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Production Methods and Materials

3.1 General

This section of the report addresses printed wiring board production methods and materials that are of particular concern with respect to waste generation and pollution prevention and control. To assist the reader in relating the survey data to specific production steps, a brief overview of the PWB manufacturing process is presented.

3.2 Overview of PWB Manufacturing Process

The most common printed wiring board manufacturing processes are the single-sided, double-sided, and multi-layer rigid sequences, which are depicted in Exhibit 3-1 (see full size graphic). These diagrams show that the number of process steps increases with the complexity of the product.

Exhibit 3-1. Overview of Rigid PWB Manufacturing Process Sequences

The main processes, common to all PWBs, are drilling, and image transfer, and electroplating. Holes are drilled into PWBs (or punched, in the case of paper-based substrates such as CEM-1) to provide layer-to-layer interconnection on double-sided and multi-layer circuits. These holes are subsequently "plated through" or made conductive by plating copper onto the hole barrels (the vertical, cylindrical surface of the hole). Since most rigid PWB substrates consist of epoxy-resin and glass, direct electroplating of hole barrels is not possible since this material will not conduct electricity. Therefore, a seed layer or coating of conductive material (usually electroless copper) must be deposited into the barrels of the holes before the electrolytic copper plating can occur. Holes are also drilled into PWBs, including into single-sided boards, to accommodate component leads that are inserted through the hole and soldered to the board. Holes drilled for this purpose do not require electroplating.

Image transfer is the process by which an image of a circuit layer is transferred from film, glass or directly from image data files, to the copper foil of PWB material. For inner-layers, this includes the application of a photo-resist (a photo-sensitive film which also serves as the etch resist), imaging, developing, and etching. For outer-layers, image transfer may include the electroplating of copper, tin, tin-lead, or nickel/gold coatings.

The predominate method of accomplishing image-transfer and layer-to-layer interconnection is called the "subtractive plating process" or simply "print-and-etch." With this process, a uniform copper foil layer is selectively etched to create a circuit pattern. Copper foil is an integral part of PWB base laminate and is applied by PWB manufacturers during the laminate stage or it is bonded to substrate by laminate makers.

In an alternative process, called additive processing, the manufacturer forms the copper image by selectively plating electroless copper onto a sensitized substrate (fully additive) or by plating a thin layer of electroless copper non-selectively onto a substrate, then applying a photo-resist and selectively electroplating additional copper onto the circuit areas (semi-additive). The fully additive process does not require etching at all; semi-additive processing only requires etching of the thin electroless copper layer. Additive processing, although attractive in terms of efficiency and waste reduction, is not a common choice among manufacturers producing the basic or low-cost PWBs. This is due to the complexity of the process compared with subtractive processing, the electrical properties of electroless copper deposits, and certain other physical properties of electroless copper (e.g., the low copper-to-substrate peel strength).

3.3 Specific PWB Production Steps

Each box in Exhibit 3-1 is a logically distinct process or group of processes. For some of the boxes, a set of competing processes, or a use-clusterc exists. For example, the hole cleaning step of desmear may be performed with a permanganate based wet chemistry, other less common wet chemistries, or in a gas-plasma chamber. The survey requested data on several use clusters that may significantly impact pollution prevention and control. Data were collected on etchants, oxide processes or alternatives, processes for making holes conductive, outer-layer etch resists, and solder masks. The responses to these survey questions are discussed in this section.

3.3.1 Inner-Layer Etchants

Inner-layers are etched during the inner-layer image transfer step of the typical multi-layer rigid PWB manufacturing sequence. Image transfer for inner-layers is accomplished in a series of processes that result in the transfer of a circuit image from film, glass, or data to the copper foil layer of PWB base material. Etching is required for all process sequence alternatives that form the inner-layer image transfer cluster within the predominate subtractive process. Panels entering the etch process have been coated with an etch resist, usually a dry film photo-resist. The resist layer selectively protects the circuit areas from etchant whereas the remaining copper foil is etched away.

