electronic Products where equipment downtime cannot be tolerated, and the circuitry must function, when required. Examples of such applications are for life support items and critical weapons systems.

Membrane Switches

The IAg surface with as little as 0.1 µm [4 µin] of immersion silver has demonstrated that it is suitable for one million actuations with negligible resistance change. However the end use atmosphere (temperature/humidity/contaminants) may degrade this performance. The end user shall determine the impact of use environments on the IAg deposit.

Metallic Dome Contacts

Data on this topic should be submitted to the IPC 4-14 Plating Processes Subcommittee to be considered for inclusion in upcoming revisions of this standard.

EMI Shielding

IAg is one of the surface finishes that may be used as an interface between electromagnetic interference (EMI) shielding and the printed wiring board (PWB). A key characteristic for this application is a consistent metal interface between the PWB metallization and the shield material. The formation of a highly conductive interface between the two surfaces will ensure excellent EMI shielding capability, which should also provide resistance to atmospheric influences on the IAg deposit. The end user shall determine the impact of the end use environment on the reliability of the shield interface. Tarnish of surrounding areas not in contact directly with the shield is NOT a reason to reject the PWB/deposit but rather an indication of the impact of the atmosphere on an active metal.

Aluminum Wire Bonding

IAg meets the requirements of MIL-STD-883, Method 2011.7. Variables that affect performance include cleanliness, substrate materials, wire thickness and surface topography. IAg is not a surface leveler; the surface topography largely depends on the conditions of the underlying copper surface. While producing acceptable wire bonds, silver, unlike the other Noble metals used for this application, is potentially not stable due to its reactive nature with the atmosphere in which it exists. Total encapsulation of the wire bonded sites is recommended in order to ensure consistent and reliable long term bonds. The committee is actively seeking additional data on the use of Immersion Silver as a suitable wire bonding metallization.

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 technique that uses an X-Ray beam aimed at an object’s 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.