Reflow Profile For Backward Compatibility

With tin lead eutectic composition one could easily get away by maintaining peak temperature between 190C to 225C, a variation of almost 35C and still achieve good reflow soldering results. This reflow process window narrows to 15C since the soldering needs to be done between 230C to 245C. With this tighter process window the importance of using product specific profile is becomes even more critical.

For developing the profile, you need the loaded board with six to eight thermocouples soldered to solder joints of large and small components across the board. In case of BGAs, thermocouples need to be attached to the inner and outer rows of the balls as well.

Commercial hardware and software packages, such as MOLE, data pack and many others are available to make thermal profile development an easy task. However, this easy task is rarely performed by many companies. For example, many companies do not even bother with using loaded board attached with thermocouples. They tend to guess the thermal mass of the board and use guess the oven settings that may work.

Such a guess work would not be so bad if they went back and checked the profiles using some failed boards because there are plenty of failed boards in such companies. Even when the failures are found, they will keep on building boards using the same profiles and blame the failures on their suppliers or the designers instead.

The problem can be further compounded by backward compatibility issues where some lead free components have to be used on a primarily tin-lead board. In such as case the profile must accommodate both tin-lead and lead free package requirements.

In a backward compatibility scenario, it is not a serious issue when using leaded components such as SOIC, PLCC or fine pitch with lead free surface finishes on a primarily tin-lead board. The real problem arises when using lead free BGAs on a primarily tin-lead board. If the tin-lead profile with maximum peak temperature of 220C is used, the BGA balls will not reflow at all or will partially reflow, creating a serious solder joint reliability. Also keep in mind that in backward compatibility you are using tin-lead solder paste since most of the components on the board are tin-lead. Such a solder paste may not withstand higher peak temperatures and may cause dewetted and open solder joints if high peak temperatures are used. And on top of that these tin-lead components may not withstand higher peak temperatures.

So a common practice is to use a peak temperature of around 230C with about 45 to 60 seconds time above liquidus or TAL of 217C to allow necessary reflow and desired microstructure of lead free BGAs without damaging all the tin-lead components on the same board. Selecting a correct reflow profile for backward compatibility is a delicate balance of peak and TAL and the importance of product and flux specific unique profile for each product cannot be overemphasized since it has big impact on product yield and reliability. In addition, not following the simple procedure such as handling of moisture sensitive components and PCBs also have big impacts on product reliability and yield.

"Selecting a correct reflow profile for backward compatibility is a delicate balance of peak and TAL"

APPEARANCES:
Ray Prasad will be teaching in-depth SMT-BGA-BTC Design and Manufacturing and Lead-free Implementation courses in Portland, Oregon on Oct 11-13, 2010. He also teaches on-site classes for companies. For details, log on to www.rayprasad.com

Ray Prasad is an SMT Editorial Advisory Board member and author of the textbook, Surface Mount Technology: Principles and Practice, and founder of the Ray Prasad Consultancy Group. Contact him at 12945 SW Glen Oak Place Beaverton, OR 97007; (503) 628-1199; Fax: (503) 628-3399; E-mail: smtsolver@rayprasad.com; Website: www.rayprasad.com.

Baking and Bagging of PCBs for Lead Free Assemblies

In my audits of various OEMs and EMS companies I generally find that very few companies are following proper handling of moisture sensitive components. Moisture sensitivity concern has been with us since the early days of high volume SMT manufacturing in mid-80s. I chaired the first IPC document on handling of moisture sensitive packages IPC -786. Now that document has been replaced by J-STD-020 and J-STD-033. They deal with handling of moisture sensitive packages including guidelines for baking and bagging in desiccant bags to prevent delamination and cracks in component packages during reflow. All the users have to do is monitor the floor exposure times on the factory floor. This sounds a very simple procedure but it is rarely followed. In one extreme case I have seen one company bake all moisture sensitive components right after receiving them even though rebaking is allowed once only after allowed exposure time has expired.

As if dealing with moisture sensitive components was not enough, now PCBs are also being treated as moisture sensitive since some users have discovered delamination in PCBs during lead free reflow and lead free rework. I have no issue with baking PCBs by users once as is the case with moisture sensitive components but the initial baking and bagging should be done by the PCB suppliers since the root cause of delamination in PCBs during reflow and rework is poor controls on curing and lamination process by the PCB suppliers. Presence of moisture and higher peak reflow temperatures simply compound the problem.

The reason for only one baking of either PCB or components should be done is that baking impacts their solderability. And solderability becomes even more critical since most users are using no clean flux these days. No clean flux is a misnomer since it requires clean and solderable components and PCBs in order to achieve good yield. IPC-1601 is a good place to start for details on baking and bagging of PCBs as are J-STD-020 and JTD-033 for components.

If your PCB supplier is not willing to bake and bag PCBs, it may be wise for you to take a conservative approach and bake the PCBs yourself if you are experiencing PCB delamination. You can either bake only the boards that you need for the day's production or bake all the PCBs in which case you will have to bag them in desiccant bags and create your own guidelines for exposure time. This will require implementation of handling procedures similar to moisture sensitive components. The focus should be on monitoring the exposure time after the bag seal is broken so that you do not have to rebake them again. Rebaking of the PCBs should be avoided since it not only impacts solderability of the surface finish but may even require you to return them to the PCB supplier for reconditioning if they have OSP surface finish- expensive and time consuming steps.

"If your PCB supplier is not willing to bake and bag PCBs, it may be wise for you to take a conservative approach and bake the PCBs yourself if you are experiencing PCB delamination"

APPEARANCES:
Ray Prasad will be teaching in-depth SMT-BGA-BTC Design and Manufacturing and Lead-free Implementation courses in Portland, Oregon on Oct. 11-13, 2010. He also teaches on-site classes for companies. For details, log on to www.rayprasad.com