Historically, ferric chloride and chromic acid were common etchants. Both have fallen into disfavor, leaving cupric chloride and ammoniacal etchants as the etchants of choice in the modern PWB shop. Ammoniacal etchants have the distinct advantage of being compatible with both organic (dry film, typically used for inner-layers) etch resists and metallic (tin, tin-lead, gold, typically used for outer-layers) etch resists making it the only choice for shops where two etching systems are economically or otherwise impractical. Although on-site regeneration methods are available for both materials cupric chloride etchant is more frequently processed on-site, a major consideration when the volume of etchant a PWB shop requires is taken into account. Furthermore, since cupric chloride is acidic, no attack on high-pH-sensitive dry film resist adhesion occurs. Sulfuric-peroxide (i.e., sulfuric acid and hydrogen peroxide) is commonly used as a micro-etchant. This chemistry was reported in use by a small percentage of respondents as a circuit etchant. Sulfuric-peroxide has much lower capacity than other etchants (approximately 38 g/l vs. 150 g/l Cu or more for ammoniacal and cupric chloride) but is easily regenerated on-site and is compatible with metallic etch resists.

The survey results relating to etchants are presented in Exhibits 3-2 and 3-3. Seventy-eight percent of the survey respondents use ammoniacal etchant for inner-layer etching, while 22% use cupric chloride. Six percent indicated that they use sulfuric-peroxide for inner-layer etching (one respondent uses both cupric chloride and ammoniacal, which is why the percentages exceed 100%). As evidenced by the data, cupric chloride use is more common among larger shops. Thirty-six percent of the shops above the median production level use cupric chloride etchant for inner-layer etching.

PWB Type
(% board ft2)
Inner-Layer Etchan
(% of inner-layer panels)
Outer-Layer Etchant
% of outer-layers)
Respondent
ID
Production
(board ft2 per year)
Rigid Non-
Rigid
Cupric
Chloride
Ammo-
niacal
Other Cupric
Chloride
Ammo-
niacal
Other
36930A nr 0 100 0 100 0 0 100 0
955099 nr 100 0 0 100 0 0 100 0
55595 nr 100 0 nr nr nr nr nr nr
44486 nr 100 0 0 100 0 0 100 0
955703 nr 100 0 0 100 0 0 100 0
6710 15,000 100 0 0 100 0 0 100 0
947745 40,000 100 0 0 100 0 0 100 0
44657 42,358 100 0 0 100 0 0 100 0
29710 57,000 100 0 0 100 0 0 100 0
502100 60,000 0 100 100 0 0 95 5 0
32482 75,000 90 10 0 0 1001 0 0 1001
25503 90,000 5 95 100 0 0 97 3 0
36930 96,000 100 0 0 100 0 0 100 0
965874 175,000 100 0 -2 - - 0 100 0
953880 180,000 100 0 0 100 0 0 100 0
33089 200,000 100 0 -2 - - 0 100 0
T3 200,000 98 2 0 100 0 0 100 0
3470 240,000 100 0 0 100 0 0 100 0
43841 250,000 100 0 0 100 0 0 100 0
279 250,000 100 0 -2 - - 100 0 0
237900 273,000 100 0 0 100 0 0 100 0
273701 280,000 98 2 0 0 1001 0 0 1001
41739 300,000 100 0 0 100 0 0 100 0
959951 320,000 100 0 0 100 0 0 100 0
42692 360,000 100 0 0 100 0 0 100 0
358000 500,000 100 0 0 100 0 0 100 0
43694 500,000 0 100 0 100 0 0 100 0
37817 540,000 100 0 -2 - - 0 100 0
42751 540,000 100 0 100 0 0 0 100 0
T2 600,000 100 0 100 0 0 0 100 0
133000 600,000 0 100 0 100 0 0 100 0
T1 936,000 100 0 0 100 0 0 100 0
740500 1,800,000 100 0 100 0 0 100 0 0
946587 1,900,000 100 0 100 0 0 0 100 0
3023 2,300,000 100 0 0 100 0 0 100 0
31838 3,000,000 100 0 0 100 0 0 100 0
462800 3,750,000 100 0 -2 - - 100 0 0
107300 5,000,000 100 0 50 50 0 0 100 0
Exhibit 3-2. Inner-Layer and Outer-Layer Etchants Used by Survey Respondents
1 Etchant type is sulfuric-peroxide.
2 Shop does not produce any multi-layer PWBs.
nr = no response

Exhibit 3-3. Distribution of Inner- and Outer-Layer Etchant Use Employed by Surveyed Respondents

Outer-layer etchant use data are discussed in Section 3.3.6.

