Subject: Time: 11:48 AM OFFICE MEMO RE>Fine Pitch in Motion Date: 3/20/96 Leo <[log in to unmask]> Early last week, Leo made a post concerning fine pitch, I prepared the following response and had trouble posting it on the technet, so I sent it directly to him. I haven't seen any other response to his post on the technet postings I've received and I though the response would be of interest to some of you other technetters. I've got a feeling the following is more than what you wanted, but it will provide you with some information that will help you to meet you and your customers requirements. In Leo's post, he had two main concerns: 1) "We are going to be placing some 15 mil quad flat packs on a pcb assy to be used in an automotive like enviornment (off road vehicle type of vibration is possible) and are wondering if there are any special processing steps that we whould take to enhance field reliability." 2) "Our customer complains that previously tested engineering samples that they made had some problems with the fine pitch. We've not encountered this type of issue before. We and our customer would appreciate any input you could give us." ------- Here goes ---------------------- In order to help you and your customer it would be best to know what your/their experience has been: Do you/they know the failure modes? Was it due to shock/vibration, thermal, component lead or solder joint failure. If component lead failure, was it at the heel bend or the knee bend? If the solder joint, how were the leads bent/formed and the configuration of the solder fillet. The following are some of the "gereral" design and assembly requirements for mounting "flat-pack", SO and similarily leaded components for "Serious, Class 3 High-Rel" applications". "Automotive" is a severe operating environment and therefore, you may want to consider "optimizing" the traditional IPC Class 3 design, manufacturing and acceptance requirements to go for additional reliability. I know that the following are not exactly easily achieved with fine-pitch components, in particular with many (most) of the commercially available components, but when the product definition requirements for quality includes reliability in adverse operating environments -- all processes may not simple to achieve the objective. Component Leads - inspect lead bends for lead and material cracks / fractures and lead forming impressions. Use "low stress" and modulus leads such as copper -vs- Alloy 42 (nickel/iron) Component Lead Angle - the optimum component lead angle for surface mounted components is 60 degrees, the acceptable angle is in the range is 55-65 degrees for Class 3 Serious Hi-Rel; but the angle can be opened up to 45-65 degrees for existing designs where the component body has been reduced in width. Reason - the 55-65 degree lead angle optimizes the strain relief for x-, y-, and z-axis displacements. DO NOT USE ANY COMPONENT LEADS WITH THE COMPONENT LEAD ANGLE GREATER THAN 65 DEGREES as the component lead (the leg portion) will not compensate for strains in the component lead (leg). The key to reliability is the heel fillet, and it is difficult to ensure a suitable heel fillet for component lead leg angles greater than 65-70 degrees. At greater than 65-70 degrees, and a minimum height heel solder fillet, the heel/land solder fillet is very subject to "peel" failure due to reverse stress thermal and power cycling. Land Pattern - the land pattern should be slightly wider than the max. component lead width. The minimum land length should be about 3X the width of the lead. The target land length condition should be about 6X the width of the lead. This allows for a slight toe fillet, a foot of up to 5X the lead width, and the length of the land behind the heel of the component lead should be ~0.3 the height of the leg. Solder Fillet - The land pattern needs to be designed to accomodate the toe, side and heel fillets. The heel fillet is the most critical part of the solder joint and is responsible for about 70-80 percent of the "strength" of the soldered connection. The heel of the lead shall be located on the land (no off-set or overhang). The continuous side fillets extending from the heel fillet and along both sides of the lead for about 2X the width of the lead are the next most critical fillets. The toe fillet is the least critical for the longer foot lengths, For shorter foot lengths become more critical as the foot approaches 2X the lead width. Foot lengths less than 2X the lead width should not be used for "Serious" Class 3 High-Rel. If you are using solder paste, care must be exercised to ensure liquidous soldering temperature is maintained long enough to ensure there are no entrapped voids in the solder fillet. The optimum height of the heel fillet is ~0.3 (one-third) the height of the leg of the component lead in order to provide the necessary stress relief due for shock/vibration/thermal stress. The optimum angle of the heel fillet between the lead and the end of the land is ~45 degrees, in other words, the length of the land behind the heel should be ~0.3 of the lead height inorder to optimize the formation of the heel fillet. The minimum height of the solder fillet (in contact with the leg should not be less then the height of the top of the foot, nor should the height of the solder fillet be greater than 0.5 the height of the component leg. For Serious Class 3 High-Rel, the stress relief bends shall not be filled with solder because they just concentrate the stress some where else. Component Mounting - the component body should have minimum standoff height of the surface of the printed board. Staking adhesives should be used with caution, they should be low expansion and should be minimal thickness, less than about 130 micrometers [5 mils (inch)]. The goal is not to have the expansion of the staking material "pumping" the component in the z-axis direction during thermal excersions (shock / cycling) and power cycling. Component Lead Coplanarity - it's very critical, don't "force" leads into coplanarity using a thermode or some other method of forcing the component leads into the solder joint. Polymeric Coatings - Solder resist and conformal coating should be used with extreme care, they can be reliability killers if they are under and "pump" the component in the z-axis during thermal excersions and power cycling. This information should give you some general help for you to improve you product. It will also give your processing/assembly personnel a challenge to develop a viable productive process to meet you production/cost requirements. But then, it all depends on how serious you (or your customer) are really serious about achieving "Serious" Class 3 Hi-Rel product definition requirements. If you need any further information, you can be contacted as follows: Ralph Hersey Lawrence Livermore National Laboratory Phn.: 510.422.7430 FAX: 510.424.6886 e-mail: [log in to unmask]