Improving Fabrication Yields by Design Print E-mail
Written by Zulki Khan   
Tuesday, 01 April 2008 00:00

The PCB designer is the architect for improved PCB yields.

A major requirement in improving board fabrication yields is doing it right the first time, because once the PCB fabrication process is complete there is really no way to go back to fix major mistakes. In some cases you can mitigate design issues during the PCB assembly process, but during PCB fabrication, once layers are laminated and the holes are drilled, you cannot easily undo the process to make corrections.

Therefore, to improve fabrication yields during and after design layout, it’s critical to follow the detailed fabrication notes and drawings, specifically calling out every item that requires any kind of explanation. This includes stack up data, layer construction information, material call outs, as well as drill charts specifying hole counts and symbols, whether drill holes are plated, and any similar information.

Notes and drawings must not have sketchy or ambiguous information, nor should they lead the PCB fabricator to make “guesstimates” about some of the directions. Fabrication notes and drawings must have clear-cut and precise information in their instructions – assumptions are not allowed. If questions arise, the OEM customer should be consulted, and the OEM should resolve any uncertainties.

A good rule of thumb is to engage the fabrication house during PCB layout/design stage. After the designer creates the stack up for impedance control, it’s a good idea to get it verified before the files are released to the fab house. Conversely, the fab house can play a reciprocal role by providing the designer with recommendations and suggestions for boosting yields. For example, a fab house may recommend material changes for a specific application that are better suited to increasing yields than those a designer specifies.

Also, it is a good idea to check the capabilities of a fabrication house before releasing a job to them. If the PCB design calls out for 3 mil lines and spaces, and the fab house does not have the capability to generate this type of feature, they can inadvertently over or under etch traces, causing open or shorts resulting in yield issues. Therefore, working together, the designer and fab house engineer can resolve any question, issue, or ambiguity that arises at an early stage in the design process, before the design ever reaches the fabrication floor.

The Importance of Fabrication Drawing

The PCB designer is the lynchpin for improving fabrication yields from the start. The fabrication drawing – the result of the PCB layout/design – is the tool they rely on to achieve this objective. A seasoned designer always finishes their layout by providing a complete fabrication drawing. An efficient fabrication drawing has four components – notes, mechanical dimensions/drawings, stack up callouts, and a drill chart seen in Figure 1.

Fig. 1

Fabrication notes include a wide range of technical details and instructions. The more complete and accurate they are, the more likely that the fabrication house will be able to produce the PCBs with requisite high yields. The following are some of the critical areas that should be covered in fab notes.

It is important to list the IPC class (I, II, or III) on the fabrication notes. Also, the designer should specify the required board materials and surface finishes such as HASL (lead-free or tin-lead type), electroless nickel and immersion gold (ENIG), immersion silver or tin. If it’s gold, what is the quantity and type? A typical soldering applications might call for 3 to 5 microinches of gold over 150 to 200 microinches of nickel. A higher thickness and different type of gold would be needed for specialized applications such as wire bonding. The designer should also include, whenever possible, a note designating a secondary (equivalent) material and manufacturer name if the primary choice is not available at the fabrication house. It may take a week or more to acquire material, causing OEM product delivery delays. That’s the level of detail the designer must include in their notes.

Ensuring that automated optical inspection (AOI) is used is another critical step in board fabrication. AOI checks inner layers to ensure there are no opens or shorts on the board, and that layer-to-layer registration is properly aligned. For example, a note to a fabricator can simply state: “Please make sure Layers 2, 3, 5, and 6 are AOI verified before laminating the layers.”

Solder mask information must also be covered, including if a solder mask is required on one or both sides of the board, halogen content considerations and also designating the color of the solder mask. In addition, it is always a good idea for the designer to mention a few preferred solder mask manufacturers. Fabrication notes must also detail maximum warpage per square inch that the designer will allow, and this should be in accordance with IPC guidelines.

