woocommerce/templates/loop/add-to-cart.php

<?php
/**
 * Loop Add to Cart
 *
 * This template can be overridden by copying it to yourtheme/woocommerce/loop/add-to-cart.php
 *
 * HOWEVER, on occasion WooCommerce will need to update template files and you (the theme developer)
 * will need to copy the new files to your theme to maintain compatibility. We try to do this
 * as little as possible, but it does happen. When this occurs the version of the template file will
 * be bumped and the readme will list any important changes.
 *
 * @see 	    http://docs.woothemes.com/document/template-structure/
 * @author 		WooThemes
 * @package 	WooCommerce/Templates
 * @version     2.5.0
 */

if ( ! defined( 'ABSPATH' ) ) {
	exit;
}

global $product;

echo apply_filters( 'woocommerce_loop_add_to_cart_link',
	sprintf( '<a href="%s" rel="nofollow" data-product_id="%s" data-product_sku="%s" data-quantity="%s" class="button %s %s product_type_%s">%s</a>',
		esc_url( $product->add_to_cart_url() ),
		esc_attr( $product->id ),
		esc_attr( $product->get_sku() ),
		esc_attr( isset( $quantity ) ? $quantity : 1 ),
		$product->is_purchasable() && $product->is_in_stock() ? 'add_to_cart_button' : '',
		$product->supports( 'ajax_add_to_cart' ) ? 'ajax_add_to_cart' : '',
		esc_attr( $product->product_type ),
		esc_html( $product->add_to_cart_text() )
	),
$product );