3.3.2 Oxide

The oxide step is also found in the inner-layer image transfer cluster. The oxide step can be eliminated by purchasing "double treated" laminate that is oxide coated by laminate manufacturers. Oxiding, which is employed to enhance the copper-to-epoxy bond strength of the multi-layer panel, is performed in a variety of similar chemistries, usually consisting of sodium hydroxide and sodium chlorite.

Survey data relating to inner-layer surface preparation are presented in Exhibit 3-4. These data indicate that al boards use the conventional oxide step for at least some of their multi-layer product and 13% also purchase double-treated material for some of their product. Of the three exclusively flex manufacturers, one does not perform the oxide step at all, one does not use the oxide step for 98% of multi-layer product, and the third uses double-treated material on all of its multi-layer product. The oxide step is not typically needed with flex manufacturing due to the different materials used. The results show that all of the respondents who do not use the oxide step are flex manufacturers.

6710
PWB Type
(% board ft2)
Inner-Layer Copper Surface Preparation Method
(% of total production)
Respondent
ID
Production
(board ft2 per year)
Rigid Non-
Rigid
Red, Brown,
Black Oxide
Double
Treat
No
Oxide
36930A nr 0 100 2 0 98
955099 nr 100 0 100 0 0
55595 nr 100 0 nr nr nr
44486 nr 100 0 100 0 0
955703 nr 100 0 100 0 0
15,000 100 0 100 0 0
947745 40,000 100 0 100 0 0
44657 42,358 100 0 100 0 0
29710 57,000 100 0 100 0 0
502100 60,000 0 100 0 100 0
32482 75,000 90 10 100 0 0
25503 90,000 5 95 0 0 100
36930 96,000 100 0 95 5 0
965874 175,000 100 0 -1 - -
953880 180,000 100 0 100 0 0
33089 200,000 100 0 -1 - -
T3 200,000 98 2 90 10 0
3470 240,000 100 0 100 0 0
43841 250,000 100 0 100 0 0
279 250,000 100 0 -1 - -
237900 273,000 100 0 100 0 0
273701 280,000 98 2 100 0 0
41739 300,000 100 0 100 0 0
959951 320,000 100 0 5 95 0
42692 360,000 100 0 100 0 0
358000 500,000 100 0 100 0 0
43694 500,000 0 100 0 0 100
37817 540,000 100 0 -1 - -
42751 540,000 100 0 90 0 0
T2 600,000 100 0 100 0 0
133000 600,000 0 100 15 0 0
T1 936,000 100 0 100 0 0
740500 1,800,000 100 0 nr nr nr
946587 1,900,000 100 0 95 5 0
3023 2,300,000 100 0 99 1 0
31838 3,000,000 100 0 100 0 0
462800 3,750,000 100 0 -1 - -
107300 5,000,000 100 0 100 0 0
Exhibit 3-4. Inner-Layer Copper Surface Preparation Data
1 Shop does not produce any multi-layer PWBs.
nr = no response

3.3.3 Clean Holes (Desmear)

During drilling, drill bits may become heated resulting in melting and smearing of the epoxy-resin base material across the inner-layer copper interface of the hole barrel (a ring of copper typically 0.0014 inches thick), which, if not corrected, would lead to a non-conductive circuit. Smeared epoxy-resin (referred to as smear) is not a concern on single-sided circuits because there is no inter-connection in the holes. Neither is it of major concern on double-sided circuits because the copper-to-copper interface extends out of the hole barrel and onto the horizontal surface of the PWB, an area that is not affected by smear. For inner-layers of multi-layer panels, the only interface with the subsequently plated copper is the portion of the inner-layer circuit residing in the hole barrel. If this surface is covered with smear, no connection to the plated copper can occur, and the PWB will be defective.

