How to improve the performance of High Density PCB Assembly Process
High-density interconnect (HDI) PCBs has recently became an integral part of the electronics industries. Electronic components are becoming smaller, faster and more lightweight but still demand of improvement is raising everyday. The best solution for this is to pack more functionality into a smaller area. That’s precisely what HDI PCBs offer.
As compared to conventional PCBs, HDI PCBs passes through different fabrication and assembly processes since these boards have high layer counts with very complex traces and via.
As the board density increases and component size reduces, manufacturers tighten the tolerances on each component, which demands for better processing methods.
While processing higher density connectors on a printed circuit board, the designers must consider the integrity of solder joints on each component for improving the producibility. This demands for proper processing temperature, suitable to the solder type under use.
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.
Quality of Solder Joint
For a SMT connectors with a double row of pins, solder joint issues can be easily addressed by the operators. In such cases an ordinary soldering iron is enough to correct it. However, in case of a SMT connector with multiple rows, addressing solder joint issues are more complicated, and a soldering iron is not enough for it. Therefore, a proper processing method that is first-time-right is absolutely necessary.
The major issues that can cause a bad solder joint are:
- Accuracy of solder paste deposit
- The volume of solder paste deposit
- Stencil opening and thickness
- PCB flatness
- Reflow temperature profile
It is not at all possible to define a single set of rules to solve all the issues listed above. Furthermore, the involvement of several variances such as Equipment underuse, Brand of solder paste and its chemical constituency and Board design and component density complicates the solution to the above problems.
Improving the Quality of Solder Joint
For the best solder joints on a PCB, it is always advised to follow the SMD manufacturer’s guidelines for 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:
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. The chances of achieving proper solder joints increase many fold, when designers use the provided footprint and stencil layouts by the manufacturer.
Solder Screen Printing Process:
For a proper solder joint, it is important that the solder covers the entire pad. To achieve 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.
However, if the centering of the solder paste is not accurate with respect to the pad, the solder will not wet the pad adequately.
Automated inspection is used to ensure proper solder coverage on the PCB. It is recommended that any solder pad assembly not completely covered be rejected, cleaned, and reprinted.
Component Placement:
It is necessary to use automated pick-and-place equipment, for proper placement of SMT components. Along with 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.
Proper Oven Profiling:
Most manufacturers make SMT components capable of withstanding the profiles necessary for lead-free soldiers as described in IPC/JEDEC J-STD-020. This specification requires that the SMT components must withstand a peak temperature of 260 °C and a temperature of 255 °C for 30 seconds.
To increase the wettability of the soldering surfaces, a low oxygen environment is provided by Nitrogen infusion. High-density component manufacturers always 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.
Manufacturers always recommend to place thermocouples through the back of the board reaching the center of the component. This is done to ensure that the solder balls or chargers are reaching the proper temperature. This also ensures that 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. The importance of proper processing methods will continue to gain importance as the electronics industry makes denser assemblies with smaller components. This is why PCBWay provides footprint and stencil layouts for all of its connector series and also provides information on how to properly process its more complex product series.
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