Surface mount device substrate

Note that the sort of oscillator circuit shown in Rgure 6.13(b) can be constructed by the interconnection of line-powered modular instnnnentaticm [30], by the use of printed-circuit board-mounted, DC-powered components [31], by the use of surface-mount devices on a suitably metallized substrate, or even on a sin-... 

As noted previously, substrate (typically PCB) design has an effect not only on board/component layout, but also on the actual manufacturing process. Incorrect land design or layout can negatively affect the placement process, the solder process, the test process, or any combination of the three. Substrate design must take into account the mix of surface mount devices that are available for use in manufacturing. 

ML-PWBs often have surface-mount devices on both sides of the board, and they receive three or more solder operations during the assembly of connectors and devices. In addition, because of the value of a completed assembly, the board must be able to withstand additional soldering operations needed for occasional removal and replacement of defective devices. Boards made with difunctional GF epoxy can suffer from lifted lands, cracked PTH barrels, or substrate blisters during these multiple soldering operations. The solution is to use materials with low moisture absorption and high thermal degradation temperature. 

This is why there are also different component shapes. Virtually the only components of interest for MID technology are those having metallized pads for connection that can be set directly on to plated lands on the surface of the substrate. These are surface mount devices, SMD for short. Through-hole devices, THD for short, are wired components and are not used for 3D-MID, so they are not discussed in more detail below. The range of SMD components can be subdivided as follows [109] ... 

Some adhesive materials and processes are used across many apphcations. For example, adhesives are used to attach bare die, components, and substrates in assembling commercial, consumer and aerospace electronic products. Adhesives are also widely used for surface mounting components onto interconnect substrates that serve numerous functions for both low-end consumer products and for high rehability applications. Underfill adhesives are used to provide stress relief and ruggedize the solder interconnects for almost all flip-chip and area-array devices, regardless of their function as integrated circuits. 

The first specifications for adhesives were generated by the staff of NASA and the DoD who were prompted by the high reliability that was required of microcircuits used in aerospace programs. These specifications covered primarily die and substrate attachments for hermetically sealed integrated circuits, hybrid microcircuits, and multichip modules. Subsequently, with the increased use of surface-mount adhesives in the assembly of commercial printed-wiring boards and underfills for flip-chip devices, industry associations took the lead in generating the requirements and test methods. 

Numerous geometrical factors of the specific interconnection will also influence anisotropic adhesive formulation and processing, including lead planarity, IC pad metallization, and IC test patterns. The planarity of the leads on the substrate and/or device and the compliance of the conductive particles will determine if anisotropically conductive adhesives can be used in a particular application. For systems with large disparities between lead height, no electrical interconnection will be formed, as shown in Fig. 5. Fine-pitch IC packages for surface-mounted applications, such as the plastic quad flat pack (PQFP), often use gullwing leads that olfer much compliance to the joint. Even if the... 

The new user of surface mount designs (SMDs) must rapidly learn the packaging sizes and types for SMDs. Resistors, capacitors, and most other passive devices come in two-terminal packages, as shown in Fig. 11.17, which have end-terminations designed to rest on substrate pads/lands. 

In the surface mount assembly process, type 11 and type III boards will always require adhesive to mount the SMDs for passage through the solder wave. This is apparent when one envisions components on the bottom side of the substrate with no through hole leads to hold them in place. Adhesives will stay in place after the soldering process and throughout the fife of the substrate and the product, since there is no convenient means for adhesive removal once the solder process is complete. Additionally, adhesives can be used to enhance both thermal and electrical conduction between device features and board features. This means use of an adhesive must consider a number of both physical and chemical characteristics ... 

These circuits are usually single- or double-sided ceramic substrates with a collection of surface-mount active components and screened-on resistors made from metallic pastes. They are most often found in hearing aids and other miniature devices. 

Materials with Enhanced Mechanical or Conduction Properties. In cases where the device package (leadless area array with ball grids) is incompatible with the CTE of a standard material, a low-expansion snbstrate mnst be nsed.This can be achieved in several ways. In the past, leadless surface-mount technology (SMT) focused on replacing the woven glass in standard FR-4 with woven quartz or aramid fibers. Although this reduced the expansion of the substrate, both materials were expensive and difficult to process. 

Clearly, it is important that the CTE of materials used in the production of components for surface mounting should match the CTE of substrates as closely as possible. The through-hole assembly process currently allows a level of imprecision and CTE mismatch since any differential expansion can be accommodated by the compliance of the component leads. Much greater precision is required in surface mounting, where any difference in CTE between mating parts can result in failure due to poorly controlled part-to-part dimensions, stressing of components, device or board damage, or poor solder joints. 

For testing convenience the device was mounted on a microscope stage in which the recording laser beam is focused from below through the glass substrate, as shown in Fig. 20a. This allows the recording surface to be directly observed from above. Since both the polymer and the top conductor are semitransparent, the performance is comparable to that obtained if the laser beam illuminates the photoconductor from the top surface. 

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