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1、BRITISH STANDARD BS 6221-20: 1984 Incorporating Amendment No. 1 Printed wiring boards Part 20: Guide for the assembly of printed wiring boards UDC 621.3.049.75 Licensed Copy: sheffieldun sheffieldun, na, Tue Dec 05 07:54:20 GMT+00:00 2006, Uncontrolled Copy, (c) BSI BS 6221-20:1984 This British Stan
2、dard, having been prepared under the direction of the Electronic Components Standards Committee, was published under the authority of the Board of BSI and comes into effect on 30 April 1984 BSI 10-1999 The following BSI references relate to the work on this standard: Committee reference ECL/19 Draft
3、 for comment 83/20739 DC ISBN 0 580 13771 6 Committees responsible for this British Standard The preparation of this British Standard was entrusted by the Electronic Components Standards Committee (ECL/-) to Technical Committee ECL/19 upon which the following bodies were represented: British Plastic
4、s Federation British Telecommunications Electrical and Electronic and Insulation Association (BEAMA) Electronic Components Industry Federation Electronic Engineering Association ERA Technology Ltd Institute of Circuit Technology Institute of Metal Finishing Institution of Production Engineers Minist
5、ry of Defence National Supervising Inspectorate Scientific Instrument Manufacturers Association (BEAMA) Society of Motor Manufacturers and Traders Limited Telecommunication Engineering and Manufacturing Association (TEMA) Amendments issued since publication Amd. No.Date of issueComments 5991July 198
6、9Indicated by a sideline in the margin Licensed Copy: sheffieldun sheffieldun, na, Tue Dec 05 07:54:20 GMT+00:00 2006, Uncontrolled Copy, (c) BSI BS 6221-20:1984 BSI 10-1999i Contents Page Committees responsibleInside front cover Forewordii 1Scope1 2Definitions1 3Handling of components prior to asse
7、mbly1 4Stoving2 5Mounting of components2 6Board assembly4 7Solderability: process considerations6 8Soldering8 9Cleaning of boards and assemblies8 10Flatness8 11Inspection of printed board assemblies9 12Storage conditions9 13Terminals and connector jacks9 14Conformal coatings10 15Assembly of surface
8、mounted devices12 Figure 1 Simple offset preformed leads3 Figure 2 Dimple preformed leads3 Figure 3 Component sequencing7 Figure 4 Single-sided assembly for SMDs only15 Figure 5 Double-sided assembly for SMDs only16 Figure 6 Mixed technologies with SMDs on only one side of PWB16 Figure 7 Mixed techn
9、ologies with SMDs on both sides of PWB17 Figure 8 Condensation reflow18 Figure 9 Conveyorized infra-red reflow furnace19 Figure 10 Dual wave soldering19 Table 1 Devices sensitive to static discharge1 Table 2 Static resistance and acceptable discharge times1 Publications referred toInside back cover
10、Licensed Copy: sheffieldun sheffieldun, na, Tue Dec 05 07:54:20 GMT+00:00 2006, Uncontrolled Copy, (c) BSI BS 6221-20:1984 ii BSI 10-1999 Foreword This British Standard has been prepared under the direction of the Electronic Components Standards Committee. This Part of BS 6221, together with Parts 3
11、 and 21, supersedes BS 5370 which is withdrawn. The text of this Part has been submitted to the International Electrotechnical Commission (IEC) for consideration as an International Standard. A British Standard does not purport to include all the necessary provisions of a contract. Users of British
12、Standards are responsible for their correct application. Compliance with a British Standard does not of itself confer immunity from legal obligations. Summary of pages This document comprises a front cover, an inside front cover, pages i and ii, pages 1 to 20, an inside back cover and a back cover.
