Archive for August, 2011

What damage does the assembly process do to a pcb? (part 6)

Wednesday, August 31st, 2011

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…)

What damage does the assembly process do to a pcb? (part 5)

Sunday, August 28th, 2011

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…)

What damage does the assembly process do to a pcb? (part 4)

Tuesday, August 23rd, 2011

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.
(more…)

What damage does the assembly process do to a pcb? (part 3)

Sunday, August 21st, 2011

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…)

What damage does the assembly process do to a pcb? (part 2)

Friday, August 19th, 2011

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…)

What damage does the assembly process do to a pcb? (part 1)

Wednesday, August 17th, 2011

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…)