Circuits Assembly - January 2008 - (Page 41)

A Fresh Look at Cleanliness What’s past isn’t prologue: A better understanding of contamination is needed. Process Doctor T he definition of board cleanliness is chang• Chemical changes in the fluxes (no high-solids ing. It is not just the general visual or norrosin to seal in fabrication residues, but new, less malized number for the whole board we insulative activators are used). Not cleaning after solhave understood it to be. Historically, it has been dering but using low-solids fluxes (with weak organic defined by total board cleanliness requirements of acids), that are left on the assembly surface, which in the military and IPC specifications of the 1970s. turn leaves a different chemical residue that does not Board cleanliness has not evolved as assembly, directly correlate to the historical response of fluxes. interconnect, flux and component packaging have • Integrated assembly techniques that were not just evolved. The difference between the historical reflow or wave solder, but 2x reflow plus selective definition of cleanliness and what is needed today pallet wave soldering plus hand soldering plus localis a direct understanding of what localized conized brush cleaning, permitting pockets of residue in tamination is around critical sensitive circuits. precise and critical areas due to secondary processing. Historically, board cleanliness was: • The direct impact of bare board, component and • The visible appearance of surface residues secondary processing residues on circuit perfor(defined by IPC-A-610); shiny solder joints mance. always looked better. Localized cleanliness. To understand an assembly’s • The general ionic measurement of contami- Figure 1. Selective cleanliness requires a look at the effect of each pronation from the entire board surface. Testing pallet soldering on cess and nearby process, especially in critical circuit was conducted in an automated 1-5 gal. con- the bottom side areas. The cleanliness of individual components, tainer of IPA and DI water recirculation over near these vias left and spacing between leads on QFP, DIPS, QFN and the board surface for 5-15 min. (normalized no-clean flux resi- connector leads, are areas for residues to impact based on entire surface area of the board’s due that passed performance. No-clean manufacturing techniques components), using a resistivity probe in solu- ROSE testing but using selective wave techniques, dirty components, caused this short tion and then creating a value of NaCl equivadirty bare boards, high levels of contamination on on a critical circuit. lence using a salt solution to establish a scale. innerlayers, hand solder residues, residues nearby Measuring the resistivity of this alcohol/water and areas around brush cleaning all rely on the solution has provided a general understanding of the understanding that these residues are benign. total board cleanliness and works as a process monitor Board cleanliness must represent the amount of residues for gross changes in the process. detected in critical areas that correlates to electrical perforWhat has changed that limits the effectiveness of this mance in the field and environmental stress screening. This type of cleanliness testing? The primary changes can be approach as a process control technique must permit the answered by looking at four key areas: assembler the opportunity to have failures (dirty boards) opti• Increased circuit sensitivity with tighter spacing (small mize the process, and then maintain this cleanliness level and amount of contamination can shift the circuit output). not have field failures from qualified processes. ■ Terry Munson is with Foresite Inc. (residues.com); tm_foresite@ residues.com. His column appears monthly. We'll Be There, Will You? Wingate by Wyndham & Conference Center Round Rock, Texas March 4-6, 2008 December 2008 Bringing you the best of PCB East and West www.pcbshows.com circuitsassembly.com Circuits Assembly JANUARY 2008 41 http://residues.com http://www.pcbshows.com http://circuitsassembly.com

Table of Contents Feed for the Digital Edition of Circuits Assembly - January 2008

Circuits Assembly - January 2008 Contents Caveat Lector Industry News Market Watch Talking Heads Focus on Business On the Forefront Screen Printing Better Manufacturing Walking on Water Flux Selection for Lead-Free Wave Soldering An A-to-Z Guide to X-Ray Inspection, Part II Evolution in Action Research Priorities for the Electronics Industry Selective Soldering Process Doctor Pb-Free Lessons Learned Materials World Product Spotlight Ad Index Assembly Insider Technical Abstracts

Circuits Assembly - January 2008

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