The desmear process is often grouped and sometimes confused with etchback because similar or identical processes can be used to perform both functions (by adjusting dwell times or concentrations). Desmear is simply the removal of smeared epoxy-resin by-products from the copper surfaces within the hole barrel to facilitate the interconnection between inner-layer copper and the electroless copper which is plated next. Etchback is the removal of a significant amount of epoxy-resin and glass from the hole barrel itself. Etchback is performed to expose a greater copper surface area which enhances the bond between the inner-layer copper and subsequent copper plating. For most applications, etchback has become unnecessary and is usually not performed unless specified. Furthermore, smear is sometimes considered to be simply a product of poor drilling practices, which, if corrected, would result in no smear and elimination of the desmear process.

Two distinct methods of desmear currently exist; wet chemical processing is most common, but plasma etching is seeing increasing use. With the wet process, sodium or potassium permanganate is the oxidizer of choice, replacing the previously used chromic acid for health reasons. Permanganate alone is a poor glass etchant; therefore, if etchback is required, a concentrated sulfuric step is necessary. Oxygen and carbon tetraflouride are among the gases used in plasma etching.

The survey results indicate that 59% of the respondents use permanganate desmear exclusively (Exhibits 3-5 and 3-6). One respondent uses a sulfuric-permanganate etchback and d process on nearly all of their product. Plasma etching is employed at 33% of multi-layer shops, but only 16% use plasma exclusively. Two respondents, one rigid and one flex manufacturer, reported they do not desmear the large majority of their product at all.

PWB Type
(% board ft2)
Etchback/Desmear Method
(% of total production)
Respondent
ID
Production
(board ft2 per year)
Rigid Non-
Rigid
Permanganate2 Sulfuric-
Permanganate3
Plasma No
Desmear
Other
36930A nr 0 100 0 0 100 0 0
955099 nr 100 0 100 0 0 0 0
55595 nr 100 0 0 100 0 0 0
44486 nr 100 0 100 0 0 0 0
955703 nr 100 0 100 0 0 0 0
6710 15,000 100 0 100 0 0 0 0
947745 40,000 100 0 0 97 3 0 0
44657 42,358 100 0 100 0 0 0 0
29710 57,000 100 0 100 0 0 0 0
502100 60,000 0 100 0 0 100 0 0
32482 75,000 90 10 100 0 0 0 0
25503 90,000 5 95 0 0 100 0 0
36930 96,000 100 0 80 0 20 0 0
965874 175,000 100 0 -1 - - - -
953880 180,000 100 0 100 0 0 0 0
33089 200,000 100 0 -1 - - - -
T3 200,000 98 2 0 0 10 90 0
3470 240,000 100 0 100 0 0 0 0
43841 250,000 100 0 0 0 100 0 0
279 250,000 100 0 -1 - - - -
237900 273,000 100 0 98 0 2 0 0
273701 280,000 98 2 100 0 0 0 0
41739 300,000 100 0 100 0 0 0 0
959951 320,000 100 0 100 0 0 0 0
42692 360,000 100 0 100 0 0 0 0
358000 500,000 100 0 100 0 0 0 0
43694 500,000 0 100 0 0 1 99 0
37817 540,000 100 0 -1 - - - -
42751 540,000 100 0 100 0 0 0 0
T2 600,000 100 0 100 0 0 0 0
133000 600,000 0 100 0 0 100 0 0
T1 936,000 100 0 80 0 20 0 0
740500 1,800,000 100 0 nr nr nr nr nr
946587 1,900,000 100 0 100 0 0 0 0
3023 2,300,000 100 0 99 0 0 0 14
31838 3,000,000 100 0 100 0 0 0 0
462800 3,750,000 100 0 -1 - - - -
107300 5,000,000 100 0 100 0 0 0 0
Exhibit 3-5. Etchback and Desmear Methods Data

1 Shop does not produce any multi-layer PWBs.
2 Desmear only.
3 Concentrated sulfuric acid etchback, permanganate desmear.
4 Permanganate etchback and desmear.
nr = no response

Exhibit 3-6. Distribution of Etchback and Desmear Methods Employed by Survey Respondants

3.3.4 Making Holes Conductive (or Through-Hole Metalizing)

This use cluster is the focus of considerable attention due to the chemicals employed, wastes generated, and overall complexity of the predominant electroless copper process. The function of the use cluster is to plate a layer of conductive material into the hole barrels, connecting inner-layer copper with surface copper. The electroless copper process includes at least seven or eight different process solutions and with the accompanying rinse tanks, an electroless copper line (i.e., entire sequence of tanks) may include as many as 20 to 25 tanks. The electroless copper process presents problems with respect to waste treatment due to the significant concentration of EDTA or other chelating agents found in all electroless copper baths and associated wastewaters. As a result, most PWB facilities use special treatment steps to separate or destroy these compounds. These treatment steps increase chemical reagent use, operating costs, and sludge production. Furthermore, most electroless copper baths contain significant concentrations of formaldehyde that result in air emissions.

