Determination Of Requirements For Ventilation System In Manufacturing Of Electronic Products

Electronic Equipment Production Processes and Hazardous Factors

Soldering of SMD components is carried out with the use of soldering pastes by heating in special devices (furnaces) while parameters of this process are set and controlled by a computing device. Soldering alloys and fluxes are used for soldering of components mounted in openings of the boards and for elimination of defects in automated soldering of SMD components. Composition of soldering alloys and fluxes includes lead with percentage that may exceed 50%.

Various phases of electronic equipment manufacture involve a wide use of lacquers, solvents, acids, mixtures and many other raw components potentially dangerous to health of personnel of enterprises.

Bonding of components is carried out by means of adhesives based on phenolformaldehyde, organo-siloxane and epoxy resins.

A characteristic feature of abovementioned processes is the emission of gaseous products making harmful impact upon personnel, dangerous for their health. In working environment, gaseous chemicals most often get into the body via respiratory tract and then via lungs get into blood circulatory system and are further spread throughout the body. Low level of labor hygiene, smoking during work (pollution with tobacco smoke, contamination of smokers’ fingers) and low level of personal hygiene may considerably increase the entire hazardous impact of chemicals upon health of personnel since their additional amount will get into the body orally (LVS 89, 2004).

The main hazard of lead is in its toxicity. Clinical lead poisoning is one of most heavy vocational disease. Lead hampers a normal functioning of cells and a number of physiological processes. The most sensitive target for lead poisoning is nervous system, which is manifested as minor behavioral changes, fatigue, reduced concentration and deterioration of motoric functions. Lead inhibits the body’s ability to produce hemoglobulin by several enzymatic steps in the heme exchange. Lead may cause two types of anemia. Acute poisoning with high lead doses causes hemolytic anemia. Lead also makes endocrinous impact, affects kidneys, reproductive function and development, as well as has cancer-inducing effect ("Occupational health," 2010).

Fluxes used in soldering also are toxic. Thus, colophony causes skin irritation and rash. Adhesives and lacquers are fire-hazardous and toxic, causing hand skin disease, irritation of respiratory tract, making adverse impact upon blood, blood-vascular organs and central nervous system (“Agency for Toxic Substances,” 2005).

Preventive measures against vocational poisoning at electronics industry enterprises include: hygienic rationalization of production process, its mechanization and hermetization. To prevent harmful impact, all soldering alloys, fluxes and other chemicals must be stored in a special tightly closed container.

A general measure, which eliminates the impact of harmful gaseous emissions at enterprises of electronics industry, is the use of local exhaust ventilation in addition to the centralized supply and exhaust ventilation. Soldering workplace is equipped with local exhaust ventilation ensuring the lead concentration in the working area not exceeding the maximum allowable value (0.01 mg/m3). Insulation stripping by firing is accompanied by smoke emission with a heavy and unpleasant odor. Therefore, upon firing of a large batch of wires, an exhaust hood with proper ventilation should be used. Work with paint-and-lacquer materials should only be carried out a separate room with supply and exhaust ventilation.

The efficiency criterion of ventilation systems is the reduction in concentrations of harmful substances in the air of the working area down to their maximum allowable values and lower. In this connection, the arrangement of the air ventilation system in the working area should meet the following requirements:

• Capability of ventilation system should be sufficient to ensure necessary conditions in the working area.

• Such air supply and exhaust methods should be used, which ensure the maximum efficiency of the working area air ventilation system.

• Ventilation system should include the air filtering system.

In initial operation phase of an enterprise of electronics industry, arrangement and specifications of ventilation system fully meet these requirements. However, with growing volumes of production and extending working area the ventilation systems are modernized without consideration of quantitative indicators, such as concentration of health-hazardous substances in the air of the working area and necessary speed of air flow created by the ventilation system. At all enterprises of Latvian electronics industry, upon increasing production volumes the working area is extended by organization of additional workplaces and laying to them the air ducts branching from already used ventilation systems. Therefore, the analysis of changes in concentration of harmful substances in the working area and appearing requirements towards modernization of ventilation system were examined by example of a Latvian enterprise producing small-size electronic boards and modules for network and household electronic devices with the use of process illustrated by chart in Fig. 1 .

