Screen printing steps

By and large, the greatest mrniber of solder-joint defects can be traced to the solderdispensing process and, specifically, the stencil or screen printing step. Therefore, process control is critical here. An absence of the control of any one of these attributes can significantly impact process yields. Important factors include the following ... 

FIGURE 6-23 Steps involved in the screen-printing process (a) deposit the graphite suspension onto the screen (b) load the screen mesh with the graphite (c) force the graphite onto the substrate. (Reproduced with permission from reference 83.)... 

The wafers are processed into solar cells, the majority of which have a diode structure, as sketched in Figure 11.4, characterized by a thin, diffused, doped emitter, screen-printed front and back contacts and a front-surface antireflective coating. Prior to the effective cell manufacturing step, a chemical treatment of the silicon wafers removes... 

S. J. Setford, S.F. White and J.A. Bolbot, Measurement of protein using an electrochemical bi-enzyme sensor, Biosens. Bioelectron., 17 (2002) 79-86. P. Sarkar and A.P.F. Turner, Application of dual-step potential on single screen-printed modified carbon paste electrodes for detection of amino acids and proteins, Fresenius J. Anal. Chem., 364 (1999) 154-159. 

Screen-printed electrodes used for the PB modification were home produced. A detailed description of the electrodes used and of the procedure adopted for PB modification is found in Procedure 17 (in CD accompanying this book). The most important thing to note about this procedure is that it does not involve any electrochemical step and, for this reason, it has been designed as chemical deposition . This procedure is also very easy to perform and could be adapted to mass production of modified electrodes (see Procedure 17 in CD accompanying this book). The suitability of the proposed deposition procedure was carefully evaluated with different electrochemical techniques and its application in real samples has been summarised and discussed here. 

The use of the electrode surface as solid phase in immunoassay can present some problems a shielding of the surface by antibody or antigen molecules can cause hindrance in the electron transfer, resulting in a reduced signal and so a loss of sensitivity. There are different ways that can be used to improve the sensitivity of the system an interesting approach could involve the screen-printed electrodes are used only for the transduction step, whereas the affinity reaction could be performed using another physical support [24]. 

This approach separates the steps relative to the immunoreaction from the step of electrochemical detection and for this reason the working electrode surface is easily accessible by enzymatic product, which diffuse onto bare electrode surface [28,33] (Fig. 25.3). Using this strategy, finding the optimum conditions for the immunoassay on the magnetic beads and for electrochemical detection on the transducer (carbon screen-printed electrodes) is much easier than in the usual one (electrode) surface systems, because optimum conditions for immunoassay do not conform with those for electrochemical detection and vice versa. 

In literature, a direct competitive assay performed using magnetic beads protein G coated as solid phase and carbon screen-printed electrodes as transducers is reported [4], The main steps of the assay are shown in Fig. 25.4. 

A system underpinned by commercially made screen-printed electrochemical cells was described by Palmisano et al. [19]. The cells were converted into biosensors for lactate in milk and yoghurt by addition of an electrochemically polymerised barrier to interference and a layer composed of lactate oxidase, glutaraldehyde and BSA. These steps appeared to have been carried out by hand. As there was no outer diffusion-limiting membrane, the linear range of the sensors was quite small (0-0.7 mM). They were incorporated into a FIA with a microdialysis unit based on a planar membrane and a buffer reservoir (earlier work used a microdialysis fibre with a platinum electrode [29]. The concentration of lactate was determined in various milks (0.27-1.64 mM), and in raw milk (c. 0.5-0.9 mM) left to degrade on the laboratory bench. The recovery of the microdialysis unit, 2.6%, implied that the sensor had an ability to return measurable currents for very low concentrations of lactate. A further implication is that the electro-polymerised layer was very effective at preventing interference. 

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