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 (more…)

I thought I would take a few minutes to clarify and highlight a few recent changes that I’ve made to the site.

1. I’ve added a site plugin named WPtouch that makes pcbdesignschool.com mobile device friendly. I’ve tried to use my Droid smart phone to show friends my content. I’ll be the first to admit that its a slow painful process waiting for the content to load. WPtouch reformats the blog so that mobile device users may enjoy the content quickly on the go. When you visit the site from a standard personal computer you can view the site normally. When you use a mobile device you automatically view the site in the mobile device friendly format.
2. We now have a polling plugin named WP-Polls that we use to ask a question of our visitors. The polls are pcb industry related. These are non-scientific polls and the results are available to anyone that wants to see them. We don’t capture, store or share anyone’s personal information. Its an opportunity to see what other people in the industry are thinking. (more…)

In this final planned post of the “What damage does the assembly process do to a pcb?” series we shall discuss copper diffusion.

What is copper diffusion?

When soldering, copper diffusion is a process in which copper atoms are removed from the copper surface and redistributed into the solder over a wide area.

Back in 2005 I was running some thermal and solderability tests on multilayer boards using various types of cured laminates, different surface finishes and various solder alloys. I was doing my RoHS and Lead-Free due diligence. I had a whole range of tests that I was running. One test consisted of solder dipping a test board in 288°C solder for 20 – 5 second intervals. I was trying to make a multilayer board built on phenolic laminate delaminate. When I reached the tenth dip I had to stop my test. I didn’t make the board delaminate but I did dissolve away much of the copper circuitry into the solder pot. (more…)

In part 4 of this series of posts I discussed the effect moisture has on the printed circuit board at soldering temperatures. I explained the material properties of FR-4 laminate and how they are hygroscopic. We also covered an acceptable practice known as dry baking used to force moisture from the product just prior to being exposed to soldering temperatures. I received some very good comments and feedback regarding part 4 of this blog series. I thought it appropriate to share this feedback in another post.

With regards to sources of moisture, in part 4 of this series I comment upon a practice where some printed circuit board manufacturers shall dry bake the printed circuit boards prior to shipping them. The important thing to add here is that common packaging materials used by the industry do not act as a 100% vapor barrier. The shrink wrap film used only slows down the process of moisture absorption by the product. Moisture in the environment outside the bag shall migrate through the protective film but at a much slower rate. Things to consider are as follows… (more…)

In part 3 of this series of posts I discussed how phenolic cured laminates are mechanically weaker than their dicey cured laminate counterparts. I pointed out some of the material properties listed on the material data sheets that explain and support this point. Whereas the phenolic systems are better at thermal management, the dicey systems are better under mechanical stress. There is no right or wrong here. The systems just perform differently under different circumstances. Understanding the differences and how they relate to the applied assembly process are important to ensure success.

On this post I would like to discuss the effect moisture has on the printed circuit board. What most people don’t realize is that printed circuit boards are hygroscopic. Boards shall absorb available moisture from the surrounding environment to the point of equilibrium. The various FR-4 laminate data sheets list a Moisture Absorption value. The values are calculated in accordance with the IPC-TM-650 2.6.2.1A specification.

The 2.6.2.1A specification basically tests a solid rectangular piece of FR-4 free of copper, no holes and the edges sanded smooth. First the sample is preconditioned, aka dry baked. Immediately after dry baking, the sample is weighed. This is the dry weight. The sample is then submerged for 24 hours in Distilled Water. The sample is removed, towel dried and then weighed. This is the wet weight. The wet and dry weights are then plugged into a formula listed in the 2.6.2.1A specification to produce the Moisture Absorption value.
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In part 2 of this post I explained how the T260 and T288 material data sheet values could be used as an indicator of how durable a laminate system (FR-4) shall be when exposed to heat. The higher the temperature applied the less time it takes to delaminate the FR-4. Traditional dicey cured epoxy systems do not stand up to lead-free assembly temperatures as well as one would think. The newer phenolic cured epoxy systems are much better suited and able to withstand the higher temperatures applied with lead-free assembly techniques.

GREAT! PROBLEM SOLVED!

Not exactly. (more…)

In part 1 of this blog post I commented upon the affect the assembly process has upon a printed circuit board. The assembly temperatures applied do in fact burn away the epoxy of the FR-4 composite material. The higher the temperature the faster the rate of burn. I touched upon the relation ship between the glass transition temperature (Tg), decomposition temperature (Td) and the Maximum continuous Operating Temperature (MOT). There is another gauge that can be used to help a designer or contract assembler understand this point and that is the Time to delamination test. These are referred to as either the T260 or T288 tests.

What is the T260/T288 Time to Delamination? This is a test defined by (more…)

A colleague contacted me the other day with a topic that would make an excellent post on this blog.

“How can we solder boards with a Tg of 180°C or even 200°C at temperatures of 225-245°C without damaging the board?  Even with leaded boards the peak reflow temperatures are way above the board’s Tg.  How is this possible?”

The answer is simple. Every time a printed circuit board is exposed to soldering temperatures it becomes damaged. This is the case not only for Lead-Free soldering applications but also for eutectic soldering consisting of tin-lead.

Tg is one of several parameters to be aware of. In the case of Tg most designers refer to the value as (more…)

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, (more…)

Envision HDI is an alternative copper deposition process that just works. Deposition or metalization is the basic foundation of a plated through hole structure. The deposition material makes a drilled hole conductive enough to allow copper to be electroplated within the drilled hole. Without metalization there would be no plated through hole. Of course there are several different chemicals available to metalize a non-conductive surface.

Electroless copper deposition was the original chemical process employed by the pcb industry to form this conductive layer. There were many downsides to this process. Chemical cost, time of operation and waste treatment to name just a few. Some of the chemicals used in the overall process are highly toxic. The process has been improved over the years.

Copper deposition was the only option for a number of years until (more…)