Posts Tagged ‘Assembly’

Simple Yield Improvement. (Part 1)

Monday, December 31st, 2012

Over the past 15 years we have seen some very amazing advancements in technology. Our electronic devices have become smaller, faster and more powerful. The capabilities of these new devices have brought science fiction to life for many of us. What the common consumer does not realize is that these advancements have occurred in a more destructive Lead-Free assembly process. Lead Free assembly methods consisting of higher assembly temperatures (around 260ºC) for longer dwell times at temperature along with a smaller processing window for success. The assembly methods today are challenged to not only maintain yields but to improve them. All the while cutting costs where ever possible. The desire to cut cost is where we see the result of unintended consequences.

I have been asked repeatedly by customers for my opinion on methods to improve yields. I evaluate designs and work with designers and assemblers on solutions to help them improve yields. This includes in process and post process yields. In other words build it right the first time and make sure it lasts in the field. A common question I am asked by customers is…

What is the one thing that we can do that can  improve our yields?

The answer is very simple… (more…)

Shelf Life Identification of a PCB.

Saturday, November 5th, 2011

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…)

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…)

Vendor Spotlight: A solution to pad cratering.

Saturday, April 30th, 2011

Lead-free assembly techniques and practices have pushed the envelope of printed circuit board technology for years. The industry has been forced to change and improve as a result of customer demands and regulatory compliance. Lead-free compliance has been a journey filled with advancement and disaster. The advancement comes in the form of new improved materials and surface finishes. The disaster came in the form of all of the manufacturing non-conformances found along the way and yet to come. Over the past six years the electronics industry has been one giant crucible. Within this crucible we have tested the limits of the process at the bare board and assembly levels.

Prior to lead-free assembly practices there was eutectic solder consisting of tin-lead (Sn63/Pb37). As the saying goes “Nothing solders like solder”. Eutectic solder wicks easily at relatively low temperatures. Solder joints have a unique property where the inter-metallic copper-tin alloy grows over time. There are decades of historical information and observation on the longevity and durability of eutectic solder joints and assembled materials. In a competitive industry where change introduces the potential of non-conformances, eutectic solders are a proven and safe technology.

Enter lead-free assembly. (more…)

Dry baking printed circuit boards.

Sunday, July 19th, 2009

What most people don’t realise is that printed circuit boards are hygroscopic. They absorb moisture from the atmosphere like a sponge does water. Although the absorption process is much slower it does happen. The amount of moisture absorbed is measured and provided on material data sheets provided by most laminate manufacturers. For example  the moisture absorption limit listed on the Isola FR406 data sheet is 0.20 % measured on a .028 inch thick piece of stock laminate. Lead free assembly compliant laminates are even more hygroscopic. For example, Isola IS410 is 0.45 % measured on the same thickness material per the data sheet. Different grades of material from different manufacturers have different moisture absorption ratings. The percentage of moisture absorption increases when you add inner layer circuity and through holes. A low humidity environment results in a slow absorption rate. A high humidity environment results in a fast absorption rate. Moisture shall be absorbed to the point of equilibrium. Once equilibrium is achieved the absorption stops.

Printed circuit boards that have moisture in them can become (more…)