High Performance Printed Circuit Boards - Lecture #4
High Performance Printed Circuit Boards By Joseph Y. Lee Samsung Electro-Mechanics
Chapter - 7Soldering and Cleaning of High Performance Circuit Board Assemblies
Introduction to Requirements Industry standardization Low cost method to bump die Cost-competitive substrates High yield assembly process
Two basic types of low melt flip- chip interconnections 63/37 Low melt 97/3 High meltchip chiplaminate laminate Conventional Pb-rich Eutectic Sn-Pb
Problems Reliability is problem for bigger dies. Failure modes are either delamination of encapsulant, most often at the die passivation interface, or chip cracking. Or combination of both delamination or chip cracking.
Definition of a solder A solder alloy consists of two or more elements that can wet to a surface, most often copper, and then react to form an adhesion layer, and, upon solidification of the alloy, produces an interconnection that has good mechanical properties.
Phase diagram on binary solder alloys L e u a ep m T eEutectictemperature, Te, Proeutectic phasethe alloy is a Te r t rhomogeneous a βliquid consisting ofA and B atoms. A XA Xe XB B Composition
No-lead solder alloys Sn-Ag-Bi Sn-Ag-In Sn-Ag-Bi-In Sn0.965Ag0.035 – melts at 221°CEnvironmental concerns to eliminate Pb.Cost too high though.No government pressure.Pb use is not high compared to industrial paint and batteries
Flux Flux; when it is applied to surfaces that are to be joined by soldering; flux cleans the surfaces and results in a better bond Helps transfer heat. Prevents oxidation. Oxidation prevents any flow of molten solder and prevents wetting.
Description of solder pastes It is a homogeneous mixtures of solder powder, flux, and a vehicle. Must be able to withstand prolonged open exposure to temperature and humidity without undergoing changes that degrade its performance. Must be able to withstand long delays or pauses in the process and resume printing with the print quality being equal to that before the pause.
Types of solder pastes No clean chemistries – consist of either rosin- or resin-based flux system with a suitable vehicle system. Water soluble fluxes – contain organic or inorganic acids in the flux.
Key factors for solder paste performance Equipment and setup parameters Stencil fabrication method Pitch of components Lead density of components Operator skills Component and card solderability Temperature and humidity
Characterize solder pastes Solder ball testing – solder paste on ceramic substrate Residue – amount of clarity of the remaining flux after reflow Solder paste wetting – hot air soldered leaded (HASL) surface is more difficult to differentiate Slump – ability of paste to stay well defined after screening Tack – certain amount of adhesive strength to hold the component in place prior to reflow operation Worklife – how long the paste can remain on the stencil before it can no longer print adequately
Conductive adhesives – 2 types Anisotropic – conductive in only one direction Isotropic – conducting in all 3 directions Materials development of the conductive adhesives Assembly process development Development of equipment set to form reliable flip-chip attach interconnections.
Post assembly cleaning Incoming hardware must be clean and highly solderable. Flux residue and other contaminations must be removed. Post assembly cleaning is required. Post assembly cleaning eliminates a lot of unknowns in the soldering process.
Environmental survey Low water usage and sewer costs have high adjusted production-based flow rates. Common pollutants: Cu, Pb, Ni, Ag and total toxic organics (TTO). Two basic wastewater treatment configurations: conventional metals precipitation and ion exchange systems. ½ of survey have a formal pollution prevention. Water is wasted by unnecessary flow rates in their rinse tanks. ¾ of survey implemented recycling, recovery, and bath maintenance. Off-site recycling is commonly used.
Problems to POT (publicly owned treatment) Pollutants that create fire or explosive hazard. That cause corrosive structure damage. That will obstruct the flow in the POTW. Biological oxygen demand, released at a flow rate or concentration. Materials hot enough to cause POTW to exceed 104°F.
Pollutions Limitations in USGeographical Federal Federal Federal State AQMDsArea Destination “Serious” “Severe” “Extreme” CaliforniaVOC Emissionsto be major 50 tons/year 25 tons/year 10 tons/year 1 lb/day VOC – Volatile Organic Compounds AQMD’s – Air Quality Management District
Pollution Prevention It is defined as the prevention of the generation of pollutants by minimizing or eliminating the steps or materials that produce them. Circuit boards require a lot of water. Reduction of water use is one goal for pollution prevention. 5lb (12kg) laptop represent about 40,000lb of materials, resources, and wastes. Few PWB manufacturers use ozone depleting substances in their products and process.
Things to Consider You do not need so much water. Determine the sources of waste. Ask employees to provide pollution prevention solutions. Environmental management system – requires documentation. Know costs of waste and pollution