Fluxes for Electronics

Fluxes perform the same functions with respect to the joint surfaces of printed circuit boards as they do when used for mechanical and other soldering and as noted in 7A3. Rosin fluxes have long been common for electrical uses but now are employed less often as the major flux element on printed circuit boards. The availability of other suitable no-clean fluxes, testing difficulties when rosin-based, no-clean fluxes and automatic test probe equipment are used, and workers’ allergic sensitivity to rosin, have all led to a reduction of rosin usage. No-clean fluxes have become dominant because of environmental factors and disposal costs related to both solvent and water-based cleaning effluents, the high density of SMT-type boards that are more difficult to clean, and the availability of effective no-clean fluxes. These do not have to be removed after the soldering operation. They typically are free of ionic materials though they contain organic acid activators (which are solids), a solvent (either water or isopropyl alcohol), viscosity modifiers (eg., methyl cellulose), surfactants, and other additives. No-clean fluxes are mixed with the same standard apparatus as are used for conventional fluxes. No cleaning or even rinsing of circuit boards is required when no-clean fluxes are properly specified.

Other Chip Configurations

Multiple integrated circuit packages (multichip devices, assemblies, modules [MCM], system in a package (SIP) or packages) - involve the assembly, in one protective package, of several integrated circuit chips (unpackaged) and, optionally, some other components connected on one substrate. This type of package is used when high speed of operation of the circuit is important. Normal connections from chips to other devices and board circuits exhibit capacitance effects that slow the rate of current flow. By putting several interconnected chips in one package, and on one common substrate, the connecting paths between them are made much shorter and the operations of the chips can be faster. Higher frequencies, better performance, and a more compact arrangement are also achieved. The cost, however, is higher than if the chips were mounted in individual packages on a circuit board. Connecting circuits are made using thin or thick films of metal or other conductive materials. This wiring is put on the ceramic or silicon substrate by using the additive method (described in A1c above) to produce a multilayer substrate. Resistors and capacitors may be formed and connected to the chips as well. As many as five chips may be included in the module. These are usually mounted on the substrate with an epoxy adhesive. The conductive films employed may be made from titanium, palladium, tantalum nitride, and electroplated gold. Electrical connections are made by wire bonding. The final package, containing both substrate and chips, may consist of a sealed metal, ceramic, or silicon rubber capsule, with external leads for connection to the circuit board. Fig. 13K5, view c, shows a simple arrangement of two chips.

Tinning

Leads, contacts, traces, pads, and other solder joint areas are coated - tinned - with a solder alloy prior to the soldering operation in order to facilitate final soldering, lessen the need for strong fluxes that may attack the circuit board, and provide longer storage life. Tinned surfaces have superior storage life and solderability than electroplated coatings. Tinning involves the following steps which may be manual or automatic: 3 1) surfaces to be tinned are degreased, 2) If necessary, surfaces are microetched with acid, 3) flux is applied, 4) the component is preheated, 5) the surface is dipped or otherwise brought into contact with molten solder, 6) The surface is held in contact with the molten solder until full wetting takes place.7) The workpiece is withdrawn from the solder, 8) cooling takes place, 9) flux residue is cleaned from the tinned surface and adjacent surfaces as necessary. 10) The tinned surfaces are inspected. Tinning by dipping is an economical method for precoating surfaces with solder, but the amount of solder in the coating is subject to variations.