If you are referring to Scanning Electron Microscopy, then before doing SEM of non conducting sample one have to coat the sample with a conducting metal like Gold or Platinum. Because without coating you will get only a blur image of the sample due to the collection of electron cloud near the sample surface, which gives rise to so called 'charging effect'. Also, if you have powder sample, then coating is a must as it also protects the powder from evaporation under high applied electric field, which subsequently increases the lifetime of the electron gun of SEM instrument.

usually aluminium wrapping is done insytead of gold and platinam as they are costly

To get a good FESEM/ SEM results of thin films you have to use Au/ Pt coating. How can you see the surface through aluminum wrapping? Also, as the method of coating is sputtering in most of the cases, the coating cost per sample thus reduces greatly. However, you can use low voltages ~1 kV for SEM of non conducting samples without coating. But, that will greatly reduce the picture quality, depending on your SEM instrument (i.e. if you have a highly sophisticated SEM instrument, only then you can see good image without coating for non conducting samples).