Analysis of Harmful Substances Concentration in Working Area Air

Laboratory measurements of harmful substances concentration in the air of the working area were regularly carried out since the time the production was started at the analyzed enterprise. Concentration of harmful substances in the air was identified by laboratory methods. Evaluation of chemical concentration in the air was carried out by comparison of its concentration with its normative shift-average and maximum single allowable values (VEL). Vocational exposure limit (VEL) is such concentration of chemical in the air of working environment, which throughout an 8-hour working day (or also at other duration of the impact but not exceeding 40 hours a week) does not cause in the body of an employee throughout its lifetime a disease or health abnormalities detectable by modern examination methods (LVS 89, 2004).

Laboratory monitoring of the air in the working area is carried out in regular production conditions, during the process in accordance with technological regulation of manufacture and the developed programme of operational control (“REACH,” 2014).

To monitor the chemical composition of the air in the working area, the air sampling was carried out within the worker’s respiration area, with maximal approaching of an air intake device thereto: at 1.5 m height from the floor at standing activity or at 1 m height at seating activity. Where a worker had no a permanent workplace, the air sampling was carried out in all spots of the working area where the worker appeared during its working shift.

Air sampling devices were placed both in fixed spots of the working area by the stationary method and by the personal monitoring method – the air sampler is attached directly at the worker’s clothing. Methods for monitoring of lead content in the air: in accordance with ISO 9855:1993: Work place air.

Air sampling is made by respiration via “Varian 3800” gas chromatograph. The measurement results of the last soldering step are presented in the table (see Table 1 and Fig. 2).

* - expanded uncertainty indicated for average values is determined as mean-square deviation multiplied by overlapping coefficient 2, providing 95% reliability level.

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Growing lead concentration in the air of the working area is explained by increased number of soldering places (currently 7) as well as increased number of soldering operations.

Monitoring of air chemical composition in the working area allows to obtain an objective picture of lead concentration and its changes in the result of output expansion. However, monitoring of air chemical composition in the working area by sampling is a costly inoperative measure carried out with intervals of several years. In case of expanded production and increased number ow workplaces, the results of monitoring of air chemical composition in the working area are a late confirmation of increased concentration of harmful substances, lead in this case, in the working area. Consequently, modernization or replacement of currently employed exhaust ventilation system, which does not provide necessary air purity at current conditions and volumes of production, will be also belated.

Use of automated ventilation systems at enterprises of Latvian electronics industry would provide the required concentrations of harmful substances, including lead, in the working area. However, installation of such automated ventilation systems requires additional expenses and therefor they are not used at enterprises of Latvian electronics industry.

As a consequence, at present time in the Republic of Latvia the monitoring of air chemical composition in the working area by air sampling is the basic method allowing to identify the necessity and degree of modernization of ventilation systems at any industrial enterprises. Therefore, of urgency is the efficiency analysis of exhaust ventilation systems upon growing number of workplaces and production volumes (number of soldering operations). Operative modernization of exhaust ventilation system in accordance with estimated technical parameters will provide a timely health protection of personnel within the working area against impact of lead and other harmful substances typical for electronics production.

Efficiency Analysis of Exhaust Ventilation System at Soldering Places

Efficiency analysis of exhaust ventilation systems at soldering places was carried out for production conditions and facilities of such enterprise where the air of the working area demonstrated the growth in lead concentration as a result of increased number of soldering places and output of electronic products (Table 1 and Fig. 2). Efficiency of exhaust ventilation systems at soldering places was carried out information way.

At one workplace one worker performs soldering with soldering allow NAC 60/40 at output rate N = 100 contact per hour. Lead-tin soldering alloy NAC 60/40 contains С = 0.4 parts of lead volume and 60% of tin. The most poisonous are lead sprays (vapors) (Alexeeff et.al., 1999). During the soldering process, up to B = 0.1% is evaporated from the soldering alloy while 1 soldering requires m = 10 mg of soldering allo. The amount of lead sprays emitted into the air:

G = c * B * m * N. /100 = 0.4 mg/h(1)

The required air exchange at emission of harmful substances is determined by:

L = G * 1000/(X_T – X_A) = 44.45 m³/h (2)

where:L, m³/h – required air exchange;

G, g/h – amount of harmful substances emitted into the air (1);

X_T,= 0.01 mg/m³ – maximum allowable harmful concentration in the air of the working area of a premise;

X_A = 0.001 mg/m³ – maximum allowable the same harmful concentration in the air of populated locations.

