Electronic Assembly Papers

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Bottom Termination Component Design Considerations to Improve Cleaning
by Mike Bixenman, KYZEN Corp., Dale Lee, Plexus Corp, Bill Vuono, Raytheon, and Steve Stach Austin American Technology – Published February 2014
Abstract: Bottom Termination Components (BTC’s) are the fastest growing package types in the electronics industry. BTCs acceptance in long-life devices exposed to harsh environments is currently limited due to reliability concerns. The large area heat sink associated with the BTC design, floods the perimeter of BTC in the soldering process with flux residue making the BTC one of today’s toughest cleaning challenges. Un-cleaned   flux residues trapped under the BTC have the potential to be hydroscopic and conductive. Concentrated ionic levels lower dielectric strength and can result in dendrite growth. From a cleaning perspective, many designers have poor insight into factors that assure a cleanable design. Variables such as solder paste, reflow conditions, component placement, component clearance (standoff), cleaning agent and cleaning equipment are important factors. Collaboration between process engineers, assembly designers, solder materials, cleaning agent and cleaning equipment experts can improve integration of the BTC design and assembly. Circuit board design plays an important role when cleaning is required. The purpose of this research is to study design considerations of BTC components in an effort to reduce flux residue under the bottom termination and improve cleaning effects.

Understencil Wipe Cleaning Yield Improvements
by Mike Bixenman, KYZEN Corp., Chrys Shea, Shea Engineering & Brook Sandy, Indium Corp. – Published September 2013
Abstract: Understencil wiping has gained increased interest over the last several years. Changes in circuit design due to miniaturized components and highly dense interconnects have increased the importance of stencils being free of solder paste deposits in the wall of the aperture. In most stencil printing processes, dry wiping has been followed by vacuum assist in an effort to clean solder paste from aperture walls. As stencil apertures reduce in size, more frequent wiping is needed to assure that stencils are free of solder paste deposits. To improve solder paste release, two technology approaches are being studied with higher levels of frequency. The first technology is a nano-scale hydrophobic, oleophobic and adhesion promoting coating.1 The objective is to treat the metal stencil surface with a nano-coating to prevent solder paste from sticking to aperture walls. The second technology is to wet the understencil wipe with a solvent based cleaning agent. The cleaning agent dissolves the flux component within the solder paste to improve release of solder balls from aperture walls.

Rinsing Study to Determine Process Factors for Removing Cleaning Agent from Bottom Terminations
by David Lober, Mike Bixenman, Ram Wissel & Jason Chan, KYZEN Corp. – Published April 2014
Abstract: Rinsing is typically the final stage in an electronics assembly cleaning process. Although it is an integral step, individuals often fail to give rinsing the attention it warrants. If this step is not successfully executed, both the cleaning agent and the residues it has removed will dry on the printed circuit and become redeposited, resulting in a contamination level that may be higher than that found prior to cleaning. Although failing to rinse satisfactorily is problematic, so too is excessive rinsing, as over rinsing proves costly due to the increase in water consumption, wasted time, increased energy consumption, and, in regions where rinse water is not allowed to go to drain, increased disposal costs. Thus, optimizing the rinsing stage of the cleaning process is of primary importance. One major cleaning challenge is successfully cleaning and rinsing under bottom terminated components. Because of the demands placed on modern printed circuits, including increased component density and circuit complexity, cleaning proves especially critical to ensure proper reliability. This study examines the interplay between cleaning agent and number of rinse cycles on the resistivity of the rinse water, the ionic contamination on the board, and the relative amount of organic residue present in the rinse water and entrapped under bottom terminated components. By understanding the nature of these interactions, the ability of different cleaning agents to be rinsed and the optimum rinsing protocol for aqueous cleaning agents can be determined.

Preventing White Residue by Controlling Upstream and Downstream Process Conditions
by Mike Bixenman, KYZEN Corp. – Published April 2012
Abstract: White residue formation after soldering is one of the most complex problems in cleaning electronic assemblies. The visual appearance of flux residue has much to do with the flux composition and solder profile. White residue is affected by a wide range of electronic assembly materials including all flux types, heat exposure during soldering and before cleaning, component spacing and pitch, solder masks, cleaning agent, wash processing conditions and mechanical cleaning energy. The ideal state, specifically for no-clean processes, is a clear translucent solid, with active ingredients fully reacted and benign. White residue left after the cleaning process is commonly the result of extracting and removing only some soluble ingredients of the residue while leaving behind an insoluble white crystal.
The most effective method for preventing white residue is controlling the upstream soldering process, post soldering heat exposure before cleaning and optimizing cleaning process variables. Over-heating the assembly during and after the soldering process can burn, char, cross-link and eventually polymerize flux residues. Changing the residue properties can lead to partial cleaning. The second key factor that contributes to white residue is the wash process. The critical variables for controlling the process are the cleaning agent, wash time, wash temperature, wash concentration, and impingement energy. The purpose of this research paper is to provide participants with an improved understanding of how controlling the assembly process and wash parameters reduce the occurrence of white residue.

Optimizing Batch Cleaning Process Parameters for Removing Lead-Free Flux Residues on Populated Circuit Assemblies
By Steve Stach, AAT & Mike Bixenman, KYZEN Corp. – Published August 2008
Abstract: Electronic assembly cleaning processes are becoming increasingly more complex because of global environmental mandates and customer driven product performance requirements. Manufacturing strategies today require process equivalence. That is to say, if a product is made or modified in different locations or processes around the world, the result should be the same. If cleaning is a requirement, will existing electronic assembly cleaning processes meet the challenge? Innovative cleaning fluid and cleaning equipment designs provide improved functionality in both batch and continuous inline cleaning processes. The purpose of this designed experiment is to report optimized cleaning process parameters for removing lead-free flux residues on populated circuit assemblies using innovative cleaning fluid and batch cleaning equipment designs.

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