Secrets of Fine-Pitch PCB Assembly

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With most electronic equipment trending towards compact designs, printed circuit boards inside them must hold a substantial number of components per unit square area. With components packed so close together, designs are usually on the extremes and manufacturers must follow tight PCB fabrication tolerances. Another feature of these designs is the use of components with very high pin count, leading to very fine pitch between the pins. This is the origin of the name fine-pitch PCB assembly, another name being high-density PCB assembly.

At Rush PCB Inc., we design and assemble fine-pitch PCBs. As per our experience, following a few simple generic rules can significantly increase the chances of a successful assembly of fine-pitch circuit boards:

  • Designing the board
  • Challenges in layout
  • Selecting proper components
  • Defining the board size
  • Component placement

Designing Fine-Pitch Circuit Boards

As stated earlier, fine-pitch circuit boards often contain many fine-pitch components. Designing such boards calls for great care and considerations from the designer, as they must not only lay tracks very close together, but also accurately space them between tiny pads and vias.

The cost of such boards goes up as the small pads require very high precision from the pick and place machines—even more so when the components are without leads and of the grid-array type. The assembly process may require X-ray inspection methods.

The designer must also maintain close relationships with the board fabricator, as the design of fine-pitch boards can easily breach the capabilities of many generic vendors. At Rush PCB Inc. we make sure our capabilities and equipment can easily meet the details and decisions of the fine-pitch board designer. We also provide our customers general guidelines on the risks related to the design and fabrication of fine-pitch PCB design and assembly. We have experienced engineers and our support team work with our customers to guide them in ensuring they obtain the highest possible quality of fine-pitch PCB manufacturing and assembly.

Challenges in Fine-Pitch PCB Layout

When there are numerous components within the limited space of a board, the designer can face substantial layout challenges. This usually results in the designer making a number of trials for achieving the perfect layout. The designer typically begins the layout by importing unplaced components from a schematic.

The challenge begins with populating the board while placing the components as close together as possible. They must also allow adequate spacing for introducing traces in between and adding power planes. While traces carry signals, power planes carry voltage and current from one component to another.

The challenge can also include circuitry operating at various speeds. For instance, the board may contain analog circuits operating at low frequencies, digital circuitry operating at medium and high speeds, and tracks carrying high power. The challenge is in placing them compactly, while not allowing them to interfere with one another. For this to be a successful design, the following steps are very important.

Selection of Proper Components

To achieve the most compact layout, it is necessary to select appropriate components that while meeting the functional requirements are also physically small enough to save board space. Usually, this is a compromise between savings offered by the component on board space and the cost savings the component offers.

Although a small component saves board space, it costs more during the assembly. Populating fine-pitch components requires more accuracy, and demands specific kinds of inspection. For instance, ball grid array or BGA components are very compact but require greater accuracy while placing them, and X-ray inspection after soldering.

In general, cost increases as tolerances get tighter. Therefore, board designers must balance cost of assembly with component size. If the priority is design compactness, the designer can use the smallest components available. Most IC manufacturers realize this trade off and offer the same component in multiple packages. The designer can select the appropriate package by reviewing datasheets.

The designer must also maintain a balance between the component size and rating while designing for high densities. This is applicable more for passive components like resistors and capacitors that are driven by power ratings.

For instance, smaller resistors dissipate less heat and therefore, are suitable for lower current ratings. However, if the design calls for a resistor of ¼ W rating, placing a ½ W resistor is not necessary, since the latter will usually be of a bigger size and take up more board space. Similarly, if the design is for a capacitor rated for 10 V, placing a capacitor rated at 50 V will take up more board space as it will be physically larger.

The designer must also be careful in introducing underrated components in the design, as these can bring on unwarranted risks.

Defining the Board Size

Once the designer has defined the schematic and specified all the components, they must define a reasonable size of the board.

Usually there can be mechanical constraints, with a physical limit to the X, Y, and Z dimensions. The designer must use these constraints from the start. However, if the constraints are still under development, then the designer has some leeway to adjust the size of the board.

The designer must first check the smallest size the board can possibly attain. The best way to do this is to place all the large components such as connectors on the outer periphery. Then they must place the largest ICs, keeping estimated space for any breakout signals. They must also keep space for sensitive traces, and arrange for isolating them from the rest of the board.

Discussing with the board manufacturer can lead to picking standard or preset board sizes, which may bring down fabrication costs. Sometimes using 3D models for the board and bulky components can help define the size and shape of the board before finalizing.

After deciding on the size of the board, the designer can start with the layout and routing. A discussion with the vendor of the board is necessary at this stage to decide on the proper design rules, as they must align with the PCB fabrication capabilities of the vendor. Major considerations include via sizes, clearances and spacing, and trace width.

At this stage, designers can consider trade-offs for cost reduction. Rush PCB Inc. offers many options that our customers can use to balance capabilities against cost. By establishing the appropriate design rules at this stage, our customers can meet their end-cost targets.

The designer must also consult the vendor about their capabilities for handling fine-pitch ICs. For instance, not all vendors can handle 0.3 mm ball pitch for a BGA or chip scale component. Likewise, the vendor must be capable of measuring and providing proper differential pair traces with impedance control. Connectors with odd shaped slots need prior discussion with the vendor for verification of their capability.

Component Placement

Once the designer has cleared the design rules and capabilities with the vendor, they can start the component placement on the board. This is mostly an effort based on trial as the components must be packed close together, but with adequate space for vias, traces, planes and more.

Rush PCB Inc. recommends starting with the placement and routing of high-speed components first, followed by those using high power. With mechanical components and connector placed at the periphery of the board, the designer has established a basic framework from where they can proceed further.

Additional Considerations

For optimizing space in fine-pitch design, designers often consider additional factors such as:

  • Via-in-Pads
  • Placement of Decoupling Capacitors
  • Adding Fiducial Marks
  • Tooling Holes

Conclusion

PCB fabrication and assembly is always a balance between complexity and cost considerations. This applies for fine-pitch PCB assembly also. Rush PCB Inc. can offer several trade-offs that save our customers substantial amounts of money and time while developing and producing their fine-pitch PCBs.

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