Eliminating Board Defects Print E-mail
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Written by Zulki Khan   
Wednesday, 30 April 2008 19:00

The assembly drawing should specify process-related issues.

The processes involved in eliminating board defects cut a wide swath through design, fabrication, and even assembly. Along the way, experience and know-how play pivotal roles because solutions to many defect-prone PCB areas are not covered in textbooks. PCB defects run the gamut, for example, from deteriorated RF signals resulting from inappropriate shielding to incorrectly structured boards due to sketchy or ambiguous fabrication notes, and on to miscalculated thermal profiles, creating BGA, CSP or QFN-based solder defects or flaws.

Eliminating board defects starts with the designer. Their experience level and design-for-assembly (DfA) knowledge has a direct bearing on the number of defects and flaws detected at PCB assembly. Certainly, the designer may have a keen sense for design, but if they don’t know how assembly and manufacturing work, have a firm grip on its processes and a good understanding of assembly equipment, the PCB design is likely to experience assembly issues.

For example, on paper, an RF design looks exactly the same as any other PCB design. However, in an RF design, a multiplicity of small differences, intricacies and issues demand careful attention. If not properly addressed at the outset, they subsequently become defects, which can keep design engineers busy debugging and trying to eliminate those defects.

Defects can be created in an RF design in such areas as an improperly designed antenna, signal deterioration at an unacceptable rate, exceptionally high noise resulting from poor shielding, improper dissipation of EMI from RF signals, and lack of attention to line-of-sight aspects. Figure 1 shows a board with five islands of circuitry shielded from one another to reduce signal-to-noise ratio (SNR) in the RF circuitry, and thus avoid defects.

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The designer must also concentrate on segregating signals to make sure signals of one particular frequency are properly separated from those of a different frequency. Otherwise, crosstalk may be created. A savvy PCB designer incorporates these and other RF design considerations, thus avoiding RF board defects at assembly and manufacturing.

Board fabrication and assembly undergo various processes and machines. Particular attention is crucial in these areas to avoid defects. Lack of PCB layout experience, for example, can lead to placing BGAs on a board’s bottom side, although there is plenty of space topside. In a situation like this, the assembly process is duplicated with two setups for pick-and-place machines and another two processes for reflow, plus using ancillary fixtures to ensure those BGAs are protected when through-hole components undergo reflow. There is nothing wrong with placing BGAs bottom-side, but if this PCB design and placement were done correctly, it would have eliminated defects and avoided creating the basis for future failures.

In cases like this, when only one side of the board is used for BGA placement, the need for using epoxy for reflow is eliminated, as is the need for a wave solder fixture.

An assortment of other defects also arises from machine limitations, incorrectly designed in tolerances, and board spacings. DfA takes into consideration physical, height, length, and package-related limitations. If a board is highly complex with 0201 or 01005 packages, and tight assembly processes are not specified, multiple board defects can result, such as tombstoning, skewing of components, and inactivation of flux with solder paste.

A designer also has to know the limitations of various fabrication and assembly process machines. Those include pick-and-place, AOI, reflow, wave solder and test systems, among others. The board should be designed with mechanical and other tolerances in mind, such as the size of the board and, in case of smaller boards, the panel size, as each machine has physical limitations. Also, there are some height-related restrictions, especially in the case of a flying probe tester, whereby using taller components might hinder and restrict movement of the flying probes.

Components can be designed and spaced too close to each other or to the edge of the board (Figure 2). Consequently, this error or oversight poses limitations for the pick-and-place machine during placement. Parts must then be placed by hand, creating reliability issues and likely leading to defects. An alternative is to redesign the board, which involves a costly and time-consuming re-fabrication.

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Insofar as component placement, a 0.004" pitch BGA, for example, requires considerably tighter placement tolerance than a 0.007" or 0.008" pitch BGA; thus, it demands more placement precision (and generally, more expensive equipment). Even then, at 0.004" pitch, reliability and repeatability can continue to be issues unless proper design measures are taken.

Fabrication. First-article is a critical step to verify virtually every aspect of PCB fabrication, including hole size verification, cutouts, slots, and all physical and mechanical dimension calculations. Eliminating defects at fabrication also takes into account well-documented fabrication notes and an extensive checklist of key factors. If any one or more factors are overlooked or blatantly omitted, chances of defects increase.

