Acronyms in the PCB business often get confusing. For instance, DfA, or Design for Assembly, is often used to mean DfM, or Design for Manufacturing, and vice versa. However, in reality, DfM is the correct acronym when used to encompass PCB design, fabrication, and assembly. Effective DfM takes into account the practice and implementation of placing checks and balances at the right places during those three major stages–design, fabrication, and assembly and test.

Advertisement

At design, components represent a potentially error-prone area causing inefficient DfM. Here, a component is defined as any part or device populating a board. The burden falls on the PCB designer to correctly make the components. For starters, he or she has to properly view, read, and thoroughly comprehend details provided in the data sheets, such as dimensions, cut outs, slots, as well as pin numbering and sequencing.

With PCBs becoming increasing complex, there can be any number of component miscues due to a PCB designer’s inexperience and not properly reading a data sheet. Those include improperly fitting a component on its associated footprint, incorrect or damaged connections into and out of a component, incorrect sequencing or pin numbering, poorly defined pad dimensions, and incorrect drill or via holes, as shown in Figure 1.

Fig 2 Signals transmitted underneath analog and digital components must be clean and well segregated. An internal layer of a multi-layer PCB is shown with power and ground layers split for proper signal transmission.

If these factors are not incorporated in component making, all kinds of mismatching issues can arise. An example includes components not fitting in the allotted hole or dimensions, actual component being too big or too small compared to the footprint on the board. Worse yet, the wrong pin numbers or sequencing on complex ICs result in re-spinning the boards, costing lots of time and money.

It’s important for a PCB designer to make sure he’s not only making the components correctly, but also to have another set of eyes to check complex components like BGAs, CSPs, DFNs, and QFNs. This increases assurances that fiducials, sequencing and pin numbering, and other associated aspects of the design are correctly performed.

Correctly splitting a PCB’s circuitry is another key aspect of DfM. The PCB designer must clearly understand that high-frequency, high-noise generating devices, like analog, which must be kept sufficiently away from noise-sensitive digital ones, especially digital clocks. Splitting PCB planes also requires special considerations, as shown in Figure 2. Will it be best from a DfM perspective to split a plane into multiple ground and power planes? Or do you place the power and ground signals on the board’s top and bottom side and reduce the number of layers bringing the fabrication cost down?

Enlarge this picture Fig 3 Fabrication drawing must be precisely documented to include stack up calculations

A simple PCB with enough real estate to avoid affecting circuitry performance can have power and ground signals on the top and bottom side without the need to create other power and ground planes. But, in most cases, separate ground and power planes are required, especially with complex, high-speed circuitry to reduce the cross talk and suppress noise.

Component placement is another critical aspect and plays a big role in PCB layout and design. Correctly placing components reduces the number of layers on the fabrication side of the board. Conversely, when components are not correctly placed, there can be long signal routes from one side of the board to the other. As a result, two to four or more layers must be added to the board, thus increasing fabrication costs. Proper placement is also important for efficient current flow and signal transfer to the external world, as well as to minimize cross-talk, signal-to-noise ratios (SNR), jitter, ground bounce, and other adverse signal effects.

PCB design experience is also at the center of precisely placing test points on a board. A test point covers a particular circuitry area and testability covers 75 to 90 percent of a board, for example. In the case of prototypes, a flying probe test covers the designed-in test points on all major ICs and modules. Therefore, during the testing process, it captures signals and functionality by probing through the board at different points.

Boundary Scan (BS) and the IEEE Joint Test Action Group (JTAG) Standard are making greater inroads into traditional PCB test. As a result, in many instances, test time is reduced because flying probe and in-circuit testing can be greatly reduced when boundary scan is incorporated in the test schemes. A savvy designer knows to incorporate BS features in the layout to assure test time can be curtailed and to save tens of thousands of dollars in test fixtures.

It’s also important to note for DfM that BS and JTAG can be used for virtually any PCB application. Currently, they are ideal test methods for continually shrinking populated PCBs after they’re manufactured. Chipmakers embed BS in many of their advanced ICs as a way to analyze and test a PCB’s wire lines or sub-blocks within an IC. It is also widely used as a debugging methodology. In this arrangement, test cells are connected to each device pin and are used to check an IC’s internal functionality. These test cells are programmed via a JTAG scan chain to drive a certain signal into a pin using an individual trace on a board.

Once the layout is completed, the seasoned PCB designer makes sure a fabrication drawing is precisely documented to include all fabrication notes, drill drawing and charts, and stack up calculations, as shown in Figure 3. The same holds true for the assembly drawing. It needs to be complete with detailed assembly notes, special techniques required, ECOs, the second operations, no clean flux usage, if any, and the use of fasteners and stiffeners, for example.

Additionally, the designer needs to make sure the silkscreen is accurate. It needs to point to the right devices in terms of their names and designators; it needs to show the right polarities and orientation. Not every component is required to have a polarity or orientation. However, if polarity and orientation are missed on those components requiring them, then the consequence is mismatched connections. Then, there’s a slight chance the board can burn to an un-repairable level. Also, if power is connected to ground, failures can occur or the board won’t operate, to the level it was designed for.

There are other detailed assembly drawing notations that may appear minor, but are major contributors to top notch DfM. For example, assembly needs to use standoffs before heat sinks; particular screws needed on the PCB chassis; thermal grease for a heat sink; tie-downs for tall components; and any special tools or wrenches, such as torque meters used in assembly process. Complying with DfM details like this can only be assured when an OEM partners with a CM that has seamless design and assembly operations.

At Fabrication

To correctly fabricate the board, a PCB designer must correctly calculate impedance control properly so that all transmit and return signals are intact. He or she then needs to send that data to the fabrication house to get an independent verification. This critical step assures the proper impedance definition and characterization. If calculations are inaccurate or not independently verified, then the fabrication house requests that particular portion of the layout to be re-done or the board stack up to be revised. A delay like this incurs a few additional days and sometimes resolving inaccurate impedance control calculations in some complex board designs can take weeks. Consequently, the OEM loses time-to-market and time-to-revenue.

To eliminate this problematic area, experienced PCB layout engineers make it a point to maintain sustained communication with their fabrication houses. By instituting these communications channels, the fabrication house can provide constant, sufficient, and valuable feedback to avoid these issues and make the fabrication process smooth.

At Assembly

Fig 4 QC time can be increased unless an AOI machine is used, especially if a PCB is populated with BGAs, CSPs, and QFNs.

At assembly, efficient DfM demands the utmost use of automated equipment with pick and place being the frontrunner. Automation brings to the table a high quality and repeatable product that can be traced back and doesn’t require human interaction and judgment, which can be sometimes questionable. AOI and X-ray also play key roles. Not using an AOI machine (Figure 4) means final QC time is increased because an effort wasn’t made initially to check the inspection process, especially if a PCB is being populated with BGAs, CSPs, and QFNs.

The same is true for X-ray. Surprises are likely to appear just when the product is ready to be shipped if X-ray is not used during the assembly process. It is vital, therefore, to maintain in-process X-rays while the PCB is being manufactured. This DfM step provides quality, repeatability, and consistency. If a CM doesn’t have all this automated equipment, then the end product will be less than 100 percent reliable.

Fig 5 First article inspection is a valuable tool for comparing the golden board with the rest of the boards, thus avoiding problems at the end of assembly.

The use of a first article inspector machine, as shown in Figure 5, always comes in handy. It is a quick way to compare the golden board with the rest of the boards, thereby ensuring that there are no surprises at the end of the assembly process.