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Experimental methods for PCB design and manufacturing

Posted: 22 Dec 2014 ?? ?Print Version ?Bookmark and Share

Keywords:PCB? printed circuit board? package-on-package? ball-grid array? BGA?

PCB technologies are going through a major new evolution unlike anything the industry has seen before. Issues relating to PCB size, shape, complexity, thermal requirements, and shrinking board space are creating design and assembly/manufacturing challenges for the original equipment manufacturers (OEMs), chip and component makers, material suppliers, and the electronic manufacturing services (EMS) providers. The effort to resolve these issues is changing the dynamics among these key players.

These challenges are pushing PCB design and manufacturing methods to newer levels and are creating demands for more efficient device packaging, newer substrate alloys, and better soldering pastes and fluxes to meet new printed circuit board (PCB) assembly and manufacturing requirements. But the major OEM requirement is for a research and development-like environment within which to conduct experiments to prove or disprove the validity of the new techniques in next-generation embedded systems.

Design of experiments (DOEs)
Consequently, EMS Providers are being called upon to take on the task of 'design of experiments' or DOEs, also known as experimental designs, requiring them to work with OEMS to do the necessary research and development. A DOE is an information gathering exercise where variation is the main characteristic. It can be performed as a fully controlled set of experiments, or as a partially controlled set of experiments that involve changing one variable and keeping everything else the same.

For example, it could be the chemical formulation of a flux that's made differently or it could be different composition of materials. In a specific case, the component of metal balls of the solder powder is different. At times, the acidic level of sulfate in the fluxes is different. This would make it a quasi-experimental design or fully natural experiment in which some of the elements in the design are changed, but others are not.

The objective is to collect as much information as possible by doing the same experiment in a variety of ways to achieve the OEM's objective. Experimental designs can be used at the point of greatest leverage to reduce manufacturing complexity and introduce new technology. When the prototype is being designed and manufactured, there is some uncertainty about how the design will work and any way to speed up up the process can reduce the time it takes to verify the functionality of the product as well as final time to market. It also reduces the design chain cycle time, product material requirements, and labour complexity. When these changes are made, an advanced type of flux may be used. Simultaneously, the associated thermal profile needs to be adjusted.

Forces driving DOEs
A key driving force for DOEs is the fact that shrinking board space is placing heavy demands on chipmakers to manufacture their devices in smaller, yet efficient packaging. Previously, EMS providers dealt with devices and components in larger packages and device packages with gull-wing leads sticking outside the actual package and onto the board, thus making it easy to assemble and inspect them. There was ample PCB real estate for earlier generation packaging.

But now, leadless packages are used to conserve board space and provide both better performance and improved heat transfer. Also, since board real estate is tapped out, components are stacked on top of each other for a package-on-package (PoP) design arrangement, as shown in figure 1. However, when packaging becomes so small, PCB design and layout become highly challenging.

 Package-on-package design arrangement

Figure 1: When PCB real estate is at a premium, components are stacked on top of each other for a package-on-package (PoP) design arrangement. (Photo courtesy of MyData Corporation).

Further, as shown in figure 2, thieving (or ground pour), the traditional copper overlay on boards to help dissipate heat, cannot be used in most cases with those shrinking PCBs. There's simply no space left to perform ground pouring.

 Thieving or ground pour

Figure 2: Thieving or ground pour, the traditional copper overlay on PCBs to help dissipate heat, cannot be used in most cases with shrinking PCBs.

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