Since the NASA report about 30 years ago about embrittlment of solder joints by the formation gold/tin intermetallics and the other 300 articles that followed, gold has been many times unjustly blamed for many solder joint failures. I am aware of one assembler who soldered HASL coated boards who accused his solder supplier of sending hin solder contaminated with 4% gold. Those of us who have been around the industry for a while and look at these thing have see intermetallic related failures with copper, nickel, silver, gold and palladium; together they form about 18 intermetallics with tin and several more with lead. Most of these are brittle and are concentration and soldering temperature related. Several modes of failure are seen: (1) Intermetallic crystals form in the bulk of the solder and form brittle structures or initiation points where cracks originate and spread (2) A layer or several layers of different composition intermetallics forming at the interface between the solder the underlying metal (copper, nickel, gold, silver, palladium). Cracks may form in one of the more brittle intermetallics or between intermetallics of different compostion. (3) As the underlying metal which often has occluded or codeposited organics, bacteria or other elements moves into the solder to form the intermetallics, it does not have a place to put these other materials as it forms the nice pure intermetallic crystals; therefore, it kicks them out into a weak debris layer between the intermetallic and the base metal or in some case between two intermetallic phases. These debis layers have been shown to be composed of compounds containing sulfur, carbon, iron, aluminum and other miscellaneous crap. All three mechanisms are related to intermetallic formation and may cause solder joint cracking. They may be formed during soldering or by solid state diffusion during operation. At room temperature the reactions are very minimal, but at boiling water temperature or under-the-hood temperatures or in the passenger compartment of my car in Arizona in the summer time the reaction is greatly accelerated. I know that many of you have seen parts that left the assembly floor in great shape and then fail 18 months later. Analysis of the solder joints on these failed units showed that the strength of the solder joint the the intermetallic layer had gone from super to zip and that any movement caused by mechanical or thermal stress caused solder joint cracking. So much for the mechanics, gold/tin forms "one" of the more brittle intermetallics and gold should be minimized as saturation ( about 4%) of the gold in the tin may occur readily in localized areas or in layers at the interface. If all of the gold in the platings could be mixed in the solder the likelyhood of reaching saturation is minimal. As you may have seen people smearing around gold plated lands with a soldering iron and a small amount of solder, they are diluting the gold with the solder. What the maximum plating thickness can be to prevent the concentration at the interface is debatable. Wave soldering often washes away or mixes the gold sufficient to prevent saturation, hand soldering is also a way of causuing good mixing and diffusion in a liquid state. Reflow or oven soldering produces the minimum of mixing or diffusion and therefore the gold must be thin. For reflow ovens, some people have a 0.2 micron (8 microinches) number; others have a 0.5 micron (20 microinches) number. All of this depends on your peak temperatures, dwell etc. For hand soldering a maximum number of about 1 micron (40 miroinches) has been used sith good success. Again going back to the theory try, try and try again. I would solder one sample your normal way, take another soldered sample and put it in the oven for about 16 hours at 150 deg. C and a third sample I would strip the gold by dipping or wicking before soldering . Try to pop the componet from the board with sharpened orange stick a small screw driver or if you are more wealthy use a Instron, pulling from one edge. If one or both or all is easily removed or the joint strength seems to be different go look for the cause. A typical acceptable one often breaks the leads or pulls the land from the board. A weak one pops clean, often with you thumb nail pressure. There are many articles written on sophisticated test methods, if you feel squeamish go do some of these. However, in closing do not automatically give "gold" a hit when you have cracked solder joints, I find it is only to blame in about 20% of the investigations that I have done. Phil Hinton Hinton "PWB" Engineering *************************************************************************** * TechNet mail list is provided as a service by IPC using SmartList v3.05 * *************************************************************************** * To subscribe/unsubscribe send a message <to: [log in to unmask]> * * with <subject: subscribe/unsubscribe> and no text in the body. * *************************************************************************** * If you are having a problem with the IPC TechNet forum please contact * * Dmitriy Sklyar at 847-509-9700 ext. 311 or email at [log in to unmask] * ***************************************************************************