Comparison of Microwave PCB Bonding Methods: What Designers Need to Know.

There are three methods commonly used for bonding multiple layers of RF & Microwave PCB laminates, such as PTFE (Teflon) materials, duroids and other RF materials from AGC Nelco,  Isola and Rogers. These three methods are 1. Thermoplastic films, 2. Thermoset prepregs, and 3. Direct bonding often called “fusion bonding”. Each have their own pros and cons that PCB designers and manufacturers need to understand to balance cost and performance.

 

Thermoplastic Film Bonding

Bonding films are designed to “flow” into the laminate stackup under heat and pressure to encapsulate the traces left on the surface of the inner layers after the etching process. This means that the bond is primarily mechanical in nature. Several thermoplastic bond films are available for use in printed circuit board applications and include Polyethylene (PE), ChloroTriFluoroEthylene (CTFE), Fluorinated Ethylene Propylene (FEP), and PolyTetraFluoroEthylene (PTFE). Major suppliers of this material over the past 40 years include Rogers and AGC Nelco. Thermoplastic bonding films are not well suited for use in hybrid multi-layer PCB due to the temperature required to melt these materials. The lower thermal degradation point of thermosetting materials may cause these materials to oxidize and decompose.

The transition (melting) temperature of these films must be considered to assure the right material match to the application. Polyethylene has the lowest melt temperature, from approximately 190°F to 250°F (88°C to 121°C), depending on the density of the resin and degree of molecular cross-linking. CTFE is approximately 380°F (193°C) which is prohibitive for printed circuit boards that will see manufacturing processes with higher temperatures such as hot air solder leveling (HASL). FEP has a transition point of 500°F (260°C) and is capable of handling hot air solder leveling temperatures. PTFE has the highest transition temperature, above 630°F (332°C), so it will survive subsequent high temperature processes.

The primary advantage of using thermoplastic films is their low electrical loss factor. PTFE multilayer pcb  boards are well known for their excellent electrical properties, but using a hybrid construction of a high loss epoxy-based prepreg would defeat the purpose of the PTFE. While a particular bonding film may not exactly match the dielectric constants of the laminate, the effect of any difference is typically negligible, or if not, the board can be designed to allow for the difference. The downside is that thermoplastic film bonding is typically limited to low layer PCBs and not suited for sequential lamination.

Thermoset Prepreg Bonding

Thermoset prepregs harden and cure as a result of a thermochemical reaction, such as the reaction that hardens the two components of an epoxy when mixed together that you can buy at the hardware store. Thermoset prepregs can also be reinforced with fillers to improve the stability of the final product.

Once hardened or “cured”, thermoset materials are typically harder than their thermoplastic counterparts. Unlike thermoplastic materials, thermoset materials go through the thermochemical reaction only once, and cannot be re-melted like a thermoplastic. Prior to the cure of thermoset prepregs, they have a limited shelf life compared to thermoplastic films.

The primary advantages of thermoset prepregs is the capability to manufacture sequentially laminated printed circuit boards, ability to produce higher layer count stackups, and a closer match to traditional laminate properties in hybrid constructions. The primary disadvantage of thermoset prepregs is a high electrical loss factor.

The first two methods require additional films or prepreg materials which function like glue to keep the multiple layers in one piece. The third approach uses heat and pressure to direct bond the material substrate layers into one piece.

Fusion Bonding

The third method for forming RF/Microwave printed circuit boards is fusion bonding, where the layers are joined together by means of extremely high temperature and precisely controlled pressure, without the addition of any bonding materials. This method has its challenges, such as the additional control over the lamination fixture, pressure and elevated temperature. But the increased performance over thermoplastic and thermoset bonding are significant in the right application. The fusion bonding results yield a fully homogeneous dielectric constant structure, with no mismatch of properties from different films or prepregs. Fusion bonding produces a single, uniform dielectric constant value throughout the PCB package, which can instrumental for high-frequency applications that must meet critical performance requirements.

Comparison of Pros & Cons

Thermoplastic Films

Pros

  • Good to excellent loss characteristics
  • Lower dielectric constant, Er than most thermosetting pre-pregs. Close to pure PTFE
  • Pure isotropic material

Cons

  • Not a good choice for sequential lamination
  • Lamination temperature unsuitable for many thermosetting pre-pregs
  • Poor drilling performance with potential for smearing
  • Cannot be desmeared or etched back

 

Thermosetting Pre-pregs

Pros

  • Possible to match resin system and electrical properties of thermosetting laminates with Er95 and above
  • Superior CTE to thermoplastic bond films
  • Can be desmeared and typically etched back

Cons

  • Anisotropic material
  • Not available in Er ≤ 2.94
  • Traditionally higher loss than thermoplastic resin systems

 

Fusion Bonding

Pros

  • Best electrical performance
  • Er almost perfectly matched to adjacent laminate materials
  • May be sequentially laminated depending on copper thickness

Cons

  • Not typically suited for bonding plated sub-assemblies unless PTFE bond ply is used
  • Long lamination cycle
  • Extremely high lamination temperature required ≥700°F (371°C)
  • Not compatible with thermosetting resin systems

 

Conclusion

As you can see, each of the discussed microwave bonding methods have distinct advantages and disadvantages that must be considered when choosing which method will be the right one for a specific application. As stated in the opening paragraph, PCB manufacturers need to fully understand these characteristics to balance cost and performance when selecting the optimum bonding method. At American Standard Circuits, we evaluate all microwave PCB bonding methods with our clients and decide the best method based on the application. For more information on PCB design, products, or solutions, call us today at (630) 639-5444.

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