Where does hikashop decide to skip credit card ?

I want to add some custom code where hikashop decides whether to skip collecting of credit card info for free products. What PHP file has this code?

Thanks for your help.

The modification needs to be done at several places:

1. in the components/com_hikashop/controllers/checkout.php in the functions before_payment,check_payment and after_payment
where you have the check:
if(empty($cart->full_total->prices[0]->price_value_with_tax) || bccomp($cart->full_total->prices[0]->price_value_with_tax,0,5)==0){

2. in the file components/com_hikashop/views/tmpl/payment.php you have a similar check at the beginning.