Shelf Life Identification of a PCB.
How does one identify the shelf life of a printed circuit board?
By convention the printed circuit board fabricator shall mark the printed circuit board with a four digit date code. The date code consists of a two digit week number ranging from 01 (the 1st week in January) to 52 (the last week in December). The year code would be 99 for the year 1999 or 01 for the year 2001. Commercial fabricators use a week/year format. Date code 0199 would be the first week of January of 1999. Military fabricators use a year/week format. Date code 0152 would be the last week of December of 2001. The printed circuit board fabricator shall place the date code on what is traditionally referred to as the circuit side of the printed circuit board.
What is the circuit side of a pcb?
To understand the term circuit side, you need to learn a little history of the printed circuit board industry. In the beginning there were single sided printed circuit boards. All of the holes were non-plated through. There were no conductors on the top side of the board. Conductors (traces and pads) were formed on the bottom side only. Pads were located at and around the non-plated holes. Traces would connect one pad to another as required by the design. Components (resistors, diodes, capacitors, etc…) were inserted into the non-plated through holes from the top side of the board. Hence the top side commonly became known as the component side. The bottom side with the conductors also referred to as circuits became known as the circuit side. The component leads were soldered to the pads on the circuit side. The circuit side was also referred to as the solder side since this was the side the component leads were soldered to.
The printed circuit board fabricator would generate artwork to help form the conductors on the circuit side of the board. The fabricator would add their identifying markings onto that artwork. A date code to identify the approximate date of manufacture was added next to the identifier. The circuit side became the conventional side where the printed circuit board fabricator added their markings. Bare board level markings of the customer were typically added to the circuit side as well.
Solder mask was soon applied to the circuit side of the board. To assist with the assembly process, component outlines and reference designators were added to the component side of the board. This was done with a silk-screening operation. The component side became the conventional side where the printed circuit board assembler added their markings. Assembly level markings of the customer were typically added to the component side as well.
Hence the marking location conventions were introduced to the industry and stuck. Component side for the assembly and circuit side for the board fabricator.
Through the years the technology advanced. Conductors were added to the component side. Eyelets were inserted into the non-plated holes. Soldered to eyelets formed an improved solder connection on both sides of the board. The double sided board had arrived. Eyelets gave way to the plated through hole commonly used today. Multi-layer constructions were introduced. Surface mount components applied to the component side and soon after the solder side as well were introduced. Ultimately components could be applied to either side of the printed circuit board. The terminology of component and circuit side became less representative of the actual product. A better description would be primary and secondary side.
Somewhere in all that history and evolution the date code resides.
What does the date code actually represent?
Neither the mil spec (MIL-PRF-55110) or IPC-6012 specify what fabrication process the date code represents. Both specifications identify the need for a four digit date code. However, both specifications provide the printed circuit board fabricator a wide degree of latitude on what the date code represents. With single sided boards it didn’t matter. There were so few steps and everything came with tin-lead solder. Over the years the process has become a lot more complex. I’ve been in the industry for a long time and have seen different fabricators define the date code to represent the following…
- The week the outer layers were imaged.
- The week the plated through holes were formed.
- The week the final finish was applied.
- The week the boards were scheduled to ship.
The specifications grant a wide degree of latitude. However, they do require that the date code representation remain consistent unless otherwise specified.
What does this mean to an assembler?
First off you need to find out how the printed circuit board fabricator defines the date code. Shelf life is all about solderability. The solderability timer starts when the final finish is applied. The type of finish determines the shelf life in months. Its either going to be 6 or 12 months.
For example, board fabricator #1 uses the date code to represent the week the board was copper plated to form the plated through holes. The board is processed through immersion tin a month later. The fabricator’s immersion tin is solderable for up to 6 months. Since the finish was applied well over a month after the date code was applied the actual shelf life would be 7 months by the date code. However, if you don’t understand the fabricator’s system you may decide to send the boards out to have the tin stripped and reprocessed through immersion tin. The date code on the board implies that the board exceeded the 6 month shelf life. You more or less just spent money you didn’t have to.
Another example, board fabricator #2 uses the date code to represent the week the boards were scheduled to ship. The boards are scheduled to be processed through immersion tin one week before the boards were due to ship. However, your buyer pushes the delivery in and out several times due to scheduling changes after the date code is applied by the fabricator. The fabricator’s immersion tin is solderable for up to 6 months. Your boards arrive well over a month early. You receive the boards with an inaccurate date code due to all of the scheduling changes. Your customer pushes the delivery out a few months. The boards don’t solder very well and you wonder why. The date code on the board indicates that the shelf life hasn’t passed the expiration date yet. In reality it has expired due to all of the scheduling changes.
Used properly the date code can be a valuable tool to monitor the shelf life of an unassembled printed circuit board. The challenge is verifying what the date code represents to the printed circuit board fabricator. Once you know that, you can better appreciate how the date code and application of the final solderable finish tie together.