13、This standard has been updated (see copyright date) and may have had amendments incorporated. This will be indicated in the amendment table on the inside front cover. Licensed Copy: sheffieldun sheffieldun, na, Tue Dec 05 07:54:20 GMT+00:00 2006, Uncontrolled Copy, (c) BSI BS 6221-20:1984 BSI 10-199
14、91 1 Scope This Part of BS 6221 provides guidance on the assembly of components on printed wiring boards irrespective of their method of manufacture. NOTEThe titles of the publications referred to in this standard are listed on the inside back cover. 2 Definitions For the purposes of this Part of BS
15、 6221, the definitions given in BS 6221-3 apply, together with the following. 2.1 taped components components attached to continuous tapes 2.2 screen printing a process for transferring an image on a surface by forcing a suitable media through a stencil screen with a squeegee 3 Handling of component
16、s prior to assembly 3.1 Static 3.1.1 It is necessary to safeguard certain types of components and devices against possible damage which can be caused by an electrostatic discharge (ESD). The susceptibility of devices to static damage can vary considerably, but generally those devices constructed wit
17、h thin oxide and extremely thin conducting paths only a few micrometres wide are most susceptible. Table 1 below gives some typical values of static discharge to which devices are reported susceptible. Table 1 Devices sensitive to static discharge It is considered that, for practical purposes, if st
18、atic potentials in the working environment can be held at less than 100 V, static problems will be greatly reduced. 3.1.2 When considering the handling of static sensitive devices, the following two general rules should be observed: a) all work areas, manual or automated in which static sensitive de
19、vices are handled should be static safeguarded areas; b) static sensitive devices/components or assemblies require the use of static protected containers during transport or storage. 3.1.3 In the assembly area, conductive items of equipment including tools, work stations, machinery and personnel can
20、 be grounded by a variety of methods obtainable from specialist suppliers. A resistance to ground of less than 1 G7 is considered sufficient to discharge to a safe level in less than 1 s; however, a minimum of 1 M7 is necessary to limit current flow to a safe level. Table 2 shows the maximum allowab
21、le resistances and discharge times for static safe operations involving personnel, bearing in mind the capacitive effects of the human body. Table 2 Static resistance and acceptable discharge times 3.1.4 Shielding of components sensitive to static should be carried out from the receipt of component
22、stage. The protection supplied by the device supplier should be maintained as long as practicable. Where dual in-line (DIL) packages are supplied in static shielded shipping tubes, these can interface directly to automatic insertion equipment and the devices need not be handled. 3.1.5 The likelihood
23、 of static damage is reduced but not eliminated once components are assembled to the printed board. Assembled boards should be transported or stored in suitably grounded conductive containers, and boards with edge connectors should be fitted with contact shunting shrouds to maintain constant board v
24、oltage levels. 3.1.6 Where boards are to be packaged in plastics bags or containers, the materials of the bag or container should be capable of preventing a static charge existing across the package. Device typeRange of ESD volts VMOS30 to1 800 EPROM100 max. MOSFET GaAsFET JFET SAW OP-AMPS CMOS Scho
25、ttky diodes Film resistors Bipolar transistors ECL (hybrid, PB level) SCR Schottky TTL 100 to 200 to 140 to 150 to 190 to 250 to 300 to 300 to 380 to 500 to 680 to 1 000 to 200 300 17 000 500 2 500 2 000 2 500 3 000 7 000 1 500 1 000 2 500 Maximum tolerable resistance from operator to ground via: Ac
26、ceptable discharge time Floor mat: 1 G7 Table mat: 1 G7 Wrist strap: 100 M7 1 s 1 s 0.1 s Licensed Copy: sheffieldun sheffieldun, na, Tue Dec 05 07:54:20 GMT+00:00 2006, Uncontrolled Copy, (c) BSI BS 6221-20:1984 2 BSI 10-1999 Generally, metallized film materials are more satisfactory as they provid
27、e true electrostatic shielding. NOTEFor more detailed information on the handling of electrostatic sensitive devices, see BS 5783. 3.2 Edge connectors 3.2.1 Edge connectors formed as part of a printed board (integral) or provided as a wired on (discrete) plug end are generally plated with a precious
28、 metal on the contact area. It is important that the surface of the contact area is protected from damage or contamination during assembly processes. 3.2.2 Where the edge connector is integral with the board, it will be necessary to protect the contact area with a heat-resistant tape or shield durin
29、g the mass soldering process. Typically, a stainless steel shield may be employed to surround the edge contacts and prevent solder wetting. However, this should be combined with a tape or heat-resistant plastics shroud that protects the contact area from flux creepage. Any tape used for this purpose
30、 should have a non-transferable adhesive. Specialist tapes are available but should be checked for adhesive transfer before use. 3.2.3 After processing any tape, etc. should be removed from the contacts before the board is cleaned. 3.2.4 Subsequent to cleaning operations the edge connector should be
31、 protected preferably with a slip-on moulded plastics shroud that combines protection with easy availability for test, etc. The shroud should be of a shape that clips onto the board beyond the contact zone. To prevent the scratching of the contact zone by grit that may have contaminated the shroud,
