The PCB interface has long been a challenge. An electronic system must endure an operational temperature range of -55 to 155 degrees Celsius. Alternatively, in extreme cases, it should withstand 250 degrees. So, they need a design with high thermal conductivity.
High thermal conductivity usually means high specific heat capacity and low thermal resistance. They are the two most fundamental properties for designing thermally conductive materials. For example, copper has high thermal conductivity and specific heat capacity. The thermal resistance of copper is relatively low. Thermal conductivity and specific heat capacity are both based on the units of kg/m.K (kilograms per meter per degree). Materials with the same weight have the same thermal conductivity. Unless they have different specific heat capacities, it means their thermal resistances will differ. All else being equal, materials with higher specific heat capacities will have lower thermal resistance than those with lower values.
We use various laminate cores with high thermal conductivity materials to develop very efficient and cost-effective PCBs. In addition, RayMing PCB and Assembly is developing many new laminates to achieve the required thermal performance that the varying markets need.
Mechanically Unsupported, High Thermal Conductivity Laminates RF-60TC PCB is a family of high thermal conductivity laminates that provide 3 to 5 times the thermal conductivity of FR-4 based alternatives.
STORAGE
You need to store the RF-60TC laminates flat in a clean, dry place. Avoid arcing or static discharge.
At temperatures below -40 degrees F, you should protect the laminate from direct exposure to moisture and store it in a dry environment.
Manufacturers anneal the High Thermal Conductivity Laminates Taconic RF-60TC PCB after manufacturing. This means heating the laminate to approximately 200 degrees Celsius, cooling, and reheating over an oven.
Shelf Life
Shelf life for the laminates is five years, but it can last over ten years, depending on the type and application. For example, use in blast furnace applications will have a shorter shelf life than those used in telecommunications.
REPAIR AND REUSING
The laminate color depends upon the processing and the age of the laminate and environmental conditions. The color of RF-60TC laminates resembles that of gold leaf. The laminates may appear to change color as they age. There is no need for concern with this color change.
You can repair or reuse the RF-60TC laminates in most cases. However, you must remove the laminate defects to make an area usable. A common practice is to cut the laminate with a bandsaw such that the damage does not exceed 8mm (3/8”).
HANDLING
PTFE is a thermoplastic material that one can reuse. PTFE is slippery. Handling PTFE requires care not to drop or scratch the material. Passive smoke particles may collect on the surface of the material over time, which will make it look darker but will not impact performance. It is stable electrically and chemically and will not contaminate other materials or cause any adverse effects. RF-60TC laminates are excellent for PTFE substrates as a substrate or top cover.
Avoid mechanical scrubbing, brushing, or sanding
Use a lint-free cloth to dust the laminate surface. Avoid using abrasive cleaners on these laminates as they are very hard-wearing. In addition, stainless steel wool may break at temperatures below 125°C (257°F) and release silica dust that can cause allergic reactions.
Suppose the laminate surface becomes contaminated with a solvent that you cannot remove mechanically. In that case, you can heat the laminate to 175°C (347°F) for a short period. This will evaporate the solvent and recover most of the strength.
It is possible to remove the solder mask from FR-4 PCBs by placing PTFE in water at 60 degrees Celsius (140 degrees Fahrenheit) for over two hours. You can then remove the solder mask mechanically with a pair of tweezers.
Do not pick up a panel horizontally by one end or edge
To inspect the laminate surface, moisten a lint-free cloth with water and hold it in such a way that light shines through but is not visible from all sides. You can obtain the best result with a magnifying glass. If you are curious about the condition of your laminates, you can also use a magnifying glass to inspect them.
It is important to note that a glossy gold or silvery coating on the laminate surface comes from an added metal layer over the material.
Prevent contaminant deposits on the material or copper
Keep the laminates clean during fabrication and assembly. Be careful not to contaminate the surfaces with fingerprints and other soils. They could cause oxidation of the copper or staining of the laminate.
Do not stack panels directly on top of each other
Adherence between the laminate surface and the PCB or PCB substrate is essential. This ensures the excellent performance of the printed circuit. Use a contact material between the PCB substrate and the laminate surface. The contact material must maintain its adhesion over a long period without damage.
INNER LAYER PREPARATION
Taconic recommends connecting the inner layer(s) of the PCB with lands and a power plane. If the inner layer(s) have no connection, vias should connect them to the outer layers.
Acclimation:
The laminate temperature depends on the temperature at which the manufacturers use. Therefore, the surface of the laminates may feel colder or warmer than normal.
The outside layer of most laminate sheets is between 55 and 80 degrees Celsius. At the same time, the glass transition temperature of the core material is between 176 and 205 degrees Celsius.
