DuPont Adhesiveless Laminate Technology Enables Increasingly Sophisticated Military Electronics Systems
Challenge: High reliability in increasingly complex designs
Solution: DuPont™ Pyralux® AP ensures plated through hole reliability in rigid flex circuitry
Rigid flex circuits are used extensively in military electronics applications such as satellite, radar and communications systems. These systems are becoming increasingly complex. Superior flexible circuit laminates are needed to meet the technical challenges of these systems while maintaining the utmost level of reliability.
Rigid flex circuits consist of rigidized sections where support is needed for components or connectors, interconnected with flexible segments where bending and turning are required. The rigid sections may contain thousands of plated through holes connecting the signal, power and ground lines throughout the board.
Most fabricators are already employing what is known as the ‘bikini’ method of rigid flex construction, where the coverlay material only covers the flexible segments and does not extend into the rigid sections. This reduces the amount of adhesive, which is known to cause problems with plated through-hole reliability, in the rigid sections of the circuit. However, this selective placement of coverlay material is not an option for the copper clad laminate, which must extend through both the rigid and flexible areas. In the past, adhesive based laminates were the only option, thus making it impossible to completely eliminate adhesive in the rigid sections of the board.
What is the problem with adhesive? The relatively high coefficient of thermal expansion (CTE) of adhesive systems creates a high level of z-axis expansion during thermal excursions. This expansion can eventually lead to cracking in the copper barrels of the plated through holes, which manifests itself as a failure in the circuit.
Additionally, a material’s glass transition temperature (Tg) plays an important role in plated through-hole reliability. Whether during the manufacturing process or in the actual application, rigid flex circuits tend to experience a wide range of thermal excursions. As a general rule, the CTE of a given material increases after surpassing the Tg. Therefore, the higher the Tg of a material, the higher temperature it can withstand before experiencing heightened levels of z-axis expansion.
The presence of adhesive in the rigid section of a board has always been an issue to some extent. In order to maintain reliable through-hole connections, circuit designs were necessarily limited in layer count, hole size and hole density. As electronic systems become more complex, these limitations become more problematic. Today’s circuits may consist of up to 30 layers or more and have aspect ratios (board thickness to hole size) of 10:1 or greater. Additionally, circuits are now much denser in both the total number of plated holes and the number of holes per unit area. Consequently, adhesive based laminates are no longer able to meet the design requirements. A solution is needed to completely eliminate adhesive from the rigid section of the boards.
Materials Selected and Why
The premier material for today’s rigid flex circuits is DuPont™ Pyralux® AP . Pyralux® AP is an all-polyimide laminate with no traditional adhesive layer. It has a CTE of 106 ppm/C in the z-axis. Most adhesive systems are four times this number. Furthermore, the Tg of Pyralux® AP is 220°C. Most adhesive systems are no greater than 40°C. The combination of these two properties makes Pyralux® AP an ideal choice for highly reliable plated through holes.
To illustrate the exceptional performance of Pyralux® AP in plated through hole reliability, a Highly Accelerated Thermal Stress (HATS) test was conducted. Two types of coupons representing rigid sections of a rigid flex circuit were manufactured. The first type contained an adhesive based laminate and the second type used Pyralux® AP. The coupons were subjected to 2000 cycles with a temperature range of –55°C to +125°C and a cycle time of approximately 12 minutes.
The results were quite dramatic. After 2000 cycles, the Pyralux® AP coupons showed little change in resistance in the plated holes, thus signifying no onset of barrel cracking. At around 75 cycles, the adhesive based coupons started showing increased levels of resistance and eventually all failed in less than 100 cycles.
Pyralux® AP has been used for over 10 years in rigid flex applications. This unique material has a proven history in military electronics systems and is the preferred material of choice for fabricators and designers alike.