We derive upper bounds on the rate-memory trade-off of cache-aided erasure broadcast channels with K w weak receivers and K s strong receivers. We follow a decentralized placement scenario, where coordination is not needed prior to the delivery phase. We study two setups: a standard scenario without eavesdropper and a wiretap scenario with an external eavesdropper. For both scenarios, we propose joint cache-channel coding schemes that efficiently exploit the cache contents and take into consideration the users' channel characteristics at the same time. We show that the decentralized placement strategy causes only a small increase in delivery rate compared to centralized strategy. Similarly, when cache sizes are moderate, the rate is increased only slightly by securing the communication against external eavesdroppers. This is not the case when cache memories are small and large.