Until the late 1980's, no commercially viable alternative for electroless copper existed. However, recently several alternatives have been developed and are being used extensively by a limited number of shops. These alternatives include carbon- or graphite-based, and, palladium-based processes. In addition, there are alternatives which use conductive polymers and inks, and one that is a non-formaldehyde electroless process. All of the alternative methods reduce copper discharges and eliminate formaldehyde, and most require fewer process baths than electroless copper. Beyond these similarities, the competing alternatives have little in common. For example, one carbon-based system and one graphite-based system require considerable capital investment in conveyorized equipment, while others can be performed in a batch mode in immersion tanks.

Based on the survey results, it is apparent that the electroless copper process is still entrenched as the predominant method of making holes conductive (Exhibits 3-7 and 3-8). Eighty-six percent of the respondents are still using electroless copper on all or nearly all of their product. Fourteen percent (14%) indicated they are using palladium-only processes on all of their product; only one respondent is using the graphite-based process, while another was evaluating it. No respondents reported using the carbon-based process, and one respondent is evaluating an electroless nickel process.

PWB Type
(% board ft2)
Through-Hole Metalizing Method
(% of total production)
Respondent
ID
Production
(board ft2 per year)
Rigid Non-
Rigid
Electro-
less Cu
Palladium-
only
Carbon-
based
Graphite-
based
Electro-
less Ni
Other
36930A nr 0 100 100 0 0 0 0 0
955099 nr 100 0 100 0 0 0 0 0
55595 nr 100 0 100 0 0 0 0 0
44486 nr 100 0 100 0 0 0 0 0
955703 nr 100 0 100 0 0 0 0 0
6710 15,000 100 0 100 0 0 0 0 0
947745 40,000 100 0 100 0 0 0 0 0
44657 42,358 100 0 100 0 0 0 0 0
29710 57,000 100 0 0 100 0 0 0 0
502100 60,000 0 100 100 0 0 0 0 0
32482 75,000 90 10 99 0 0 1 0 0
25503 90,000 5 95 97 3 0 0 0 0
36930 96,000 100 0 100 0 0 0 0 0
965874 175,000 100 0 100 0 0 0 0 0
953880 180,000 100 0 100 0 0 0 0 0
33089 200,000 100 0 100 0 0 0 0 0
T3 200,000 98 2 100 0 0 0 0 0
3470 240,000 100 0 100 0 0 0 0 0
43841 250,000 100 0 0 0 0 100 0 0
279 250,000 100 0 0 100 0 0 0 0
237900 273,000 100 0 100 0 0 0 0 0
273701 280,000 98 2 100 0 0 0 0 0
41739 300,000 100 0 100 0 0 0 0 0
959951 320,000 100 0 100 0 0 0 0 0
42692 360,000 100 0 100 0 0 0 0 0
358000 500,000 100 0 100 0 0 0 0 0
43694 500,000 0 100 0 100 0 0 0 0
37817 540,000 100 0 0 100 0 0 0 0
42751 540,000 100 0 100 0 0 0 0 0
T2 600,000 100 0 100 0 0 0 0 0
133000 600,000 0 100 100 0 0 0 0 0
T1 936,000 100 0 99 0 0 0 1 0
740500 1,800,000 100 0 nr nr nr nr nr nr
946587 1,900,000 100 0 100 0 0 0 0 0
3023 2,300,000 100 0 100 0 0 0 0 0
31838 3,000,000 100 0 100 0 0 0 0 0
462800 3,750,000 100 0 -1 - - - - -
107300 5,000,000 100 0 100 0 0 0 0 0
1 Shop produces only single-sided PWBs.
nr = no response
Exhibit 3-7. Through-Hole Metalizing Methods Data
Exhibit 3-8. Distribution of Through Hole Metallization Methods Employed by Survey Respondents

