I am finding a serious failure problem with MAX232 ICs, used in a product.

The product uses a 89C52 with MAX232 (Surface mount) to interface with a battery operated handheld terminal , bit serial interfaces to some sensors and LCD display and RS485 interface to another external device which works on 230V AC. The product has a built in linear power supply and is operated from 230V AC. There is no isolation between the product and external RS485 device, but only DATA+ and DATA- lines are used. Ground line is not available for connection on the extrnal device.

The earthing on the 230V supply point is not good and some times a voltage of 90+V is found between Neutral and Earth.

Over a period of time I noticed that a whole batch which was using SIPEX brand MAX232 had field failures. Now the rate of failure has increased, even with MAXIM make ICs. I am not able to find a reason for the same. MAX232 circuit is a standard circuit using four numbers of 1uF MLC capacitors and 0.1uF decoupling capacitor.

There is no failure on RS485 side.

PCB tracks have enough clearances and there is no possibility of interference. 5V power supply for 89C52 and MAX232 is derived from 7805 and has negligible ripple. There is no failure in any other IC used on the PCB

MAX232 failure is also of a funny nature. RX out (TTL) signal pin keeps pulsing at a low frequency even when there is no incoming data and RX inputline is at steady -12V.

I am told there are a lot of duplicate MAX232s floating around in the market. Does any one have any idea about such a problem?

Thanks in advance for any kind of input on this.