THERMODE TYPES

PCB Design

• Width of the PCB traces should be 50% of the pitch to avoid short circuits
• Make PCB traces wider than flex traces to allow solder to flow and fillets to form
• Tooling holes in PCB and Flex allow easy alignment of traces
• Provide room for thermode to overhang both ends of pad area by 0.040”(1mm)

 

 

PROBLEM

Differences in heat sinking from pad to pad can cause uneven heating:

Unequal trace area and plated through holes act as a heat sink, causing uneven heat distribution.

 

Equal trace sizes (mass) leads to equal heating and better solder joint quality. Minimum .08" wide solder area is recommended.

 

 

 

Flex Circuits

• Most common type is polyimide (also known under the trade name of Kapton)
• Two layers of polyimide encapsulate the copper traces(normally 0.5–2oz)
• Copper is Rolled Annealed or Electro-Deposited (most cos teffective and widely used)
• Thickness of copper traces ranges from 0.0007–0.004 inches (18-100microns)
• Operating temperatures ranging from130–200°C (can with stand soldering temperatures up to 300°C for a short time)
• Thickness of the polyimide ranges from 0.001–0.0047inches (25-120microns)

	Type	Advantages	Disadvantages
	Exposed Lead	Lower thermode temperatures.	Flux contamination on thermode. Leads can be damaged or bent when handled.		Maximum polyimide thickness, the top layer, needs to be .002 inches or 50 microns. Loss (thermal barrier) of 60–80°C per 25 microns thickness
	Single Sided	Easy to handle. Minimal flux contamination on thermode.	Higher thermode temperatures due to thermal barrier.		Minimum pitch needs to be.008 inches or 200 microns to avoid short circuits.
	Open Windowed	Lower thermode temperatures.	Flux contamination on thermode. Part alignment is critical.		Minimum pad length needs to be .080 inches or 2 mm to allow fillets to form.

 

Flux

• Flux is a chemical agent that assists in the soldering process by removing and preventing oxide formation.  
• Flux is a catalyst that does not become part of the final metal composition.  
• Flux improves the wetting action of solder by lowering the surface tension of the solder.
• Each flux has an optimum working temperature where it reacts chemically with oxidized metal surfaces

Flux activation or working temperature is an optimum range of temperatures where it chemically reacts with oxidized metal surfaces.

Flux for electronics applications:

• Must be heated to remove oxides
• Relatively “inert” at room temperatures

Electronics Fluxes

Water Soluble

• Most active type of flux
• Residue is corrosive and conductive
• Requires cleaning equipment and hot water treatment

Rosin Activated

• Very active flux
• Requires saponifier (chemical) and hot water to clean

Rosin Mildly Activated

• Promotes good wetting on oxidized surfaces
• Requires saponifier (chemical) and hot water to clean

No Clean

• Least active type of flux
• Reflow process must occur within 5 minutes of application
• Low solids content leaves little residue
• No cleaning required or possible