Here, planning at the CAM stage calls for checking, double-checking, and even triple-checking such factors as layer stackup structures’ thicknesses and impedance control calculations to ensure they are precisely correct and are within specified tolerances. The fabrication description chart (Figure 3) shows a four-layer stackup, with all key elements precisely described.

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A board’s innerlayers also can be a breeding ground for defects. Proper design uses correct prepreg materials and core thicknesses to prevent innerlayer registration problems. Similarly, applying correct board surface finishes is of the utmost importance, especially for Pb-free product. Where OEM cost is an issue, Pb-free HASL or ImAg may be the right way to go for a Pb-free board. However, ImAg has a tendency for corrosion.

In this case, pending corrosion would not be immediately apparent. However, over time, it can begin to occur in the field, creating one or more defects and subsequent product failure. Therefore, a better choice could be ImAu. Again, experience and know-how play into situations like this to make the right call.

Fabrication notes also cover IPC class, according to which the board is going to be built, such as Class 1, 2 or 3. Mistakes can occur if the fabrication class is not properly called out in the fabrication notes. It’s also important to document the percentage of warpage allowed. Defects can arise from excessive warpage (bow and twist), improper etchback, plated versus non-plated hole errors, and incorrect mechanical tolerances of drill bits. Plus, drilled holes must have sufficient tolerances so they become full holes without creating half moons or having stubs. These and many other fabrication factors set the stage for defects.

It’s also crucial to define the number of panel stackups for the drill and routing stages. For example, in a six-board stack, specified drill and routing tolerances may be achieved on boards 1, 2, 3 and 4, but not 5 and 6, due to mechanical drill limitations. To avoid this, the number of panel stackups is defined as a process. Defining processes like this within well-documented fabrication notes provides an extra measure of assurance for eliminating defects. AOI can also be defined as a process for inspecting innerlayers, especially for high-count layer boards using extra thin prepregs.

Assembly. Multiple factors can eliminate board defects at assembly stages. Like fabrication notes, there should be a comprehensive assembly drawing detailing all the necessary assembly processes. For example, a process can clearly spell out that all components, including BGAs, are to be machine placed, rework or ECO callouts, and the use of any special process. Figure 4 shows an example of a properly created assembly drawing, which can eliminate confusion, answer questions, and reduce board defects.

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Assembly processes must be repeatable. The first article is important at the assembly level as well, so technicians can check for polarities, missing components and other key areas. Planning must be conducted to determine and document the processes that need to be defined, the machines needed, engineering change orders (ECOs) and use of special equipment such as arbor press for press-fit connectors, AOI machines or flying probe testers.

Documentation is important for technicians in the field who need to read and decipher ECOs that deviate from original build. Likewise, rework instructions, if any, need to be clearly spelled out with solid quantitative data for measurement and verification purposes.

At times, instructions can be issued in an assembly drawing to avoid board defects, which could be process-related issues. Also, depending on how progressive an EMS provider is, post reflow inspection can be specified as part of the process for all BGAs, CSPs and QFNs, as opposed to at QC at the end of assembly. At this point, actions can be taken to avoid board defects such as improper board reflow, poor orientation, wrong thermal profile and improper flux activation, among other problems.

This is important to eliminate defects. If the thermal profile is incorrect and the board does not heat properly, the result will be cold solder balls on the BGAs, CSPs or QFNs (Figure 5). If the thermal profile is too hot, bridging can result.

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During first article post-reflow inspection, while boards are going to reflow, the process can be stopped; it can be changed and a set of one or two boards as second articles can be run to correct the process. But if some are not caught during the process, they are found at the end of it when it can be too late. A shipment could be missed, or rework may be too large, taking more time, resources and extra dollars.

Thus, it is important for EMS providers to build QC steps within the assembly/manufacturing process. It’s also important for OEMs to specify process-related issues as part of the assembly drawing to avoid defects.

Zulki Khan is president and founder, NexLogic Technologies, Inc. (nexlogic.com); This e-mail address is being protected from spambots. You need JavaScript enabled to view it .

Last Updated on Friday, 25 April 2008 08:53
 

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