The 2007 IPC specification IPC-4554 is applicable to the production of immersion tin (ISn) as a surface finish for printed circuit boards. It relates specifically to the solderability of the finish for reliability and reproducibility in printed board manufacture and tackles the difficulty in extending shelf-life for over six months for this surface finish type.
The immersion tin finish is a single layer of tin that is deposited directly over the copper surface of the printed board. The function of the layer is twofold: to provide a protective surface to prevent oxidation of the copper, and to create a reliable solderable surface. Tin has also been used for press fit connections and as the interface for Zero Insertion Force (ZIF) edge connectors. However, the specification from IPC focuses solely on tin as a surface for soldered joints.
The main issue with tin as a surface finish is that copper and tin have a strong affinity for each other. This means that over time the two metal layers will diffuse across the copper-tin boundary. Eventually, this will compromise the integrity of the tin over the copper, and oxidation of the underlying surface will occur. Delaying this inevitable diffusion as much as is practicable is key to extending the shelf life of the component.
Fundamentally, it’s the thickness of the tin deposit that determines the longevity of the part. Therefore the aim of IPC-4554 is to provide a standard tin thickness that manufacturers can use to achieve a reliably solderable finish for all surface-mount and through-hole assembly applications. Adherence to the specification should result in components with a category 3 durability per J-STD-003 for a shelf life of six months.
As mentioned above, the tendency of the copper to diffuse through the upper tin layer has a negative effect on solderability and shelf-life. It also makes it potentially difficult to accurately measure the thickness of the tin layer. This is a serious issue, as it’s the thickness of this layer that needs to be controlled to achieve a good quality component.
While IPC-4554 differentiates between total tin thickness and usable tin thickness, on its own, XRF can only determine total tin thickness. In order to determine usable tin thickness, a destructive test using another technique must be performed. XRFs used for analyzing tin thickness can be set up in different ways depending on the kind of detector found in the instrument – either a proportional counter or a semiconductor detector like a silicon drift detector (SDD). Using an XRF that is not set up correctly may result in inaccurate readings. The IPC-4554 specification helps manufacturers to ensure they’re carrying out accurate measurements by giving detailed XRF instrumentation calibration instructions, including the use of XRF standards specifically suitable for this type of measurement. A full discussion of the influence of detector choice and analytical line selection, as well as the use of foil standards rather than electroplated standards (to prevent the impact of intermetallic layer formation) is given, with recommendations on how to incorporate this into production-scenario XRF calibration.
The specification covers visual inspection of the finish, as well as thickness parameters, porosity, adhesion, solderability and tin whisker issues. You can obtain a full copy from IPC here.
Hitachi High-Tech Analytical Science are members of the IPC and fully recommend following IPC guidelines for maximizing the quality and reliability of the immersion tin surface finish. Our XRF instruments are developed to keep up with the rapid advances of PCB technology, and are designed to help you achieve consistency and reliability in production.
We have a range of instruments that can help you conform to IPC specifications. Find our more here: