Soldering Component leads

For standard components, an accuracy of 100 /xm is sufficient. Complex components must be placed with an accuracy better than 50 /xm. For special applications, high-precision machines are used with an accuracy of 10 /xm related to a normal distribution and at a standard deviation of 4o-. This means, for example, that only 60 of 1 million components will be outside a range of 10 /rm. Due to the necessity for using a small amount of solder paste with fine-pitch components (down to 200 fxm pitch), minimal vertical bending of the component leads (about 70 /am) causes faulty solder points and requires cost-intensive manual repairs. 

Surface-mount technology replaces previous methods of inserting component leads into plated through-holes of PWBs, wave soldering from the back side to flow solder into the holes, and simultaneously forming both mechanical and electrical connections. SMT is highly automated and currently the most widely used production process for the assembly of single-layer, double-sided, and multilayer circuit boards. 

The recommended lead-free solder formulation is Sn-Ag-Cu for board assembly but there are other formulations such as Nickel-Palladium (NiPd), or Nickel-Palladium with Gold flash (NiPdAu). Passive components, to be compatible with a lower temperature Lead process (which is 215°C for 50/50 Tin/Lead formulations and 230°C for 40/60 formulations) and the higher lead-free process of up to 260°C, use pure matte Tin for their contacts. The use of lead in solder is partially based on several potential reliability issues. Pure Tin component leads have been shown to result in inter-metaUic migration in the termination of the electronic component and the growth of tin whiskers which could cause short circuits (which is why there is a exemption for military use (only) components). 

Pin in paste (PIP) technology, also know as alternate assembly and reflow technology (AART) by Universal Instruments and others, allow the use of THT parts with solder paste and reflow soldering. Paste is deposited onto the board at through-hole locations, the leads of a THT part are placed through the paste, and at the conclusion of the placement process the board and components are reflowed. The two major concerns with the use of the PIP process are the deposition of an adequate volume of solder paste and the use of THT components whose body can withstand reflow soldering temperatures. In the wave solder process, only component leads were raised to soldering temperatures, but in the reflow process the entire part must withstand the process temperatures. 

Few technical issues have hit the electronics industry as hard in recent years as the switch from Sn-Pb solder to lead-free attachment. Both by mandate, e.g., the European Commission requirement to eliminate lead in consumer products, as well as voluntarily, electronic component manufacturers and assemblers must come to grips with issues surrounding the transition to lead-free attachment of components. While various industry, academic, and governmental agencies that have studied alternatives to lead in solder now seem to agree on SnAgCu as the preferred alternative, as discussed by Bradley, Handwerker, and Sohn, there is still much to be done to create a minimal-problem transition. 

Leadcoplanarity is defined as follows. If a multilead part, for example, an IC, is placed on a planar surface, lack of coplanarity exists if the solderable part of any lead does not touch that surface. Coplanarity requirements vary depending on the pitch of the component leads and their shape, but generally out-of-plane measurements should not exceed 4 mils (0.004 in.) for 50-mil pitch devices, and 2 mils for 25-mil pitch devices. 

Reflow The solder paste temperature exceeds the liquidus point and reflows, wetting both the component leads and the board pads. Surface tension effects occur, minimizing wetted volume. 

Through hole Also a plate through hole (PTH). A hole in a substrate that extends completely through a substrate, is solder plated, and is intended for a component lead. 

All component leads attach to the PCB by being inserted into holes that pass through the PCB.The components may be secured by wave soldering or by pressing into holes that result in an interference fit (press fit). Assembly involves a component placement operation followed by a wave-soldering operation. This method is still the workhorse of the low-cost consumer electronics industry. 

Vibration. Vibration is a term that describes oscillation in a mechanical system, and is defined by the frequency (or frequencies) of oscillation and ampUtude. PBAs that are subjected to extended periods of vibration will often suffer from fatigue failure, which can occur in the form of broken wires or component leads, fractured solder joints, cracked conductive patterns, or broken contacts on electrical connectors. The frequency(s) of vibration, resonances, and amphtude(s) all influence the rate to failure. 

The fundamental mode is the primary mode of concern because it has the large displacements that cause fatigue damage to solder joints, component leads, and connector contacts. 

Continuous flexing of a PBA will fracture component leads and, more important, surface-mounted component solder joints, due to mechanical fatigue failure. (Mechanically induced flexing or vibration in assembled PBs is used under controlled conditions to induce failures in solder joints for quality and reliability studies.)... 

FIGURE 40.1 (a) Photograph of a through-hole printed circuit board, (b) Optical micrograph of the component lead as it is soldered into the circuit board hole. (Courtesy ofSandia National Laboratories)... 

Component Lead and Circuit Board Hole Sizes. Designing the correct component lead hole diameters begins by referring to the appropriated industry standard(s) (e g., IPC, Electronic Industry Association [EIA], etc.). Hole tolerances must take into account runout by the drill, etch-back, barrel-plating thicknesses, and the need for a nominal gap of 0.07 to 0.15 mm between the pin and hole to support the capillary flow of the molten solder. In addition, there are added tolerance considerations due to the variation in component lead diameters as well as the positioning accuracy of the equipment. 

A minimum temperature of 230°C is required to ensure adequate wetting and spreading by the Pb-free solder on circuit board pads as well as on component leads and terminations. Temperatures exceeding 245°C increase the likelihood of thermal damage to larger, plastic-molded packages (e.g., BGA and QFP devices). When temperatures exceed 260°C, there is the potential for thermal degradation of passive chip components (chip capacitors, indnctors, or filters) as well as to circuit board structures (e.g., vias) and laminate materials. In the case of Sn-Pb eutectic solder that melts at 183°C, the minimum process temperature of 215°C ensures adequate solderability, which is 15°C lower than that of a nominal Pb-free process. The available Sn-Pb process window is 215°C to 260°C, or a AT equal to 45°C, rather than the AT of 30°C for the Sn-Ag-Cu Pb-free solders (T di = 217°C). 

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