PCB SOURCING


regardless ofwhich AVL component is selected in the BOM.


The precision of the fit between the


component lead contact-area (the portion of the surfacemount lead that makes contactwith the corresponding pad on the PCB) is crucial to the quality and long termreliability of the product. The geometric relation between the pin contact-area of the component in the BOM/AVL and the pad used in the PCB design is a critical factor in the ultimate solder joint reliability. A solder joint formedwith too little volume of solder in the heel or the toe (depending on the package)will be less reliable, and put thewhole assembly at risk for poor yield or field failures due toweak solder joints than break the circuit due to stress fatigue (Figures 3 and 4).


Engineering or component specialists should


validate the correct package to pad selection for every design. This can be done as soon as the PCB design has completed the component placement phase of the CAD flow. In parallel with the CAD flow, smart software can simulate actual SMT placement by merging the CAD layout data as is with the BOMand AVL data already chosen. The actual component shapes and pin contact areas fromthemanufacture’s part number in the AVL are layered automatically over the CAD design onto the intended pads. Any deviation from set numerical rules for pin-to-pad analysis are instantly found and highlighted.


At this point, purchasing can participate


in the trade-offs between correcting the AVL to parts thatmeet the design requirements or engineering changing the pads in the designed foot print. Eitherway ismajor cost avoidance because a form-fit mismatchwill not be sent to the production floor. Figure 4 shows a typical example of a slightmismatch thatwill createmassive problems if not corrected before production. Many times part height can also be a critical issue for process considerations for reparability, testability or tolerance conflicts in box build assemblies, and thereforemust be included in building up the perfect AVL.


Figure 5: A foolproofmethod is needed to automate the capture of incomingmaterial bar code data and register this information into thematerials database for every reel, and to link this data to a unique barcode ID number placed on every reel and tray


statusmust be linked to every unique reel ID number so that as these reels are issued to the floor, themanufacturing execution systemcan monitor theMSD exposure times automatically, and restrict the use of non-RoHS compliant material fromuse on RoHS lines.


As these reels and trays are added into the


Figure 3: Correctlymatching the pin dimensions of the component package to the surfacemount pads designed by engineering is critical to building a quality product. For a gull-wing device as shown above theremust be enough heel distance to create a strong joint


Figure 4: A slight dimensionalmismatch between the component leads fromparts approved in the AVL and the designed footprint on the PCB can create huge problems and expense formanufacturing and long term product reliability


component database, it nowbecomes possible to track and control the use and consumption of each individual reel and part number, issued from stock, placed into service against a work order, issued to amachine on a production line, held in dry storage on the floor or in the warehouse, or related to test and inspection failures on the PCB assembly. There is technology today to automatically scan and decodemultiple barcodes on a reel or tray, and register all data to the materials database (Fig 5). There will be a lotmore on this subject in another article in this series, but providing the professional buyer visibility to real-time operational inventory data is crucial not just for improving inventory levels, but to alsomonitor and improve the AVL


as issues or ECNs related tomaterial performance arise on the shop floor.


There is an unfortunate connection


betweenMSD devices and RoHS compliance that createsmany challenges for the process engineer. RoHS compliance requires the use of lead free solder paste,which reflows at a higher temperature the standard eutectic tin-lead solder alloy. The higher reflow temperaturesmean that tighter control on MSDs is needed. This is due to the fact that damage to anMSD that has expired its allowed open time in the ambient air, has a


higher risk of damage in during the higher temperatures of RoHS processing. The higher temperatures elevate the violence of escaping moisturewhich can damage the component. It is also a foregone conclusion that RoHS compliancewill only becomemore strict over time, as fewer and fewer exclusionswill be allowed.


MSD and RoHS status Other criteria are equally importantwhen building up the perfect AVL. These are themoisture sensitivity level of the device and the compliancewith RoHS directive for heavymetal content (i.e. “lead free”). Each of these crucial factorsmust be captured as an attribute in the component database. MSD level is crucial because certain packagematerials and thickness absorb ambientmoisturemore quickly than others. If the process is running along based onworse case ofMSD level 3 (168 hours allowed as open time in standard ambient conditions before re-bake is required), and suddenly the materials teamissues an alternate component to the floor that is MSD level 5A (24 hours open time allowed), the impact to the production flowcould be catastrophic.


BothMSD and RoHS statusmust be captured for every reel and tray as they enter thewarehouse through incoming inspection. This


Summary Professional buyers today are looking formoreway to add value to the success of the factory. Oneway to do this is to improve the quality of the AVL and sowork in concertwith newproduct designs as early in the design phase a possible. When the AVL can be validated early and completely, including the troublesome physical attributes, form-fitmismatchedwill be avoided on the shop floor saving both time andmoney. The buyer can also helpmove the factory forwardwith improved compliancewithMSD and RoHS regulations by including these factors in the perfect AVL.


Bruce A. Isbell ismarket developmentmanager for the Valor Division ofWilsonville,Ore.-based EDA tool vendorMentor Graphics Corp.


www.mentor.com September 2010 | 49


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