SMT: Manual Soldering And Repair Process

7 Manual soldering and repair

1. 1. Introduction
2. 2. Use of soldering iron
   1. 2.1 Hot gas soldering
   2. 2.2 Resistance soldering
3. 3. Basic rules for hand-soldering
4. 4. Desoldering and removing of components
   1. 4.1 Desoldering of DIL-type packagesg
   2. 4.2 Repair of printed boards carrying SMCs
   3. 4.3 Removal of SMCs from PCBs
   4. 4.4 Methods of remounting
5. 5. Repair of damaged surface mount pads

1. Introduction

Manual soldering with a soldering iron is used in the following occasions:

1. 1. small production runs

2. 2. laboratory assembly work

3. 3. space applications

4. 4. rework / repair

5. 5. for the addition of extra components to a machine soldered assembly.

In most cases the solder is applied in the form of flux-cored wire. In making soldered joints by a manual method, the operator can watch the progress of the joint formation, and adaptation of the soldering process to varying conditions, such as of solderability. With this in mind, hand soldering is still used for the making electrical connections with the highest reliability requirements, e.g. for space applications. When mass soldering is involved, quality hand soldering is not economically viable.

To assure reliable manually soldered joint requires considerable management involvement. It must

1. * Assure selection and training of operators

2. * Provide written procedures

3. * Research for proper equipment selection

4. * Establish equipment knowledge

5. * Train in handling ESD, and

6. * Ensure environmental cleanliness.

2. Use of the soldering iron

Hand soldering is a skill, obtained by training and experience. Soldering irons are heated by an electrical element. The heat must be transported from the element to the tip with which the soldering is done. The metallic piece with a tinned tip, acting as heat store and heat conductor, is called a soldering bit. The temperature of the tip must be higher than the soldering temperature, because the heat transfer from the tip to the joints is relatively poor. The soldering iron should not be used as a lever. A well tinned soldering tip is indispensable for making good joints. Cleaning should never involve a file or a steel brush, but only a cloth or a wet sponge. Bits for soldering irons are commercially available in a variety of shapes, including special hollow bits for suction desoldering iron. Standardization of the tips has been proposed but has not yet been realized.

Three aspects of manual soldering are important.

1. i) how to operate the soldering iron

2. ii) how to connect wires to various connecting parts, terminals, etc.;

3. iii) how to solder various components to printed boards.

In working with surface mount components, in addition to ordinary soldering irons as detailed above, hot gas soldering and resistance soldering are used widely. Though hot gas soldering is non-contact type, resistance soldering is of contact type wherein the heating elements come in contact with the point of soldering. Typical operation and applications are given below.