At all times during the handling and processing of QFPs, special care must be taken to make sure that all the leads are co-planar. QFP leads are small and very fragile and the smaller the lead pitch the more fragile the lead. Any mishandling during prior processing renders the leads prone to lack of co-planarity.

It is most unlikely that the act of reflow soldering wil cause a lead to "lift". There is a much greater probability that a lead has become bent upduring one of the many handling functions to which the component is subjected , i.e.the cropping, loading, unloading, transport, picking and placing etc.,of the components.

Suspect components should be inspected prior to reflow to check for co-planarity or lack thereof. However, with less than 1% fall out it may be an arduous task to identify defective components.

Harold Hyman Consultant, VJ Electronix Harold Hyman has been involved in metallurgical aspects of the electronics industry since the 1950's, and in semiconductor development and engineering for STL, Ediswan & RCA. He later joined HTC, a pioneer of vapor phase soldering and continued industry experience Dynapert, GenRad, Teradyne, SRT and VJ Electronics.

The lifted leads may very well be a lack of perfect planarity of the leads, in that all the leads are not in the same horizontal plane. Each component actually has a co-planarity specification, which is typically around 100 microns, but could be larger depending on the type of QFP package.

 It is also possible to experience damage to the pins, especially the corner pins, since they are the most likely to be damaged during handling. The ratio of the solder paste height and the lead coplanarity could be problematic, in that if the solder paste height is modest, there may not be enough solder volume to "catch" the worst non compliant parts.

A slight overprint of the solder paste could provide the small additional solder volume. As the paste reflows, it pulls onto the exposed pad, and since the pad controls the x-y coordinates, the solder will rise in the z direction, and catch the non-compliant pin.

Another option is to place a small solder preform with a chip shooter out of tape and reel packaging into the solder paste if room for overprinting on the card is not available.

Paul J. Koep Global Product Manager, Alpha - Cookson Electronics Mr. Koep is responsible for product planning and technical marketing for the Preform Products at Alpha, part of Cookson Electronics. He is the co-author of several patents in the areas of soldering applications focusing on reflow and alternative methods.

I have no data on fall out rates so I can't comment however I have seen what sounds like the same problem before, in these cases it was due to a coplanarity issue with the problem leads.

To be sure I suggest a cross section on the affected lead and adjacent leads to look at the stand off of the lead from the pad.

What may help is a solder paste with an extended activity life so that when the component is pulled down on to the solder when it wets the other leads there is still sufficient activity to clean the problem lead and allow the molten solder to wet the lead.

There are a few things to review, the flatness of the pads in relationship to each other, the co-planarity of leads of each component and the amount of solder paste deposited on the land areas.

Boards which have been hot air leveled (HASL) will have a rough topography on each pad and if the component is place on one of the high spots it may allow the lead enough height to miss being soldered and exhibit itself as a lifted lead problem.

Using immersion plating such as an OSP coating, or an ENIG plating will make the pads much flatter in relationship to each other and will help in reducing and eliminating this problems.

As for the components, an audit may be in order to determine if the components are within the acceptable tolerances of the component co-planarity requirements.

As for the paste deposition process, check the amount or height of paste deposited and the size of the aperture on the stencil.

Leo Lambert Vice President, Technical Director, EPTAC Corporation At EPTAC Corporation, Mr. Lambert oversees content of course offerings, IPC Certification programs and provides customers with expert consultation in electronics manufacturing, including RoHS/WEEE and lead free issues. Leo is also the IPC General Chairman for the Assembly/Joining Process Committee.

Are the leads lifted or has the solder wicked up the lead leaving the pad/lead interface dry?

If this is the case it is likely the differential heating of the leads - the lead elbow/shoulder heating faster than the foot. This can be traced back to reflow profiling and solder deposition issues.

Allen W. Duck CEO, ATEK llc Allen Duck is a 20-year Electronics Industry veteran with Global experience in multiple fields of technology and management. He started A-Tek in 2006 to provide a sales and service channel for international equipment companies wishing to offer value based solutions to USA companies.

This is likely a co-planarity problem with the parts. Some parts have lead displacements before you place them. Many fine pitch placers have a co-planartiy option.

The machine will inspect the leads on each part prior to placement. Several of our Juki systems have this option.

Bob Black President and CEO, Juki Corporation Mr. Black was the President and Co-Founder of Zevatech in 1977 and introduced first Pick and Place System at Nepcon West 1980. Bob is now the President, CEO and Co-Founder of Juki Automation Systems. He is also a Co-Founder of the SMEMA Council of IPC. He serves as a member of SMTA and SEMI..