This issue of The Board Authority is a first. It combines two distinct, yet closely related disciplines: Assembly and Solderability. They go together like wine and cheese, Laurel and Hardy—you get the picture. The solderability preservation of the bare board is generally the responsibility of the PWB fabricator. However, quality of the assembled PWB is generally measured by the strength and long-term reliability of the solder joint. Of course, there are a number of factors and variables that affect the solder joint. The surface finish quality is only one such factor. The other factors and variables are introduced in the assembly process itself. Wave solder parameters, the IR reflow profile, type of flux or solder paste used, temperature of assembly, flux application techniques, etc., can all influence the quality of the solder joint. That is why we were so determined to bring the readers a combined issue that would not only serve as a reference, but would also provide a better understanding of issues that both the fabricator and assembler face. We hoped to accomplish this through the support of CircuiTree and the expertise of industry leaders who agreed to commit this expertise to writing.
George Milad of Shipley Company and Gerard O'Brien of Photocircuits teamed up to provide a well-written overview on surface finishes and solderability. This article serves as the cornerstone for this issue. One must first understand that solder joint reliability is very dependent on the solderability of the PWB. The fabricator and end-user have numerous choices to accomplish this task with respect to final surface finish. George and Gerard provide some much needed insight on this very critical subject.
Immersion silver has gained momentum in the last two years as a replacement finish for hot air leveling. Don Cullen of MacDermid frames the argument that it is acceptable to provide PWBs with silver. His extensive article details the solderability testing that has made silver a household name among some major OEMs and EMS providers.
We are very pleased to provide the readers with key information relating to assembly equipment. Did you ever wonder how the EMS industry evolved from a basic labor-intensive hand-soldering operation to the efficient supply chain manager it has become today? Read Mark Apell and Keith Howell's article on the "Evolution of Mass Soldering" to find out. Speaking of evolution, the technology shift to fine pitch surface mount designs has necessitated a major change in SMT equipment. Nigel Palmer updates the reader on these equipment requirements, including the latest in pick and place, chip shooter, reflow furnaces, and solder paste printers.
What does a major EMS provider require from a board finish? Shelf life? Joint reliability? Low cost? Wire bondable? Ability to maintain solderability through multiple soldering cycles? Press fit compatible? There are many requirements that must be met. Industry veteran Bruce Houghton of Celestica presents an EMS provider's view of the surface finish debate. Read Bruce's article to find out what can be encountered when assembling PWBs in mass quantities, and with a variety of surface finishes. To complicate matters, as Bruce will mention, all chemistries for a particular finish do not always yield the same results from a solderability standpoint.
No discussion on assembly and surface finishes would be complete without articles on testing and reliability. Duncan Gray provides a protocol and guidelines for using In-Circuit Test (ICT), Functional Test, and finally System Test. He stresses the need to balance these various tests, based on system complexity. Combining X-ray and in-circuit test to detect, diagnose, and repair manufacturing faults in assemblies is the subject of Joe Kirschling's extensive work. He proposes that this combined test strategy will better isolate defects on complex printed wiring boards and assemblies.
With the lead-free soldering debate raging on, there remain the concerns over long term reliability of not only the lead-free solder joint, but the PWB as well. These lead-free solders will require significantly higher temperatures to bring the lead-free pastes and solders above liquidus. What effect will this have on the vias? A noted expert on reliability, Joe Smetana, investigates what effect higher soldering temperatures has on through hole reliability. Smetana looks at board designs as a way of mitigating the effect of higher temperature soldering. Reading this comprehensive dissertation is a must for anyone concerned about end product reliability. Raby and Heller approach the subject of lead-free soldering by evaluating tin silver bismuth and tin silver copper lead-free solder pastes. The joints were X-rayed and sectioned to determine the presence or absence of voids in the fillet. This makes for interesting reading.
Dan Feinberg is an industry leader who is always concerned about the competitiveness of the PWB fabrication industry. Dan was asked by the co-editors to take on the "Green" controversy by interviewing the individuals who make their living in this industry. Dan asked the questions on environmental regulations and lead-free, and what that will do to our industry. Dan will provide his own views as well. You will be surprised to read what he uncovered.
We hope you enjoy reading this issue of The Board Authority as much as we had putting it together. We certainly wish to thank all the authors for the hard work and professionalism. And a big thank you to the staff of CircuiTree for getting this work published and in your hands.
MIKE CARANO |
Assembly and Solderability Issue