Total output rate of local exhaust system, which is used for air exhaust from soldering places and from equipment installation places, TE (Technological Exhaust) Ln = 1000 m3/h.

The number of workplaces in the soldering section is 7, thus the required air exchange for the whole soldering section:

L₇=7* L1=7*44.45 m3/h = 311.15 m³/h

TE exhaust system has also thereto connected microwave oven Wave Soldering Seho, which in accordance with technical documentation of the oven requires the air exhaust 500 m³/h from the oven.

Thus, TE exhaust system should have output rate equal to:

W_N = K_k (L₇+500)

where K_k = 1,3..2,0 – reserve coefficient. Choosing the average reserve coefficient, we determine TE exhaust system should have output rate:

W_N=1.7*811.15 =1,378.955 m³/h

Diagram of TE exhaust ventilation system is shown on Fig. 3 .

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Air duct of ventilation system consists of 10 straight-line sections. Using the length and diameters of straight air duct pipes and air velocity at these sections, total pressure losses in the air duct network were calculated by formula (Petrov, Volhin & Petrova, 2006):

$p=\stackrel{10}{\underset{i=1}{\sum }}\left({\mathrm{R}}_{\mathrm{i}}*1_{\mathrm{i}}+{\mathrm{z}}_{\mathrm{i}}\right),{\mathrm{P}}_{\mathrm{a}}$(3)

where R_i – abrasion pressure losses at estimate i-section of air duct (Pa) per 1 m;

l_i - length of i-section of air duct, m;

z_i – local resistance pressure losses at estimate i-section of air duct, Pa.

Abrasion pressure losses R_i, Pa/m, in round air ducts are determined by formula (4)

$R_i=\frac{{\lambda }_i}{d_i}\mathrm{*}\frac{\rho \mathrm{*}{V}_i^{\mathrm{2}}}{\mathrm{2}}$,(4)

where λ_i – abrasive resistance coefficient at i-section of air duct;

d_i – diameter of i-section of air duct, m;

V_i – air velocity at i-section of air duct, m/s;

ρ – density of air moving in air duct, is equal to 1.2 kg/m3;

ρV²/2 – velocity (dynamic) pressure, Pa.

Abrasive resistance coefficient is calculated by Altschul formula (5):

${\lambda }_i=\mathrm{0.11}\mathrm{*}{\left(\frac{K_a}{d_i}+\frac{\mathrm{68}}{{Re}_i}\right)}^{\mathrm{0.25}}$(5)

where Кa – absolute surface roughness of steel plate air duct Кe = 0.1 mm;

di – diameter of air duct, mm;

Rei – Reynolds number for i-section of air duct.

Reynolds number is a nondimensional characteristic of liquid flow, which is determined by proportion between dynamic pressure (ρv2) and contact tension (μv/L) and calculated in the following way:

${Re}_i=\frac{\rho \mathrm{*}{V}_i\mathrm{*}{d}_i}{\mu }$(6)

where Re – nondimensional Reynolds number;

ρ – air density, which is equal to 1.2 kg/m3;

Vi – air velocity at i-section of air duct, m/s;

μ – dynamic air viscosity, which is equal to 0.00185 Н*s/m2;

di – diameter of i-section of air duct, m.

Pressure losses on local resistance zi, measured in Pa, with bends and elements of the air duct structure: grilles, valves, manifolds, etc. Local resistance in TE1 exhaust ventilation system is created by the bends in all sections except sections 3 and 10. Therefore, local resistance at these sections is equal to 0 while for other sections is determined by formula (7):

$Z_i=k_p\mathrm{*}\frac{\rho \mathrm{*}{V}_i^{\mathrm{2}}}{\mathrm{2}}$(7)

where kp – local resistance coefficient at i-section of air duct, with air duct bend angle equal to 900.

Vi – air velocity at i-section of air duct, m/s;

ρ – density of air moved via air duct, which is equal to 1.2 kg/m3.

Total pressure losses in TE1 air duct network are determined in Microsoft Excel (see Table 2 ).

Losses for the last section (N=10) were determined with consideration of all eleven soldering workplaces.

po = 1.184

mu = 0.0000185

Thus, total pressure losses in in TE ventilation system air duct network are H = 344.62 Pa.

Therefore, TE system fan should ensure compensation for pressure losses 344.68 Pa in the air duct network. However, fan used in the exhaust system in accordance with technical documentation creates the initial air pressure in the air duct network, which is equal to 290 Pa.