Backplane PCBs Fabrication, Tips and Solutions

Backplane PCBs are amongst the most demanding segments of printed circuit boards manufacturing. These are huge and intricate circuit boards that are the primary interconnect of other daughtercards or modules in any electronic system, and are thus needed in telecommunications equipment, servers, data centers, aerospace systems and in industrial control. Backplane PCB manufacturing requires special skills, equipment and strict quality control due to the complexity (in a number of layers) that frequently goes above 20, and a thickness exceeding 5mm.

This guide examines the most important issues that manufacturers struggle with when developing backplane PCBs, and offers a set of solutions that have been used to address the challenges.

Getting to know of Backplane PCB complexity

Backplane boards do not contain signal processing as well as standard PCBs. They operate as complicated interconnection systems that allow power distribution and high-speed transfer of data between several daughterboards instead. Backplane designs nowadays have two or more layers, with counts as large as 40+ layers, board-thicknesss of 3.2mm-8mm, hundreds of high-speed differential pairs, several power planes to support different voltage rails, and high-density connector arrays that need to be carefully aligned.

Such features make the production of these products unique and need special solutions.

In this critical challenge, layer alignment and registration control is required:

The Alignment Problem:

The dimensional stability is hard to sustain with increased number of layers used with repeated lamination cycles. Registration errors are caused by temperature changes during pressing, material expansion coefficients, and mechanical stresses and increase with every added layer.

Manufacturing Solutions:

• Sequential Lamination: In sequential lamination, the layers are not laminated together in a single process, but are constructed in a controlled sequence, which ensures better registration. This method minimizes cumulative errors and enhances accuracy of alignment.

   

• Improved Registration Systems: Have the installed automatic optical check systems with a micron level accuracy to check the alignment between the layers to the layers before moving on to the next manufacturing process. The modern systems are capable of detecting the deviation in alignment as small as 25 microns.

   

• Material Selection: Select low coefficient of thermal expansion, low dimensional stability core and prepreg materials. Such materials as Megtron 6 or Isola I-Speed have better stability than regular FR-4.

   

• X-ray Drilling Verification: X-ray inspection of the drilled holes should be done after drilling to ensure that holes are drilled in the correct position by passing through internal pad targets before going into plating operations.

High Aspect Ratio Drilling and Plating:

High aspect ratio drilling and plating represents a critical challenge, which is also known as ultra-thin plating.

Backplanes are very thick and thus aspect ratio is an issue of concern. Aspect ratio, which is a ratio between drill depth and hole diameter, is an issue that is more problematic with an increase in backplane thickness.

The complications of drilling and plating

Vias and connector drilling at high-speed works well to the thickness of 5-6mm, but larger drill sizes are necessary to drill through thicker PCBs. At aspect ratios above 10:1, a number of issues arise such as drill bit deflection that leads to positional errors, poor chip evacuation, more drill bit breakage and poor ability to maintain cylindrical hole geometry.

When electroplating is not done deep enough, then the copper in the hole will not be sufficient and excessive copper at the hole opening will also influence hole diameter and lead to mismatch between hole diameter and copper thickness.

Practical Solutions

• Design with Realistic Aspect Ratios: Have aspect ratios of 10:1 or less. In the case of 6mm thick backplanes, the minimum via diameter should be 0.6mm in order to preserve reasonable aspect ratios.

   

• High-Density Plating Processes: Adopt pulse plating technology as opposed to DC plating. Pulse plating offers enhanced copper deposition in the high aspect ratio holes because it enables enhanced circulation of electrolytes and diffusion of ions.

   

• Multiple Drilling Passes: To drill especially thick boards, make several drilling passes on each side, and then make the final hole, to get the desired accuracy and overcome aspect ratio issues.

 

• Improved Cleaning Methods: Use improved desmear and cleaning methods that are specifically made to high aspect ratio holes to assure good copper adhesion during plating.
  

Fundamental Question: Back-Drilling and Stub Removal

In cases where only high-speed signals are connected to inner layers of a PCB, unused through-hole vias form signal stubs which cause reflections and impedance discontinuities. Even short stubs can be a major contributor to signal integrity degradation at multi-gigabit data rates.

