Simple Yield Improvement. (Part 1)
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…
Many readers of this blog have come to understand that I am a strong advocate of Dry Baking. Many electronic materials are hygroscopic. Materials absorb moisture to the point of equilibrium. Moisture must be removed from electronic materials prior to them being exposed to assembly temperatures. This is done through a baking operation. For printed circuit boards and sensitive electronic devices this is critical. High end manufacturers in the military and aerospace industries understand the need and benefit for dry baking. Commercial level manufacturers tend to view dry baking as a cost to be reduced and eliminated. The consequences of eliminating dry baking are either not properly understood or are not viewed as critical enough for the manufactured product. To appreciate the need for dry baking it helps to understand what happens to moisture bearing materials (printed circuit boards and electronic components) when they are exposed to assembly temperatures.
What happens to moisture bearing materials when they are exposed to moisture?
Printed Circuit Board (PCB) materials expand when exposed to heat. Different materials expand at different rates when they are heated. Printed circuit boards are composite structures consisting of metal, glass and polymers (epoxy resin). Metals and glass expand at a consistent rate up to their melting points. Polymers have two states of material expansion. The temperature where the rate of expansion changes is referred to as the glass transition temperature (Tg). Below the Tg of the polymer, the material is solid and hard. Above the Tg of the polymer, the material becomes soft and pliable. Above the Tg the rate of expansion is also greater by a factor of 10 compared to the expansion rate below Tg.
In a PCB the distribution of copper, glass and epoxy is not uniform. One layer of the PCB may be a continuous ground planes. Another layer of the PCB may be a split power plane. The board may be an unusual shape to fit into a tight enclosure. The top side of the PCB may be very busy with several Ball Grid Array (BGA) devices and other IC devices. More than 50% of the surface area on top is copper. The secondary side could be sparsely populated with capacitors and resistors. Less than 50% of the surface area on the bottom is copper. There are holes drilled into the PCB and a few slots added in to eliminate noise from surface creep or even areas susceptible to Conductive Anodic Filament (CAF) formation.
When a PCB is exposed to assembly temperatures the copper, glass and epoxy all expand at different rates. The epoxy has a very high rate of expansion. The Glass has a much lower rate of expansion compared to the epoxy. In a PCB the glass takes the form of woven fiber glass cloth and provides a compression force to counter the force of expansion in the X and Y axis (Length and Width) of a PCB. The copper also has a much lower rate of expansion compared to the epoxy. In a PCB the copper on a given layer helps to provide a compression force that helps to counter the force of expansion in the X and Y axis of a PCB. The copper in a plated through hole helps to provide a compression force that helps to counter the force of expansion in the Z axis (Thickness) of a PCB. Copper does not provide much of a compression force in the Z axis since copper stretches. When copper stretches beyond its point of elasticity it may crack resulting in an open.
As a PCB is heated through the assembly process there may be some small amount of warp and twist observed. This is the result of the difference between the different rates of expansion. When the resultant expansion force (expansion – compression) of one area of the PCB is greater than the resultant force of another area of the PCB we see a slight warp or a twist. The distribution of the copper and material removed from holes and slots affects the direction and severity of the warp and twist. Some amount of warp and twist is to be expected in any given design. The amount visible on some designs may be very small.
How does moisture affect the system?
Copper and glass do not absorb moisture. The moisture however can be absorbed into the epoxy, at the epoxy/glass interface, into micro-cracks and into resin voids. When the moisture is heated at assembly temperatures the moisture vaporises. When the moisture vaporises the result is an explosive expansion force. The added expansion force of the moisture can be observed as severe warp and twist. The amount of warp and twist from explosive moisture expansion is enough to interfere with component assembly. Some surface mount devices may shift towards the pad edge and form a poor solder joint since the device is too close to the pad edge. The corner of a large surface mount device may not make contact with the pads to be soldered to since the device can not rest flat on the bowed surface. These are the obvious effects. What should be a concern are the hidden effects we cannot readily observe.
When the industry converted to Lead-Free assembly practices the materials commonly used for printed circuit board manufacturing were found to be inadequate. Traditional laminates rely on a curing agent known as Dicyandiamide (Dicy) to harden the epoxy into a hardened material. Dicy cured epoxy systems are acceptable for traditional tin-lead soldering. Dicy systems are not able to withstand the higher assembly temperatures at the longer dwell times for multiple temperature excursions and survive intact. At the higher temperatures the epoxy burns away at an accelerated rate. When enough of the epoxy burns away the structure of the PCB is compromised. The PCB either delaminates or cavities form that become locations for moisture to be absorbed into. These could potentially adversely affect the circuit performance in the field. Voltage leakage between conductors and even CAF may form as a result.
Phenol novolac (Phenolic) cured epoxy systems have been agreed upon by the majority of the members of the IPC as the preferred Lead-Free assembly compliant material to replace Dicy cured epoxy systems. Phenolic cured epoxy systems have been found to be able to withstand the higher assembly temperatures, at the longer dwell times and through multiple thermal excursions. However, whereas the Phenolic cured epoxy systems can withstand Lead-Free assembly temperatures they are more brittle, mechanically weaker (lower bond strength) and more moisture absorbent than their Dicy cured counterparts.
What this means in simple English is that Phenolic cured epoxy systems are even more susceptible to moisture born delaminations. Even the warp and twist that occurs to a PCB with absorbed moisture is damaging. The Phenolic cured epoxy is more brittle and mechanically weaker. It does not take as much force to fracture the bonded epoxy when the PCB is deformed. Mechanical born delaminations may be observed between plys of pre-preg or between the pre-preg and core or a PCB.
It is important to point out that PCBs absorb moisture readily. Once exposed to the environment the process of absorption begins. Assembly environments are humidified to cut down on static electricity that can result in damage to sensitive components. Area humidification is a source of moisture the PCB and electronic components shall readily absorb.
Dry baking removes the moisture from the PCB. The explosive expansion force from the moisture is removed from the PCB. The potential of warp and twist is greatly reduced. The potential for moisture and mechanical born delaminations is reduced if not eliminated. In process assembly non-conformances related to component placement and solder joint formation are reduced. Hidden damage to the PCB not readily observed as a result of the explosive moisture expansion are eliminated.