The designer also should include a note on the thieving process that allows for even copper distribution on the board. Thieving adds non-conductive copper material to the board that balances copper weight on the board’s entire surface, so that when etching is performed, it is uniform over the board’s surface. This minimizes the creation of either opens or shorts. Figure 2 shows a PCB design with copper thieving added.

Fig. 2

Designing For Odd-Shaped Boards

When it comes to fabrication notes, mechanical dimensions and drawings require equal attention from the designer. This is especially true for odd sized or unusually shaped boards that use radial angles to define curvatures or slot dimensions. When creating a fabrication drawing for such boards, the designer must specify even the smallest detail in lengths, widths, and angles. The designer must carefully designate any holes that reference each other, especially if they are at a specific angle or run concurrent to each other.

Gold fingers, like cables and chassis wires, are used to connect daughter cards to a motherboard or other subsystems, and when providing details on the beveling of the gold fingers at the edge of the connectors, the designer should specify the angle and at what levels the gold fingers should be beveled (see Figure 3).

Fig. 3

The datum point 0,0 must be specified, so that every other dimension is measured from that original point. Cutouts, slots, and holes must also be meticulously designated. On the drawing, there are two sides for reference designators. At times, the designer might note a special feature on the bottom side that isn’t required on the top side. This distinction must be made in the fabrication drawing. If not, the fabrication house is left to make what perhaps can be a faulty assumption. Other details the designer must include would be any special features, like counter sink holes or sequential lamination.

Stack up callouts specify board thickness, composition of internal layers, pre-preg thickness, and copper ounces per square inch used on the board. At the layout stage, the designer should perform precise calculations on the amount of current that will flow through the board. The callout specifies the thickness of copper (measured in ounces) to comply with current requirements. What must be avoided is the fabrication house relying on their own judgment, or making a decision using no calculations at all. (Note: an “inaccurate judgment” is not the fault of the fabricator, it’s a design error.)

When calling out for impedance control requirements, a precise tolerance, such as 5% or 10% should be specified. For high-speed designs, impedance requirements could be single ended, or multiple differential impedances. Since there are many factors that can change the impedance on a board, a seasoned designer is always aware and mindful of these factors, which may include the stack up of the board, number of ground planes, trace width and thickness and the dielectric constant.

Lastly, the drill chart covers four aspects – the symbols used, the size of the tools, the quantity of each drill size, and if holes are plated or non-plated. When defining drill symbols to distinguish multiple drill sizes used on the board, a separate precisely defined symbol should be used to make this distinction clear and without ambiguity.

Accurate and comprehensive attention to detail should result in as near an ideal fabrication drawing as possible. A drawing of this caliber eliminates most or all of the uncertainty in the planning and computer-aided manufacturing (CAM) stages, as well as many that may be raised by the fabrication house.

The CAM stage allows the fabrication house to review the different files generated in the PCB layout process. This includes Gerber files, which generates renderings that show layers, power and ground planes, drill holes, etc. At this point, oversights and potential problems can be corrected, as the efficient use of a sophisticated CAM tool will uncover discrepancies, such as half moons, stubs, or missing connections.

Assumptions and Common Sense

Wrong assumptions can cover many aspects of the PCB fabrication process that may include surface finishes, board material, copper plating, and other smaller details such as plated-through holes (PTH) versus non-plated through holes. In the case of the PTH, it can create a short between the chassis and screw tightening the board, if specified incorrectly. At times, surface finish may not be specified by the OEM customer, and an assumption may be made to apply a HASL surface finish, but in a lead-free application, a silver or gold finish might be required to withstand the high thermal profile during reflow.

Improving fabrication yields requires the designer to use plain old common sense, as well as applying their extensive experience and know-how. A seasoned designer knows precisely where the pitfalls exist in a board design, and applies all the tricks and techniques they have learned from previous experience. A considerable amount of that design knowledge results from practical hands-on experience, rather than from textbooks or formal training. PCD&F

Zulki Khan is president and founder, Nexlogic Technologies and can be reached at This e-mail address is being protected from spambots. You need JavaScript enabled to view it .

 

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