3.3.5 Outer-Layer Etch Resist

Outer layer etch resist application is a process found within the outer-layer image transfer cluster. This cluster is different from other PWB use clusters in that many of the alternatives that form the cluster are not available to the manufacturer as true alternatives. For example, the choice of etch resist is determined by economic considerations, downstream processes, and end-user specifications. If electrolytic gold is specified by the end-user, for example, gold must be plated as an etch resist. If gold is not specified, it is almost never considered as an alternative to tin or tin-lead due to obvious economic considerations (thin electroless gold finishes have been considered as a replacement for tin-lead from time-to-time). Therefore, genuine choices among the cluster elements are rather limited.

The elimination of lead plating has been a goal of many PWB manufacturers due partly to strict local discharge limitations. Tin-lead is plated as an etch resist, then, on panels subsequently processed with solder-mask-over-bare-copper (SMOBC), the tin-lead coat is promptly stripped. Therefore, when the predominant SMOBC process is specified, tin-lead is easily replaced by tin as the etch resist of choice. Unfortunately, a minority of PWBs still require tin-lead reflow and these panels must be processed with a tin-lead etch resist, which is subsequently fused into solder. Many shops cannot, for economic or other reasons, maintain both tin and tin-lead plating lines and are thus unable to employ tin-only plating on that portion of their product which is SMOBC. In short, the transition from tin-lead plating to tin-only has been slow.

The survey results indicate that 60% of respondents use tin-lead plate for at least a portion of their production, including 35% that use tin-lead plate for the majority of their work (Exhibits 3-9 and 3-10). The results also show that 41% avoid the use of tin- plating completely. Dry film is the predominate resist for 19% of the survey respondents and 38% use dry film as an outer-layer etch resist on at least a portion of their product. Forty-three percent of the respondents indicated that they employ nickel-gold plate as an etch resist on some of their product, although none plated nickel-gold on more that 20% of their total production.

PWB Type
(% board ft2)
Etch Resist Type
(% of outer layers)
Respondent
ID
Production
(board ft2 per year)
Rigid Non-
Rigid
Tin Tin-Lead Dry Film Nickel-
Gold
Other
Resist Type
36930A nr 0 100 0 95 0 5 0
955099 nr 100 0 33 67 0 0 0
55595 nr 100 0 0 0 0 0 0
44486 nr 100 0 100 0 0 0 0
955703 nr 100 0 0 0 100 0 0
6710 15,000 100 0 0 100 0 0 0
947745 40,000 100 0 0 95 2 3 0
44657 42,358 100 0 0 95 0 5 0
29710 57,000 100 0 100 0 0 0 0
502100 60,000 0 100 0 5 95 0 0
32482 75,000 90 10 90 5 0 5 0
25503 90,000 5 95 1 2 96 1 0
36930 96,000 100 0 96 2 0 2 0
965874 175,000 100 0 85 0 15 0 0
953880 180,000 100 0 0 100 0 0 0
33089 200,000 100 0 0 95 5 0 0
T3 200,000 98 2 0 80 10 10 0
3470 240,000 100 0 75 10 10 5 0
43841 250,000 100 0 0 85 10 5 0
279 250,000 100 0 0 0 65 0 351
237900 273,000 100 0 0 100 0 0 0
273701 280,000 98 2 50 45 0 5 0
41739 300,000 100 0 0 0 100 0 0
959951 320,000 100 0 65 35 0 0 0
42692 360,000 100 0 95 5 0 0 0
358000 500,000 100 0 98 0 0 2 0
43694 500,000 0 100 0 0 100 0 0
37817 540,000 100 0 0 0 2 0 982
42751 540,000 100 0 80 0 0 20 0
T2 600,000 100 0 100 0 0 0 0
133000 600,000 0 100 0 95 0 5 0
T1 936,000 100 0 0 99 0 1 0
740500 1,800,000 100 0 0 0 100 0 0
946587 1,900,000 100 0 94 1 0 5 0
3023 2,300,000 100 0 0 100 0 0 0
31838 3,000,000 100 0 0 0 0 0 0
462800 3,750,000 100 0 0 0 0 0 1001
107300 5,000,000 100 0 98 0 0 2 0
Exhibit 3-9. Outer-Layer Etch Resists Data

