SMT Perspectives and Prospects
December 21, 2011 |Estimated reading time: 4 minutes
Editor's Note: This column originally appeared in the October 2011 issue of SMTMagazine.
Manufacturing PCB assemblies containing grid array packages in the co-existing RoHS lead-free-compliant and RoHS-exempted industry sectors poses a great challenge to both OEM companies and EMS providers. The grid array packages referred in this article include ball grid array (BGA) packages, fine-pitch BGA (CSP) and other array packages using solder balls/solder columns/other physical forms of solder as the board-level interconnection technology.
Over the last 10 years, I have received many questions about best practices when a PCB comprises lead-free packages, tin-lead packages using lead-free solder paste or lead-free solder paste. Concerns often arise regarding the resulting performance and reliability level in each scenario. The commonly occurring scenarios cover the following combinations of package solder ball and SMT solder paste are:
- Assembly containing BGAs with SAC305 solder ball using SAC305 solder paste;
- Assembly containing BGAs with low-Ag SAC solder ball using SAC305 solder paste;
- Assembly containing BGAs with SAC305 solder ball using SnPb(E) solder paste;
- Assembly containing BGAs with low-Ag SAC solder ball using SnPb(E) solder paste;
- Assembly containing BGAs with both SAC solder ball and SnPb solder balls using SAC305 solder paste; and
- Assembly containing BGAs with both SAC solder ball and SnPb solder balls using SnPb(E) solder paste.
Today the industry often refers to the “ball and paste” combinations as having backward and forward compatibility: Backward compatibility combines lead-free solder balls with tin-lead solder paste; forward compatibility combines lead-free solder balls with lead-free solder paste.
Considering both solder joint reliability and PCB assembly integrity, what are the best practices to ensure the ultimate reliability of a PCB assembly? What is the rationale behind such best practices?
From a reliability perspective, one unique delineating factor is whether a PCB assembly is destined for high-reliability or harsh-environment applications. Packages including BGA, CSP and WLP have been deployed and used in the industry for a long while. These architectures using solder balls in an array pattern demand new or different requirements from those of leaded components, such as QFPs or SO packages. The differences are fundamental: The solder joints of leaded components are made of metal leads and solder paste; in contrast, the solder joints of BGA-type components are made of solder ball alloy and solder paste. Therefore these two types of packages introduce different issues in production defects during manufacturing and in reliability during a product’s service life.
The conversion from tin-lead solder to lead-free solder has generated much confusion regarding the assembly of BGA-type packages within the SMT manufacturing infrastructure. Many questions still remain and the answers, coming from many diverse sources of information, are confusing—even misleading. Questions include:
- Should the reflow profile be set to specifically melt BGA solder balls?
- Can an SAC305 ball and a SnPb solder paste work well together?
- Can a SnPb ball and a SAC305 solder paste work well together?
- Can an SAC305 ball and another lead-free solder paste work well together?
- Can a low-Ag SAC ball and a SnPb solder paste work well together?
- Can a low-Ag SAC ball and a SAC305 solder paste be used together?
- Can a low-Ag SAC ball and another lead-free solder paste work well together?
- Will an SAC ball and an SAC solder paste work for harsh-environment electronics?
- Can other lead-free solder balls and other lead-free solder pastes work well together?
- Can other lead-free solder balls and other lead-free solder pastes work for harsh environment electronics?
- How does the process affect solder joint reliability and PCB integrity?
- What does it take to produce acceptable lead-free BGA assemblies? and
- What does it take to produce a reliable BGA assembly for harsh-environment electronics?
Questions 1 and 2 are the ones I see most often. This is largely due to the fact that some sectors of the industry are still exempted from the RoHS mandate and are not required to convert to lead-free solder. Sectors charged to produce mission-critical, high-reliability products cannot use SAC solders to assure solder joint reliability to target levels under harsh service environments. These sectors continue to use SnPb solder paste, while BGA components manufacturers have switched to lead-free solder balls.
I realize that, so far, I’ve only discussed questions, but, as American businessman Robert Half once said: Asking the right questions takes as much skill as giving the right answers. Future columns will answer the above questions and dive much deeper into the subject. Dr. Hwang, a pioneer and long-standing contributor to SMT manufacturing since its inception as well as to the lead-free development, has helped improve production yield and solved challenging reliability issues. Among her many awards and honors, she has been inducted into the WIT International Hall of Fame, elected to the National Academy of Engineering and named an R&D Stars to Watch. Having held senior executive positions with Lockheed Martin Corporation, Sherwin Williams Co., SCM Corporation and IEM Corporation, she is currently CEO of H-Technologies Group providing business, technology and manufacturing solutions. She is a member of the U.S. Commerce Department’s Export Council, and serves on the board of Fortune 500 NYSE companies and civic and university boards. She is the author of 300+ publications and several textbooks and an international speaker and author on trade, business, education and social issues. Contact her at (216) 577-3284; e-mail JennieHwang@aol.com.