High Frequency 5G Telecom PCB Assembly

Which Frequency Band Sets the Laminate Choice?

A sub-6 GHz control or support section will be able to accommodate a different material than the 24-40 GHz RF path or specialized mmWave test path. When working with the new 5G technology, separate out the telecom band and specialized mmWave cases. Multiple types of RF laminates such as RO3003, PTFE, LCP, and other RF laminates may be used in the same project; however, most of the time they will be performing different functions for the RF paths.

Dk and Df Before Loss Budgeting

Df is the measure of how much energy a material loses, whereas Dk affects impedance, trace width, and field behavior. Therefore, prior to creating a loss budget, it is essential to understand how to read both of those properties.

For high-frequency laminate, Df 0.004 or less is typically a target threshold; however, it is not the sole determining factor in the decision-making process. Although lower Df materials will yield less overall energy loss, these materials can present challenges regarding processing, moisture, bonding, and thermal performance.

Rogers Materials for Balanced RF Loss

Rogers RO4350B is a good example of a material that sits in between standard FR-4 and more specialized PTFE-class materials. Even though Rogers RO4350B provides lower loss than FR-4, it still stays closer to standard fabrication behavior than PTFE-class materials.

RO3003 indicates a transition to lower Df and more demanding microwave paths. When you have a situation where insertion loss is more critical than material cost, using RO3003 becomes advantageous; however, RO3003 also requires careful processing to control dimensionality and physical properties during fabrication.

When Does FR4 Stop Working?

FR-4 should not be your go-to option when it comes to RF paths that are sensitive to RF loss. While Df values in the range of 0.015-0.020 may be acceptable for control or low-frequency support boards, they will not provide a mmWave answer.

FR-4 should not be eliminated too soon from the project. Any section of your project that has acceptable RF budgets should incorporate FR-4; conversely, any sections that require low-loss laminates should reserve low-loss materials for those specific applications.

PTFE and LCP at the Edge of mmWave

PTFE and LCP solve different problems in regard to mmWave applications. PTFE materials fit very low-loss microwave and mmWave frequencies. LCP materials assist in producing compact or flexible RF structures that require both low loss and a bendable material.

If any aspect of the process, such as copper bonding, drilling, laminate handling, or material availability, is incompatible with your design, then you will be unable to benefit from the advantages that PTFE and LCP provide.

Conductor Loss and Copper Roughness

At high frequencies, the skin effect causes the current to crowd near the surface of the conductor. The rough surface of the copper gives the signal greater surface area to travel across and increases the insertion loss of the RF path.

The copper weight of the conductor is still important for current distribution and thermal effects, but the fact that the copper weight has to be sufficient does not always translate to providing a high-performance RF path. For high-speed RF layers, the profile of the copper, trace geometry, and impedance characteristics must all align.

How Return Paths Break Signal Integrity

When an RF trace is split by a plane, gap, via hole, or bad transition, you lose continuity on the return path. The result is an impedance mismatch for the signal, and it will be reflected, distorted, or poorly returned by way of that discontinuity. In a good reference plane, this problem can be minimized.

Via Stubs as a Signal-Loss Source

Via stubs form an unused transmission line. Stub reflections may not be significant at low frequencies, but when they are present at higher frequencies, they can return energy into your signal path.

EMI and Crosstalk Around Dense RF Routing

When RF routing is densified, the analog, digital, power, and control paths are closer together, and EMI and crosstalk occur as layout failures rather than compliance concerns.

Which HDI Features Help 5G Modules?

When RF routing must be compact, have a narrow pitch escape, or have a short interconnect path, HDI provides help to a module.

The use of blind vias and buried vias will help reduce routing pressure.

The use of backdrill can reduce stub behavior in cases when using a through via would leave too much unused via barrel.

Backdrill and Blind-Via Fabrication Limits

The problems solved by backdrilling and blind vias are different.

Backdrilling is used to remove unused length in vias that can reflect RF energy.

Blind vias and buried vias are used to shorten transitions and create routing space in dense modules.

The use of a 4mil laser drilling capability will be important when using microvia interconnects, but the registration, plating, and stack-up discipline must still be maintained.

Fine Trace Limits and mSAP Options in Dense 5G Hardware

Fine trace and space limits provide benefits for the escape path of compact modules; however, fine trace and space limits also increase the manufacturing sensitivity of a design.

