Monday, December 21, 2009

The Tin Commandments - Just in Time for the Holidays!

The Tin Commandments
Copyright © DfR Solutions, College Park, MD, 2009 All Rights Reserved.

1. Thou shalt have no Lead (Pb) in the solder for EU (among other) applications.
The European Union, the USA, most industrialized Asian nations, etc., all require that all commercial products to be sold in their Countries must have hazardous materials (like Mercury and Lead) removed from PC Boards. The deadline for compliance was 2006 with elimination of some exemptions forthcoming. The most common Pb-free solder alloys consist mostly of tin with small additions of silver an copper. Some question why the electronics industry, which accounts for less than 0.1% of the lead in land-fills, was targeted by the EU. It matters not, since the train has now left the station. Even exempt industries such as telecommunications, medical, and the military are finding it increasingly difficult to continue using Pb.

2. Thou shalt not make Tin (Sn) whiskers that will cause short circuits.
Pure Tin tends to make “whiskers”, a short growth from the main body of Tin, and small Tin lands make the whiskers grow the most. Historically 1-2% Lead (Pb) was used to prevent whiskering in tin coatings. Various methods are used to mitigate whisker growth without use of Pb but the only way to prevent it is use coatings such as Pd or Au.

3. Thou shalt not take Pb-free ball grid array components and solder them onto a Pb-based solder pad.
Though it is not impossible to achieve a reliable mixed solder joint, the component must reach 230°C to do so. The PCB and other components must be designed to survive the higher temperature.

4. Remember the Sabbath Day (and 2nd and 3rd shift workers).
Building high quality products with Pb-free solder requires consistently hitting a much more narrow process window. Your off hour workers must be trained just as well as the 1st shift workers.

5. Honor thy supplier qualification and process control requirements.
In order to ensure that you get the materials you specify, trust the documentation from your suppliers.

6. Thou shalt not kill your boards with the higher temperatures in reflow ovens in a Lead-free application.
Notice that the temperature difference in most SMT re-flow ovens between a Leaded Solder and a Lead-free Solder is about 35 degrees C. Specifying the appropriate board properties is critical to prevent delamination, warping, cracked vias, or worse. Components must also be specified to survive reflow conditions (JST-020C should be followed religiously).

7. Thou shalt not mate female Gold connectors with male Tin connectors (or vice versa).
The reason for this is because of the inter-metallic that is made between Gold and Tin (or a solder Tin alloy) when they are repeatedly rubbed together. Build up of this inter-metallic can cause an open connection. If you need a good connection, use gold with gold. Tin with tin is also acceptable for applications where fretting is not a concern.

8. Thou shalt not steal published quality and reliability data.
Since your environment is unique to you, don’t expect to get similar results when you application differs from that of published reports.

9. Thou shalt not bear with the same rework standards after going Lead-free.
Experience shows that the re-work stations in most PC Board manufacturing facilities are often overlooked as an essential area that must be controlled. Hitting the small process window requires well controlled rework profiles and often new equipment. “Touch up” areas are becoming extinct because of the extra heat of the solder iron that’s required in a Lead-free process. The quality of all rework processes should be validated with close inspection.

10. Thou shalt not covet the Tin mines in China, Indonesia, and South America.
It’s true that our planet’s main resources for Tin are in China (40%), Indonesia (20%) and South America (20%) and yet it seems that Chinese companies are buying up another Tin mine every week. Tin is available in mines across our world, but the best mines are not in the Americas or Europe.

Copyright 2009 by DfR Solutions and VALUE Engrafting. All Rights Reserved.