Back-Drilling Solutions

• Controlled Depth Back-Drilling: Once the PCB has been fabricated, apply the precision drilling tool to cut out the unused sections of the via barrels on the backside of the board. The manufacturer also removes through copper by re-drilling the target holes using a different drill size.

   

• Laser Ablation: Laser ablation technology is used to remove copper with minimal mechanical stress and depth control to very narrow stubs compared to mechanical drilling, and is very precise in stub removal.

   

• Process Control: Introduce strict depth measurement and verification procedures by means of cross-sectional analysis of test coupons to guarantee the uniformity of back-drilling depth of production batches.

Critical Challenge #4 Signal Integrity Management

Signal integrity through long traces and multiple connectors is also important in the design of backplanes. Long trace lengths enhance signal attenuation, crosstalk and electromagnetic interference.

The choice of material to use depends on the high performance.

Laminates with low-loss are required to reduce the loss of insertion on long interconnects. FR-4 materials are no longer sufficient at the frequencies above 10 Gbps. Depending on the speed, it is recommended to use Rogers 4350B at moderate speeds up to 25 Gbps, Panasonic Megtron 6 at high-speed designs at 25-56 Gbps and Isola Astra MT77 at ultra-high-speed requirements of above 100 Gbps.

The strategies of impedance control will be discussed here

• Stackup Design: Stackup Design tools and work with your PCB manufacturer to make sure that it is feasible. Correct impedance modeling needs to have accurate material properties, copper weight and manufacturing tolerances.

   

• Test couples Implementation: Add representative impedance test couples to every production panel. Time-domain reflectometry values confirm that production processes meet the target values of impedance with acceptable tolerances.

Critical Challenge #5: Thermal Control in the Process of Manufacturing

Backplane PCBs have large thermal mass due to the thick copper requirements and large number of layers that makes lamination and soldering more difficult.

Thermal Solutions

• Long Pressing Cycles
  Long Pressure backplane stackups need longer heating and cooling periods during lamination to guarantee that all the resin moves around and that layers bond correctly.

   

• Preheating Protocols: Preheat before wave soldering or selective soldering To avoid thermal shock, preheat joints prior to wave soldering or selective soldering.

   

• Copper Balance Design Distribution of copper uniformly across the layers to avoid warping during thermal cycling and enhance heat distribution during the manufacturing processes.

Critical Challenge 6: Quality Control and Testing

Backplane PCBs are complex and expensive to manufacture and therefore extensive testing is absolutely required before it can be shipped to the customers.

During the evaluation, the following types of tests are necessary:

• Electrical Testing: Perform 100 percent electrical testing by flying probe or dedicated test by using a flying probe or dedicated test to ensure continuity, isolation, and correct interconnections between all layers.

   

• Impedance Measurement: Determine controlled impedance traces are specifications using TDR testing on production coupons and key signal paths.

   

• Microsectioning Analysis: Cross-sectional analysis of samples of each production lot to ensure copper thickness in vias, layer registration, and material bonding.

   

• Dimensional Verification Dimensional verification is performed using coordinate measuring machines or automated optical inspection to verify that the dimensions and location of holes, as well as connector alignment, are within the tight tolerances needed by backplane applications.

Backplane Manufacturing: Best practices

• Early Involvement with the Manufacturer of your PCBs: Involve your PCB manufacturer in the design phase. Potential problems are spotted at the initial design-for-manufacturability reviews, and these problems are transformed into expensive problems.

   

• Prototype Validation: It should always be prepared to develop and test prototype backplanes carefully before volume development. These boards are so complicated that refinement at each stage is necessary.

   

• Material Quality Control: It is important to insist on materials of reputable suppliers whose properties are consistent and whose material certification records are complete.

   

• Process Documentation: Have a comprehensive process documentation and specifications to make the process consistent across production runs.

   

• Statistical Process Control Implement SPC to control critical parameters such as copper thickness, hole size, and impedance to enable the early identification of process drift.
  

Conclusion

Production of the backplane PCBs on a successful basis implies tackling a set of intertwined issues such as layer alignment, high aspect ratio drilling, stub elimination, signal integrity control, and thermal control. With sequential lamination and pulse plating, advanced manufacturing technology with strict control of its process, early design team work, and improvement through continuous evaluation of the testing feedback and analysis of production data.