Copper Foil mechanical fatigue in flexible printed

circuitssubject to high cycle vibration

04/22/2009Don Blanchet3B Associates339-234-3544

Scope Present information on the

metallurgical structure of 2 copper foil structures used in flex circuit design for a vibration environment– RA, rolled annealed– ED, electrodeposited

“Superflex” from OAK-Mitsu

Metallurgical Factors Copper foils have grain structures that vary widely

depending on the manufacturing process

My research has found that they do not behave as does bulk copper both in the annealed and cold worked states

Foils developed for flex circuit applications are sometimes tailored for the type of flex and expected number of cycles

Specific Cycling Tests have been developed to determine fatigue properties for various common applications such as:

– Disk drives– Cellular telephones– Printers

Reference copper foil fatigue characteristics (1/2 oz)

LongCrack propagation

length

shortCrack propagation

length

Superflex copper

Is “SuperFlex” right for a vibration application?

Interviews with OAK-MITSUI tech support:

this material was developed for primary use in cellular telephone applications – large bend-to install and moderate flexure cycles.The last fatigue test was performed over 10 years ago to certify to IPC 4562 grade 11Our application sounds like a misapplication due to the number of cycles accumulated during our vibration testing They have never encountered a similar application

Hitachi copper foil research

Crack microstructure

“RA”

“ED”

Fatigue Life - vibration

~12 Hr qualification test

Conclusion Rolled and Annealed copper foil is

the clear choice over electrodeposited metallurgy for a high cycle flex application.

Goals Characterize the modal

frequencies.

Examine the response to Qualification level sine vibration 10G over 20-2000hz.

Look at 3 mutually perpendicular input axes.