<a href="#" class="cp_btn">button</a>

a.cp_btn {
	display: block;
	position: relative;
	width: 160px;
	padding: 0.8em;
	text-align: center;
	text-decoration: none;
	color: #fff;
	background: #455A64;
	border:1px solid #fff;
	overflow: hidden;
}
a.cp_btn:after {
	content:"";
	position: absolute;
	top: -100%;
	left: -100%;
	height: 100%;
	width: 160px;
	background : #fff;
	opacity: 0.3;
	transition: .4s;
}
a.cp_btn:hover:after {
	top: 0;
	left: 0;
}