CPF官方网上说：
CPF contributions are not mandatory for Singapore citizens or Singapore Permanent Residents who work overseas. If you wish to continue making CPF contributions for your existing employees who are posted overseas, these are deemed as voluntary contributions. You have to register for a new CPF Submission Number (CSN) for such payments.

>>你随时都可以交CPF，无限制。

IRAS官方网上说：
If you work overseas, it is not mandatory for you and your employer to pay CPF contributions.
However, you may wish to build up your retirement savings by making voluntary CPF contributions, up to the approved limit. However, if your employer in Singapore is paying CPF contributions for you, the employer's contributions will count towards the limit.

Do note that voluntary contributions do not qualify for tax deduction.

>>如果是在国外工作，国内无收入，就无需交 CPF。如果自愿交 CPF，自愿交代部分无法抵税（这个比较 tricky，因为你都在新加坡没有收入了，当然不需要缴税，也不存在需要抵税的。由于这部分收入是国外收入，应该交国外的税，把税后的钱来交 CPF 是不收税的，因为 CPF 不是收入，是付出，是存款。不过以后你再取出，可能会要缴税，这是根据 取 CPF 的规定收税的 ）