Shelf Life = Solderability = Supply Chain Management
I had an interesting conversation with a colleague the other day. They wanted to return some printed circuit boards to an off-shore manufacturer because not all of the surface mount pads would solder. I help friends and customers troubleshoot problems all the time. Its just good business.
“Was it the entire lot of boards or only a few?”, I asked.
“Well, when we soldered the first batch the pads wet with solder with no problem. The second batch from the same lot has solderability issues. The pads don’t wet properly. I can’t explain it but nothing has changed here. The boards have ENIG on them so it must be the boards.”
“Well, if the first batch soldered and the second batch from the same lot didn’t then there’s a problem. How much time has passed between batches?”
“Hmmmm. About three years”
We then proceeded to have a conversation about shelf life. This is a topic that I find many people don’t know about or understand properly. Put simply, shelf life is a guaranteed period of time that the soldereble surface finish shall remain solderable. The shelf life timer starts when the solderable finish is applied to the copper. Standard shelf life times range from six months to a year. Beyond the shelf life period there are no guarantees that the printed circuit board shall remain solderable.
Its all about the solderability.
One very important thing to point out is that the same printed circuit board design manufactured by two different board manufactures shall have a variance in performance. Both manufactured boards should meet the minimum customer or industry standards for performance and acceptability. However, no two board shops are alike. We all use different materials, chemistries and equipment to build the product. A board manufactured by one shop shall be better than the board manufactured by another. Likewise the board manufactured by one shop may generously exceed the minimum shelf life by many months. Another manufacturer may exceed the minimum shelf life by one or two months. The overall manufacturing operation and process of each manufacture has an effect on the shelf life of a given product.
Beyond the overall bare board manufacturing process there are several key variables that determine the shelf life solderability of a printed circuit board. The first is the type of finish. How well does the design interact with the bard board manufacturing process. Next, what environment are the boards subjected to in manufacturing, in transit, in storage or at the assembly facility? What type of device is being soldered onto the board? What type of equipment is being used for soldering? Are the boards dry baked? What is used to clean the bare board surface? What type of flux is used for soldering? How much intermetallic growth is there. How much surface oxidation is there? More or less anything that affects solderability or the soldering process either improves or subtracts from the solderability shelf life. There are a lot of variable to consider. Variables that can occur at the board manufacturer, in transit or at the assembly facility.
When you consider the number and types of variables that effect solderability, standard shelf life times are more of a conservative time period. If you are a prototype contract manufacturer shelf life time limits are less of a concern. Product is manufactured and assembled in a matter of weeks. Standard manufacturing volumes are a different story. Boards can be manufactured months in advance to being used. Spare parts become an inventory item to be used at a later date. That later date can be months or a year later. Tight control over your supply chain becomes critical. The larger the organization the more critical a knowledgeable bare board / assembly specialist becomes. This is someone that understands the bare board process and assembly techniques. These people are worth their weight in gold and they are few and far between.
In a perfect world manufacturing yields are at 100%. In the reality of manufacturing there shall always be non-conformances. Yields are never at 100%. Companies are always cutting costs. Putting off maintenance or letting people go in hard economic times. When this happens variables are introduced into the manufacturing supply chain. Non-conformances result in assembly lines with partially assembled product coming to a halt because of part shortages, a perceived problem or a quality question. A manufacturing line at rest produces no product, ties up capitol and in the end costs money. Having a knowledgeable individual manage the manufacturing supply chain ensures product reliability, improves the shelf life, ensures solderability and in the end saves money. They more or less pay for themselves and I know and have worked with some of the best. If you are looking for an individual of such caliber, send me an email at firstname.lastname@example.org and I’ll point you in their direction.