32、the shroud should be pushed on to the board from the side. NOTEOnly plastics with stable plasticizers should be used to prevent transfer of plasticizers onto edge contacts. 3.2.5 Discrete edge conductors, not subject to mass soldering, should be protected either as in 3.2.4 or by a tape having adhes
33、ive edges and a clear centre section positioned over the contacts. 4 Stoving 4.1 General. Boards should be placed in an air circulating oven such that there is free circulation of air round each board and stoved at 120 5 C for a maximum of 2 h. 4.2 Double-sided printed wiring boards. In order to min
34、imize blow holes in joints of mass soldered plated-through hole double-sided printed boards, it is advisable to stove before assembly and within one week of the soldering operation. 4.3 Stoving multilayer and flexible printed wiring boards. It is recommended that multilayer and flexible printed wiri
35、ng boards be stoved before being soldered. This action is necessary to drive out slowly the absorbed moisture and promote any gassification that may otherwise result in delamination or outgassing during mass soldering. NOTESpecifications for multilayer circuits and flexible circuits normally require
36、 specimens to be stoved at 125 5 C for a minimum of 1 h before testing interlaminal bond using thermal shock. 5 Mounting of components 5.1 Fitting 5.1.1 General. Horizontally mounted, non-polarized components should be oriented so that component marking and colour codes may be “read” in the same dir
37、ection. Vertically mounted, non-polarized components should preferably be mounted so that the markings can be read from top to bottom. All polarized components should be oriented so that their positive (+) or negative () symbols are visible. Components should be mounted on only one side of the board
38、, generally opposite the major circuit pattern, unless otherwise permitted by design, e.g. surface mounted components. The projection of the outline of the component on the board should not extend over the edge of the board or interfere with board mounting. It should preferably be a minimum of 1.5 m
39、m from the edge of the board and board guide or mounting hardware. 5.1.2 Accessibility. The placement of any component on the board should not prevent the insertion or removal of any hardware (tool clearance included) used to mount the assembly, unless the assembly is considered disposable and is no
40、t intended for repair. 5.1.3 Centreing. The body of the component should be approximately centred between the lead span, whenever possible and practical. 5.1.4 Horizontal placement of axial-lead components. Axial-lead components should be mounted so their axis is parallel to the mounting base. Unles
41、s otherwise specified by the user, components may have a portion of the body in direct contact with the printed wiring board. 5.1.5 Components mounted over conductors. Components should not be placed in contact with more than one conductor unless the board surface is suitably protected from moisture
42、 traps. NOTEThis recommendation applies to components with or without sleeving. Licensed Copy: sheffieldun sheffieldun, na, Tue Dec 05 07:54:20 GMT+00:00 2006, Uncontrolled Copy, (c) BSI BS 6221-20:1984 BSI 10-19993 Care should be taken when mounting metal-cased components to ensure the casing does
43、not come in contact with the conductive pattern. Sleeving may be used to insulate metal-cased components. The sleeving should be sufficiently transparent so that the identification markings remain visible and legible. Sleeved components should not be mounted over or in contact with sharp solder conn
44、ection points. Adhesive pads of insulating material may also be placed under the component instead of sleeving but it is not good practice to rely on a solder resist film in this context. Thermal degradation of components may result from contact with conductors on lands during mass soldering; adhesi
45、ve pads may also be used as a thermal barrier. 5.1.6 Vertical component mounting. Axial-lead components mounted perpendicular to printed wiring boards should be installed with a minimum of 0.4 mm and a maximum of 3.0 mm clearance between the end of the component body and the surface of the circuit b
46、oard to prevent entrapment problems. There should be no strain on the component lead when a spacer is used. Vertically mounted radial-lead components should have their leads parallel to each other and perpendicular to the edge of the component-mounting base. Such components should have their vertica
47、l axis within 15 of the plane that is perpendicular to the component-mounting base, unless otherwise restricted by design. 5.1.7 Moulded or encapsulated ceramic or plastics components. Moulded or encapsulated ceramic or plastics components should be mounted so that the coating or sealing material do
48、es not enter the solder joint area. Components installed in plated-through holes should maintain a minimum clearance of 1.5 mm between the surface of the board and the end of the lead coating. When the holes are non-plated-through holes, the minimum clearance can be reduced to 0.75 mm. 5.2 Lead form
49、ing. The purpose of preforming parts for retention to the board is to combine the advantages of labour reduction achieved in straight through leads and the mechanical reliability of the clinched lead. Simple forms are used only to hold parts of the board prior to soldering; complex forms can be used for mechanical strength. The forming of leads can be from a minimum of a simple offset to a complex compound form. The major factors affecting the forming of leads are board thickness, lead diameter, lead material, hole size, and tooling system req