1 Screened ink.
2 SES-R (liquid polymer).
nr = no response

Exhibit 3-10. Distribution of Outer-Layer Etch Resists Employed by Survey Respondents

3.3.6 Outer Layer Etchant

The choice between cupric chloride and ammoniacal etchants for outer layer etching is determined to a large extent by compatibility with the various etch resists employed. Metallic etch resists are generally incompatible with cupric chloride, greatly limiting its use on outer-layers regardless of whatever advantages it may offer. Ammoniacal etchant is generally compatible with all etch resists and therefore is very common as an outer-layer etch resist. Sulfuric-peroxide is also compatible with metallic resists.

Only 13% of the survey respondents indicated they use cupric chloride on any outer-layer panels, while 87% indicated they use ammoniacal etchant exclusively for outer-layers (see etchant data presented previously in Exhibits 3-2 and 3-3). Two respondents (6%) indicated that they use sulfuric-peroxide on all outer-layers as an outer-layer etchant.

3.3.7 Solder Masks

Solder mask application is found in the surface finish use cluster. Most PWBs, including nearly all high-technology circuits, require solder mask. The survey results show a very substantial use of liquid photoimageable masks (LPI). Seventy-six percent of respondents apply LPI to at least a portion of their product (Exhibit 3-11). Thermal masks are used by 74% respondents. Forty percent use dry film masks on at least some of their product. A significant percentage of respondents indicated that they use all three common mask types (26%).

PWB Type
(% board ft2)
Solder Mask Types
(% of total production)
Respondent
ID
Production
(board ft2 per year)
Rigid Non-
Rigid
Thermal
Masks
Dry Film LPI1 Screened LPI Curtain-
Coated
36930A nr 0 100 2 0 98 0
955099 nr 100 0 60 1 39 0
55595 nr 100 0 90 0 10 0
44486 nr 100 0 10 5 80 0
955703 nr 100 0 5 15 80 0
6710 15,000 100 0 10 0 0 0
947745 40,000 100 0 58 2 40 0
44657 42,358 100 0 20 0 80 0
29710 57,000 100 0 0 100 0 0
502100 60,000 0 100 0 0 0 0
32482 75,000 90 10 65 0 35 0
25503 90,000 5 95 0 0 0 0
36930 96,000 100 0 5 5 90 0
965874 175,000 100 0 80 0 20 0
953880 180,000 100 0 40 20 40 0
33089 200,000 100 0 100 0 0 0
T3 200,000 98 2 40 5 50 5
3470 240,000 100 0 50 10 0 40
43841 250,000 100 0 0 50 50 0
279 250,000 100 0 95 0 5 0
237900 273,000 100 0 1 0 99 0
273701 280,000 98 2 60 0 0 40
41739 300,000 100 0 80 0 20 0
959951 320,000 100 0 70 0 30 0
42692 360,000 100 0 5 14 0 80
358000 500,000 100 0 1 0 8 90
43694 500,000 0 100 0 0 0 0
37817 540,000 100 0 50 50 0 0
42751 540,000 100 0 0 0 100 0
T2 600,000 100 0 0 0 40 60
133000 600,000 0 100 5 95 0 0
T1 936,000 100 0 15 5 0 80
740500 1,800,000 100 0 nr nr nr nr
946587 1,900,000 100 0 0 0 15 85
3023 2,300,000 100 0 23 11 66 0
31838 3,000,000 100 0 0 0 100 0
462800 3,750,000 100 0 5 0 95 0
107300 5,000,000 100 0 35 0 65 0
Exhibit 3-11. Solder Mask Data

1 Liquid photo-imageable.
nr = no response

3.4 Chemical Usage

Chemical usage data were collected from respondents for their most commonly used chemicals. These data have been organized according to the process in which they are used and are presented in Appendix B. It is anticipated that these data will be used in future projects to aid in determining cost savings associated with the implementation of pollution prevention options.


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