High Density PCB Assembly

Electronics are becoming smaller while it is getting faster. For enabling smaller component footprints on high-density printed circuit boards or PCBs, Rush PCB recommends considering the integrity of solder joints on each component for improving the producibility. This calls for proper processing temperature, suitable to the solder type underuse. As the board density increases and component size reduces, manufacturers tighten the tolerances on each component, which calls for better processing methods.

With circuit functionality and board density increasing, designers must use IO connectors with higher pin counts. However, these now occupy the same space that lower pin count IO connectors once occupied. This increased pin density often creates complications during assembly. For an error-free soldering joint on each pin, it is necessary for the soldering process to achieve the optimum processing temperature.

Regular SMT connectors cannot go beyond a specific number of pins per square inch that high-density PCBs can effectively use. For increasing the pin count significantly, connector manufacturers are offering various designs to reduce the component footprint. These designs include BGA, solder charge, and alternating pitch.

High-Density Connectors

BGA: Connector manufacturers are now offering high-density connectors in the BGA footprint. Like the BGA ICs, these connectors also use spherical solder balls attached to the pins on the underside of the connector. The arrangement requires very little solder paste deposit to form a good solder joint.

Solder Charges: These connectors use high-density open-pin field arrays, providing a solution like that offered by the BGA setup. However, solder charges offer an improved edge bonding between the connector pins and the PCB pad.

Alternating Pitch: Solder charges with alternating pitch is another offering for high-density connectors. With alternating pitches of 1.2 mm and 0.8 mm, the design offers the board designer additional space for trace routing between the rows.

Solder Joint Quality

For SMT connectors with a double row of pins, operators can easily address solder joint issues—an ordinary soldering iron is enough. However, for an SMT connector with multiple rows, addressing solder joint issues are more complicated, and a soldering iron is inadequate. Therefore, a proper processing method that is first-time-right is absolutely necessary.

Although many issues can cause a bad solder joint, the major ones are:

• Accuracy of solder paste deposit
• The volume of solder paste deposit
• Stencil opening and thickness
• PCB flatness
• Reflow temperature profile

With manufacturing set-ups being unique for each assembly shop, it is impossible to define a single set of rules to solve all the issues listed above. Furthermore, the involvement of several variances complicates the solution to the above problem. Chief among them being:

• Equipment underuse
• Brand of solder paste and its chemical constituency
• Board design and component density

Improving Solder Joint Quality

For the best solder joints on a PCB, following the SMD manufacturer’s guidelines offers maximum success. Most manufacturers provide a PCB footprint for their components, along with the necessary tolerances. Some also suggest the optimum layout and thickness for the stencil, guidelines for the printing process for solder masks, tolerances for component placement, suitable profiling for reflow ovens, and considerations for rework.

Stencil and Footprint: Rush PCB recommends PCB designers must download the component footprint and stencil layout from the component manufacturer’s website. Component manufacturers offer more than a hundred thousand symbols and footprints that designers can download for popular EDA tools. When designers use the provided footprint and stencil layouts, their chances of achieving proper solder joints increase manyfold.

Screen Printing Process: For a proper solder joint, it is necessary that the solder covers the entire pad. For achieving this, the manufacturer suggests an aperture in the stencil larger than the pad. This ensures the solder ball or charges under the component make contact with the solder paste. However, this requires accurate registration of the stencil with pads on the PCB. High-accuracy registration ensures the solder paste location with respect to the solder balls or charges offers good contact between them.

If the centering of the solder paste is not accurate with respect to the pad, the solder will not wet the pad adequately. Rush PCB uses automated inspection to ensure solder covers the entire pad. We reject solder pad assemblies not completely covered. We clean, and reprint them after improving the registration of the stencil with the PCB.

Component Placement: For proper placement of SMT components, it is necessary to use automated pick-and-place equipment. Along with the centroid data, or the X-Y coordinates, it is also important to program the Z-axis dimension for truly seating each solder ball or charge of the component on its solder deposit.

As the solder melts in the reflow oven, the weight of the component makes it settle in its proper position on the board. This also helps to overcome any coplanarity in the component.

Reflow Oven Profiling: Most manufacturers make SMT components capable of withstanding the profiles necessary for lead-free soldiers. According to the standard IPC/JEDEC J-STD-020, SMT components must withstand a peak temperature of 260 °C and a temperature of 255 °C for 30 seconds.

Nitrogen infusion provides a low oxygen environment, and this increases the wettability of the soldering surfaces. High-density component manufacturers recommend completing the soldering process in an environment rich in nitrogen.

It is vital that the fully populated PCB assembly undergoes a proper profiling process. Overlooking the reflow process that forms the solder joints can lead to various defects in the joints and makes the board non-functional.

Rush PCB places thermocouples through the back of the board reaching the center of the component. This way we are sure the solder balls or chargers are reaching the proper temperature. This also ensures the reflow process is achieving the solder paste reflow profile parameters recommended by the manufacturer.

Conclusion

Although processes will always have flaws, with proper processing it is possible to reduce the need for reworking the assembly, scrapping it, and generating lower profits. Importance of proper processing methods will continue to gain importance as the electronics industry makes denser assemblies with smaller components. Rush PCB offers superior stencil layouts for SMT components while matching their footprints to those the manufacturer supplies.