Flex & Rigid-Flex Circuit Assembly Pre-Bake Specifications

Flexible polyimide materials used in flex and rigid-flex circuits absorb moisture, typically up to 2-3% by weight. This moisture absorption is an inherent material property and applies across all manufacturers and polyimide brands. To prevent assembly-related failures, a thorough prebake is required prior to any soldering or assembly process.

Industry standards recommend that all flex circuits be pre baked immediately before assembly without exception. Receiving parts in vacuum-sealed packaging or pre baking prior to shipment does not eliminate the need for prebaking again before assembly.

At a Glance: Pre-Bake Specifications

• Flex and rigid-flex circuits must be pre-baked prior to assembly to remove moisture absorbed by polyimide materials, which can reach up to 2-3% by weight.
• Pre-baking at approximately 120-121°C helps prevent delamination by eliminating entrapped moisture that can convert to steam during soldering and reflow processes.
• Bake duration varies by design and construction, with typical times ranging from 2 to 10 hours based on factors such as layer count, stiffeners, and overall circuit thickness.
• Moisture management in flex and rigid-flex circuits is a process control issue, not a packaging issue. Pre-bake every time, size the bake duration to the complexity of the design, and do not rely on external indicators to determine readiness for assembly.

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IPC 2223C Sectional Design Standard for Flexible Printed Boards

Entrapped moisture within the dielectric can vaporize at soldering and reflow temperatures. Depending on the moisture content, this vaporization may cause severe delamination that can physically rupture the circuit, or milder blistering that may later develop into larger defects. For this reason, bakeout is considered a standard and necessary process to mitigate moisturerelated blowout risks.

Flex and rigidflex printed boards generally require longer bake times than equivalent rigid printed boards. In addition, the time elapsed between baking and soldering is critical, as moisture can be rapidly reabsorbed after baking.

IPC FA251 Assembly Guidelines for Flexible Printed Circuits

Flexible dielectrics and adhesives absorb moisture at varying rates depending on material composition. In some cases, entrapped solvents remaining from manufacturing may also be present. Assemblers are therefore advised to include a prebake cycle to remove absorbed moisture or solvents accumulated during storage, handling, or fabrication.

Bake temperature and duration vary based on the combination of adhesives, dielectric materials, and board construction. A commonly referenced cycle is 121°C for 30 to 60 minutes, though requirements change with design complexity.

Flex and Rigid-Flex Circuit Assembly Pre-Bake Requirements

Flexible polyimide materials used in flex and rigid-flex circuits are inherently hygroscopic and can absorb 2 to 3 percent moisture by weight. This is a material property of polyimide and applies across all manufacturers and material systems. Moisture absorbed by the material must be removed prior to any assembly process.

Pre-baking is not optional. It is a required step immediately before assembly, regardless of how the product was packaged or how recently it was received. Even circuits delivered in vacuum-sealed or moisture-barrier packaging will absorb moisture over time once exposed, and baking prior to shipment does not eliminate the need for a pre-bake at the point of use.

If moisture is not fully removed, it will convert to steam during reflow. The rapid expansion of that steam creates internal pressure within the circuit construction. This is the primary cause of coverlay delamination, layer-to-layer separation, and stiffener delamination. In many cases, these failures are not visible until after assembly, leading to latent reliability issues in the field.

Recommended Pre-Bake Guidelines

A standard starting point is 120°C in a controlled oven environment. The required bake time depends heavily on the construction of the circuit. Simple single or double-layer flex circuits may require as little as 2 hours, while complex multilayer rigid-flex designs with stiffeners and thicker constructions can require 6 to 10 hours or more. Higher layer counts, adhesive systems, and the presence of stiffeners all increase the time required for moisture to fully evacuate the structure. When in doubt, longer bake times are preferred over risking retained moisture.

Do Not Rely on Humidity Indicator Cards

Humidity indicator cards included in packaging are not a reliable method for determining whether flex or rigid-flex circuits are ready for assembly. These cards measure the ambient humidity inside the packaging, not the moisture content absorbed within the polyimide material itself. A card may indicate acceptable conditions while the material has still retained significant internal moisture.

Relying on these indicators can create a false sense of security and lead to skipping the pre-bake process, which directly increases the risk of delamination during reflow. The only reliable approach is to follow a defined pre-bake process based on the construction of the circuit, regardless of packaging indicators.

DuPont Pyralux^® Flex Materials Baking Recommendations Prior to Reflow

Boards manufactured using Pyralux^® flex materials should be baked prior to solderrelated processes such as solder leveling and reflow. Typical bake conditions are 250°F (121°C) for two to ten hours, depending on board thickness and overall design.

Polyimide films absorb moisture rapidly after baking. As a result, soldering and reflow operations should be performed within 30 minutes of bake completion. Vacuum ovens may also be used to remove moisture at lower temperatures, typically between 150 and 175°F (65-80°C), which can additionally reduce oxidation on exposed copper pads.

Recommended Pre-Bake Temperature and Duration

Flex circuit boards should be prebaked prior to all soldering methods, including hand, wave, infrared, and vapor phase soldering. While 121°C is the recommended bake temperature, prebake duration varies significantly based on circuit construction.

Factors influencing bake time include layer count, stiffener presence, board thickness, copper distribution, overall board size, and ambient conditions such as humidity. Prebake durations typically range from a minimum of two hours up to ten hours for highlayercount designs.

Pre-Bake Reference Parameters

Parameter	Value	Units
Moisture absorption (polyimide)	2-3	% by weight
Recommended bake temperature	120-121	°C
Typical bake duration	2-10	hours
Alternate vacuum bake temperature	65-80	°C
Example dielectric breakdown voltage	>3	kV

Moisture Absorption Characteristics by Material

Absorption levels vary by material type and construction. Representative values include:

• Kapton^® NH: 2.8%
• Pyralux^® LF: 1.8%
• Pyralux^® RF: 1.8%

These differences reinforce the need to tailor prebake cycles to specific materials and constructions.

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Frequently Asked Questions

Quick Links

• Why do flex and rigid-flex PCBs require pre-baking before assembly?
• Is pre-baking required even if boards arrive vacuum sealed?
• What happens if moisture is not fully removed?
• What temperature is recommended for pre-baking flex PCBs?
• How long should flex circuits be baked?
• How soon must soldering occur after baking?

Why do flex and rigid-flex PCBs require pre-baking before assembly?

Polyimide materials absorb moisture, which can convert to steam during soldering and cause delamination or blistering if not removed.

Is pre-baking required even if boards arrive vacuum sealed?

Yes. Industry standards recommend prebaking immediately prior to assembly regardless of packaging condition.

What happens if moisture is not fully removed?

Remaining moisture may boil during reflow, potentially causing mild blistering or severe delamination that can damage or destroy the circuit.

What temperature is recommended for pre-baking flex PCBs?

A bake temperature of approximately 120-121°C is recommended, with duration determined by the circuit design.

How long should flex circuits be baked?

Bake times typically range from two to ten hours depending on layer count, stiffeners, material stackup, and board thickness.

How soon must soldering occur after baking?

For moisture sensitive polyimide films, soldering and reflow should generally be completed within 30 minutes after baking.

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Avoid Delamination and Boost Assembly Success

Incomplete moisture removal is the leading cause of coverlay, layer-to-layer, and stiffener delamination in flex and rigid-flex circuits. Moisture converts to steam at assembly temperatures, and its expansion can separate layers or create internal blisters.

Including a controlled and validated pre-bake step helps reduce assembly defects, improve yield, and protect long-term circuit reliability. For design-specific guidance, Epec recommends reviewing prebake requirements early in the assembly planning process.

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