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IN THE 21ST CENTURY
INDUSTRY TERMS AND ACRONYMS
Printed Circuit board. This term is usually applied to the completed product consisting a fiberglass core with copper traces n’ spaces on the outer surfaces.
Trace n’ Space
The pattern applied to the layers of a PCB. Trace and space widths come from the design software and are contained in the “Gerber File” to the fabricator. They are usually designated by a thickness (in mils) of the trace followed by the thickness of the isolating space between the traces. For example, 5/6 refers to a 5-mil trace and a 6-mil space.
A thin piece of fiberglass laminate with copper cladding on both sides. The core has fully cured epoxy and is intended to be buried within a multilayer PCB structure. It is available in a wide range of thicknesses and of copper foil weights.
A photosensitive material applied to the copper foil of the core or the outer layers of a PCB to allow the transfer (photographically) of the computer-generated (CAD) image in the Gerber file to the copper surface of the core or outer layer of the PCB
A process of removing a base layer of copper leaving only the desired traces n’ spaces. The etching solution is masked from the traces n’ spaces by a photomask such that only the exposed base layer of copper is etched. This process continues to the surface of the laminate until all of the traces n’ spaces are un-shorted and there is no residual copper left between the traces.
The process in a multilayer PCB where the cores are sandwiched between layers of glass matt saturated with an uncured epoxy. The layers provide the insulating material between copper layers of traces n’ spaces and they also provide the “glue” that bonds the inner layers together.
Thin sheets of a woven glass matt that are saturated with a “B-stage” epoxy that is dry to the touch but uncured. When exposed to temperatures above the melting point of the epoxy, the B-stage flows around the copper circuitry (above and below) removing any air, filling all geometry, and bonding the surfaces together as it cures. Prepreg is available in a wide variety of thickness due to the size and type of glass used to weave the matt. The variety is further expanded by the types of epoxy resin saturating the matt. The resin varies depending upon the amount of flow that is desired when the epoxy is liquid.
The copper foil used by a fabricator is sold to them by weight. The weight is based on a uniform film of copper spread over 1 ft2 of surface. If 1 ounce of copper is uniformly spread over the 1 ft2, the thickness of the resulting copper is 1.4 mils (.0014”) thick.
NOTE: A customer request for “1 ounce copper” on the surface is due to a misunderstanding of how the term is actually applied. While the fabricator does purchase the copper foil by weight, (1 oz, 2 oz, etc.) once the fabrication process begins, this designation is no longer correct. The purchased foil is either eroded chemically and mechanically or increased by plating (electro or electroless). Fabricators ignore this customer misapplication of the terms and instead defer to the IPC–A-600 definition: “thickness after processing”, which re-defines ounces into a thickness of copper (For example, 2 Oz is defined after processing as being 2.2 mils instead of 2.8 mils.)
This is a fabricator term for the double-sided, copper-clad structure that comes out of a laminating press once the core, prepreg, and copper foil have all been pressed together and the epoxy has cured. To the untrained eye, the “brick” looks like a piece of double-sided, copper-clad laminate.
Sequential lamination refers to a repetition of the lamination process. If sequential lamination is employed, its purpose is to create an “intermediate brick” that is intended to be used for additional processes. In sequential lamination, multiple intermediate bricks or a combination of intermediate bricks and double-sided cores are bonded together. This process can be repeated multiple times depending upon the process capability of the fabrication shop. The typical number of laminations is 3, but a few very specialized shops can perform 5–6. These are rare.
Is there a future for the soldering process?
The drive to miniaturization comes from many corners.
Obviously, smaller more powerful devices fill a need for more conveniences that are portable. However, speed is also size-driven, and speed is everything when talking about the capability of digital systems. But, is the present state of electronic manufacturing nearing the end of its ability to use the soldering process?
Manufacturing has struggled to keep pace with the accelerating technology.
Manufacturing has struggled to keep pace with the accelerating technology. Manufacturing is all about dealing with real-world constraints while technology is about dreams. Today’s CAD systems can easily create designs that manufacturing technology cannot produce.
While this may be frustrating, it is also a necessary part of the evolutionary process. It is also the essence of capitalism. As Technology creates a demand, the manufacturer that can address the demand is the one that is generously rewarded by the market place.
Today, such a challenge of Technology vs Manufacturing is at a critical phase. The smallest components are 01005 chips (12 mils long and 8 mils wide.) For anyone struggling with deciding how small is the 01005, it is the equivalent of 3 hairs long and 2 hairs wide! For most of us, our eyes can resolve down to about 5 mils (a little over one hair in width). That means that many people simply cannot see the 01005! The mechanical and optical systems have kept pace with this “race to the infinitesimal”, but the physics of soldering has not.
When one examines the current process for soldering SMT parts, two factors should cause questions to arise:
1. “What happens to the SMT component when the solder becomes molten”?
2. “Is there a bottom limit on the size of the components”?
The first question is easier to answer. When the solder becomes molten there is a battle among the four primary forces that act on the component:
• Gravity tries to pull the component closer to the PCB.
• The surface tension of the solder tries to pull the solder into a ball under the component.
• The Molecular attraction tries to overcome the surface tension and spread out the solder over the exposed surfaces.
• The buoyancy forces push the component “up” in opposition to gravity.
Basically, this is a race condition as to which force wins long enough for the molten solder to solidify. As the component size gets smaller, so does the mass. As the size of the component (mass) continues to shrink, molecular attraction (wetting force) will begin to lose the battle and the component will not solder. We are already in that window with the most prevalent solder alloy (SAC305) and the smallest component (01005). The yields at the assembly level are not good for generic high-mix lines, such that many assemblers will not take designs that use these components.
The answer to the second question is “Yes there is a bottom limit”. However, this answer is incomplete. A more qualified answer would be, “Yes there is a bottom limit with soldering technology as it stands today”.
CAN TECHNOLOGY OVERCOME PHYSICS?
Throughout our history, we have faced the challenge of technology demanding more than manufacturing can deliver. And, we have always found a solution. Not by trying to defy the laws of physics with current processes, but by developing new processes that follow the laws of physics.
When we examine the four primary forces of soldering, we find that two are universal laws (Gravity and Buoyancy). I will not delve into a possible divine intervention to change the laws of our physical world. Therefore, we must address the other two forces, molecular attraction, and surface tension. To change the impact of either of these forces, we must change the materials (metals) involved. This is the challenge that manufacturing faces to develop the next revolution in electronic miniaturization. Some things are immediately suggested such as conductive adhesives. I would recommend a quick review of “volume resistivity”. At the same time, I also recommend a review of the physics that creates the capacitive or inductive effect (mathematical inverse analogs) or even resistance itself. These hints are to invite discussion about the materials as well as the process.
Stanley L Bentley, P.E. is the Senior Technical Advisor to RapidProto.com, a stand-alone supplier of Rapid Electronic Assemblies.