Siloxane-Modified Polyimide Adhesive for High-Temperature PCBs
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Solution Overview
Problem
Existing flexible printed wiring boards lack sufficient heat resistance at 150°C for 1,000 hours, which is necessary for engine control units in automobiles, due to poor heat resistance of adhesives used in these boards.
Innovation Solution
An adhesive composition containing siloxane-modified polyimide with specific structural units, epoxy resin, and inorganic filler, which provides improved heat resistance and mechanical strength, is developed for use in printed wiring boards, bonding films, coverlays, and copper-clad laminates.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Temperature
If existing general-purpose adhesives are used for stacking components in flexible printed wiring boards, then the board can be manufactured with standard materials, but the heat resistance at 150°C for 1,000 hours is insufficient for engine control unit applications
Solution Approach 1:
The adhesive composition uses a composite system combining siloxane-modified polyimide (with specific structural units containing siloxane groups and aromatic rings), epoxy resin, and inorganic filler. This composite structure leverages the high-temperature stability of polyimide, the crosslinking strength of epoxy, and the thermal resistance of inorganic filler to achieve 150°C heat resistance for 1,000 hours required for engine control units.
Solution Approach 2:
The invention modifies the chemical structure of polyimide by incorporating siloxane groups (Si-O-Si bonds) which have higher thermal stability than traditional C-C bonds. The specific structural units with siloxane-modified aromatic rings change the thermal degradation behavior, raising the glass transition temperature and maintaining adhesive strength at 150°C over extended periods.
2Temperature
If the siloxane-modified polyimide has higher molecular weight to improve heat resistance, then thermal stability increases, but processing difficulty and viscosity increase
Solution Approach 1:
The invention optimizes the molecular weight parameter of siloxane-modified polyimide to a specific range (25,000-150,000 g/mol). This parameter optimization balances thermal stability (requiring higher molecular weight) with processability (requiring lower molecular weight for lower viscosity). The epoxy resin addition further modifies the rheological properties, enabling proper spreading and bonding characteristics.
3Strength
If inorganic filler content is increased to improve heat resistance and mechanical strength, then thermal and mechanical properties improve, but the adhesive composition becomes more complex and harder to process
Solution Approach 1:
The adhesive formulation uses a balanced composite system with siloxane-modified polyimide, epoxy resin, and inorganic filler in specific proportions. The inorganic filler (such as silica, alumina, or titania) provides thermal conductivity, heat resistance, and mechanical reinforcement. The epoxy resin acts as a matrix that binds the filler particles and polyimide, ensuring proper dispersion and preventing aggregation, thus maintaining processability while achieving enhanced peel strength and thermal stability.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
The adhesive composition enhances the heat resistance and peel strength of printed wiring boards, enabling their use in high-temperature environments, such as near automobile engines, without compromising oil resistance or flexibility.
Implementation Method 1
the siloxane-modified polyimide has a weight-average molecular weight (Mw) of 25,000 or more and 150,000 or less, and a content of the inorganic filler is 10 parts by mass or more and 100 parts by mass or less relative to 100 parts by mass of the siloxane-modified polyimide
Implementation Method 2
An adhesive composition containing siloxane-modified polyimide with specific structural units, epoxy resin, and inorganic filler, which provides improved heat resistance and mechanical strength
Implementation Method 3
a content of the inorganic filler is 10 parts by mass or more and 100 parts by mass or less relative to 100 parts by mass of the siloxane-modified polyimide
Data Source
AI summary
An object of the present invention is to provide an adhesive composition for a printed wiring board, the adhesive composition having a good heat resistance. Another object of the present invention is to provide a coverlay for a printed wiring board, a copper-clad laminate, and a printed wiring board, all of which use the adhesive composition for a printed wiring board. The present invention provides an adhesive composition for a printed wiring board, the adhesive composition containing a siloxane-modified polyimide that includes structural units represented by formulae (1) and (2) below, an epoxy resin, and an inorganic filler. The siloxane-modified polyimide has a weight-average molecular weight (Mw) of 25,000 or more and 150,000 or less. A content of the inorganic filler is 10 parts by mass or more and 100 parts by mass or less relative to 100 parts by mass of the siloxane-modified polyimide. In formulae (1) and (2), Ar represents a tetravalent aromatic tetracarboxylic acid residue, m is 0.35 or more and 0.75 or less, and n is 0.25 or more and 0.65 or less. R1 in formula (1) represents a divalent diamine siloxane residue, and R2 in formula (2) represents a divalent aromatic diamine residue.