SSL Certificates marked with the SHA-1 algorithm to certificate signed with SHA-256 are set to start in November 2014 to meet refreshed federal and PCI compliance measures. SHA-2 certificates are exponentially more secure, which are more secure. The underlying cause of the issue is a known vulnerability that opens it to collision attacks. Such attacks could enable an attacker to produce extra certificates that have an indistinguishable digital signature from a unique digital signature. Google and CWI Amsterdam and Google declared they had played out a collision attack against SHA- 1, publishing two disparate PDF records which create the same.”]
Source: https://gbhackers.com/microsoft-boycott-sha-1-ssltls-certificates-edge-internet-explorer/