A design that uses a 2.7mil/2.7mil trace / space or 2 mil trace / space will have less margin for process drift.

mSAP allows for straighter and more accurate traces than subtractive processing when geometry stability is important.

Rigid-Flex Options in Telecom Devices

Rigid-flex offers an alternative method to interconnect the antenna, module, and enclosure geometry of a compact telecom device without introducing an awkward board-to-board transition.

Achieving an electrical goal requires consideration of bend radius, copper fatigue, antenna clearance, and assembly handling.

Why High-Frequency Telecom PCBs Need Precision Assembly

Due to the high frequency of the RF signals, the precision of the assembly of the PCB becomes of increased importance. A very small deviation from placement, soldering, or routing can cause an RF failure, whereas the same PCB manufactured at low frequencies could withstand this drift without issue. Conversely, a dense RF module will have less tolerance for even minor deviations.

RF Testing With S-Parameters

S-parameters connect the assembled board back to its RF behavior. By using S-parameters, you can measure insertion loss, return loss, and how your signals will behave throughout the entire operational frequency range.

AOI and E-Test Before RF Testing

Before proceeding to RF tests, both AOI (Automated Optical Inspection) and E-Tests (Electrical Tests) have a role to play. AOI reveals the visible defects in the assembly, such as incorrect placement of components, incorrect soldering, and board defects. E-Tests reveal issues related to open and short circuits as well as electrical faults at the circuit level.

Thermal Cycling and EMI Testing

Thermal cycling evaluates the effect that temperature variations would have on the performance of the board. Thermal cycling is also related to thermal expansion (CTE), movement of the dielectric materials, solder joint reliability, and thermal paths in the PCB design.

EMI testing determines how interference occurs due to the routing and shielding options utilized for closely placed RF, digital, and power traces.

Traceability in Telecom PCB Assembly

In many cases, it is even more important to have traceability for RF PCBs than for general PCBs. Most RF failures are either intermittent in nature or influenced by the environment. If an RF PCB has passed the S-parameter test at some point in time but has drifted out of specification after deployment into service, the only way to trace that failure back to the cause is by using traceability.

How to Compare RF Microwave PCB Manufacturers

When comparing RF microwave PCB manufacturers, it's important to note that it's not enough just to look at a capability list. Just because two manufacturers check every box on that capability list, it does not mean that they will produce the same type of RF outcomes. This is because the various categories listed on the capability list all interact with one another — for example, a manufacturer could fabricate very fine-featured parts, but if they do not have any RF test data to back it up, then the manufacturer will not have an actual RF impedance result to confirm the accuracy of their fine-featured component fabrication on a real board. This makes it critical to tie the various records together across categories to get a complete view of a given manufacturer. The various records should show that the RF test data came from the same product lot as the fine-featured fabrication claim; otherwise, it becomes impossible to know if the claims were ever tested against each other.

DFM Support Before RF Production

Before a manufacturer is ready to make RF boards, DFM support should catch any potential stack-up, geometric, laminate, via, and assembly issues. For 5G and microwave circuit boards, DFM review can help to mitigate the cost of rerouting the boards due to assembly problems or unstable test behavior.

It’s just as critical to consider the ability to assemble RF products properly, and DFA issues associated with RF circuit board build limitations will have an impact on assembly. If a manufacturer produces RF boards to meet performance limits but has not produced a board to that level of performance with densely compacted components, BGA packages, or mixed RF-digital assembly, then that manufacturer may face assembly challenges related to impedance control achieved during fabrication. If the impedance control achieved during RF board fabrication is lost during assembly, it becomes impossible to produce a functional RF product.

Which Certification Documentation Matters?

There are various certification documents that are important when starting a new RF circuit board project. If you are working on a new RF project, such as 5G, and you do not have documentation for IPC-6012 Class 2/3, UL, RoHS, REACH, and ISO 9001, you have a major gap in your supply chain. Document types such as AS9100D, ITAR, JCP, and MIL/aerospace belong to a different group of certification and regulation; when the final application for the equipment produced under these certifications is a regulated product, i.e., defense, aerospace, or export-controlled hardware, these document types may be required. When one reviews the certification level by tier, one can easily avoid disqualifying a supplier simply due to the supplier mainly serving different market segments and not having the required documents at the time of bid.

When Lead Time Claims Need Engineering Review

Claim lead times for engineering review: “As fast as 5 days” will depend upon material availability, HDI density, impedance testing, thermal cycling, and EMI testing.