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”

“ouch”

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 david.duross@pcbdesignschool.com and I’ll point you in their direction.

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2 Responses to “Shelf Life = Solderability = Supply Chain Management”

  1. rksalman Says:

    Can you please go one step further and supply me with data on the shelf life of a pwb that does have the components already installed? I realize there are many variables but am trying to find information on how long an FR4 board will last barring of course the components do as with all I have read the components would need to be replaced long before the board in a non-corosive enviroment. Is there any good reading out there on this topic?

  2. David Duross Says:

    Once the components are installed the shelf life is no longer a factor. With regards to this post the term “shelf life” refers to how long the board remains solderable. I believe what you are referring to is product longevity or product life expectancy. How long shall the component last in the field under load or in use. As you mentioned, there are many variables that affect the life span of a given product.

    Is the assembly lead free or non-lead free? How complex is the design? What types of materials are used? How was the product assembled? What type of operating environment is the product in? Critical electronics are tested through longevity simulations.

    Some sites that may interest you are…

    http://www.army.mil/article/40712/Army_works_to_decrease_lead_free_electronic_components/

    http://www.retra.co.uk/code.asp?p=13

    http://www.calce.umd.edu/general/software/calcePWA/iest2002r2.pdf

    If you Google search for “electronic life expectancy simulation” you should find some leads on the information you seek. Variations of these search terms should also provide you with some good results.

    Now, as far as the printed circuit board is concerned…

    In a non-corrosive environment I believe the biggest challenge to the printed circuit board shall be moisture and heat. Once the board is assembled the board should last forever. Once you start using it you add wear and tear to it. How much depends upon where the device is used.

    When you turn a device on and give it power the board heats up. You turn it off the board cools down. A computer in an air conditioned room is kept cool. The same computer on a hot summer day in an house without air conditioning runs hot. When the computer temperature temperature increases past the Maximum Continuous Operating Temperature (aka, MOT) of the weakest component, that weakest component slowly burns up. The damage is cumulative and may take months to reach a point where the device doesn’t function anymore. The weakest link may or may not be the board.

    Humidity comes into play if the board is damaged enough in the assembly process and the E-glass and epoxy separate. The separation fills up with moisture. If the separation forms between two unlike conductors where one is positively charged and the other is grounded you can have CAF. CAF is short for Conductive Anodic Filament growth. You effective electroplate a copper sliver between the positive and negative conductor in the separation until a short forms.

    Longevity simulation testing of a given product line is the only way to know how well a product shall last in the field. I do have some information regarding MOT and CAF listed on this site if that is of interest to you. One of the other site members may have something to contribute to your question as well.

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