Scaling:
The thickness of the lamination will relax because of several factors. Reduced cooling will reduce thermal conductivity by 10% from the rated value. Short-term humidity and temperature changes can also affect thermal conductivity. This causes the laminate to have a more abrupt temperature change at one location than another.
LAMINATION
Lamination of the PCB is when you apply a plastic film over a copper clad board to insulate it from temperature, moisture, and vibration changes. The most popular laminate form used today is polyimide with glass fibers. We use other materials for special applications, such as holding metal plating sheets and substrates. Glass fiber laminates are also excellent for separation layers for printed circuit boards.
Flow Patterns / Thieving:
Lamination will have flow patterns different from the ideal planar flow pattern. This occurs because of the lack of proper operation of the laminating machine. Poor adhesion will also cause the feathering of the lamination.
Padding and Conformance Materials:
The laminating process will allow some air trapped on the laminate with gaps between the laminate and the board substrate. Air in this area will cause a decrease in the efficiency of the laminate.
Pressure:
Small cracks may form after removing the laminate’s stress. Therefore, you should not stuff the laminated board with pressure or press it onto it for several minutes after laminating it.
Temperature:
The outer-most layer will have a different thermal conductivity than the core material. This is because the temperature of the outer-most layer has varied during processing.
DRILLING
It is a necessary process that allows for the population of the board with components. Drilling is also used to allow vias between layers and remove material from around features like pin headers and mounting holes. Glass fiber reinforced laminates have excellent drill strength.
Drill Bits:
Polyimide is harder than copper, so that the bit will dull more easily. The composition of the polymer layer may also affect the lifespan of the bit. In addition, the material may have a greater coefficient of thermal expansion than copper. As a result, it will cause durability problems when drilling into polyimide.
Chip Load:
Chip load on the drill bit will affect how quickly it will wear. A high chip load helps generate friction and heat, which may cause the drill bit too dull quickly. You may also scratch the laminate surface, which is especially important for a solder mask that is only 100microns thick. When dull, the bit will also leave rough edges after drilling.
Cutting Speed:
The cutting speed of the drill bit will affect how quickly it will wear. For small hole sizes, a high cutting speed worsens the situation. A lower cutting speed is better for large holes because it diminishes plastic shavings from forming.
Dwell Time:
After drilling a hole, you will have to re-enter the drill bit in the hole. Drill bits that have gotten dull will make this harder. The material may also spoil due to the thermal shock from thermal cycling.
Peck Drilling:
Drilling will generate heat. The polyimide and glass fibers will also generate heat. The thermal expansion and conductivity of the materials will cause thermal cycling. It causes the hole to become oval-shaped and may lead to cracking.
PLATING
The most common plating process is tin deposition, where you deposit a thin layer of the tin onto the surface of the PCB. For solder masks and thin layers of copper, the plating will cause damage to the PCB. Therefore, you should avoid thinner layers. Otherwise, post-plating cleaning may become hazardous.
Metal:
The surface of the copper-clad board does not have to be 100% pure copper. Larger amounts of lead or other metals may cause corrosion problems for electrical connections. The surface may also become pitted after electroplating, especially if there is a large difference in the plating thickness.
Tin Thickness and Coverage:
It is essential to know whether the plating system used will build up in on all areas of the PCB. For example, copper-clad boards with wider than 0.1mm features may have full tin coverage over some areas while having no tin coverage over other areas.
Base Material:
Use the glass fiber reinforced laminates for plating. They are more conductive than their PVC counterparts and do not have the same problems with cutting.
Drill Thickness:
If the drilling process is too aggressive, it may cause damage to the PCB.
Overflow:
The plating tank may not adjust its acid levels, which will cause the plating to overflow onto the board.
SOLDER MASK
Solder mask is a photo-sensitive polyimide film applied to the PCB before placing the components. The solder mask will protect the areas of copper and prevent the solder from flowing there. In addition, the solder mask is dielectric, so the solder will not stick to it. This is important because if solder masks cover the surface of vias and other features such.
After soldering, you will expose the mask to a light source. The polyimide will convert to its original composition. A chemical reaction will occur between the polyimide and the exposed solder. The polyimide will decompose and oxidize in a process referred to as “exposure.” Paste-down:
Conclusion
Lab testing confirms that the High Thermal Conductivity Laminates and High Thermal Conductivity Boards are very efficient in dissipating heat. It allows for an excellent performance of the PCB in a range of applications. This family of laminates will revolutionize PCB thermal performance at Taconic. In addition, it will provide new opportunities for design engineers seeking high-performance laminates.
