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Repair of printed boards carrying SMCs Reflow soldering

4.4 Methods of remounting:

4.4.1 Soldering iron plus solder wire:

a) Components with few connection leads:

The small solder tip for small components must be placed on the solder pad, not on the lead of the component. Thin (0.5-0.8mm) solder wire is used, solder alloy tin60-lead40 with a mildly activated rosin flux, maximum soldering time 3 sec.

b) Components with many connection leads:

These components are soldered by placing the component on the solder pads with a pair of tweezers and reflowing the solder at the edges of two pads on opposite diagonals. The component is now held in place. Soldering is then completed as usual with solder wire as given above.

4.4.2 Reflow with hot air:

This method is used for medium or large quantities. Special tools are used. No excess solder is allowed on the solder pads. Reflow excess solder with the help of hot air and a little and a little flux. Pre-tin the solder pads. Apply a small amount of flux on the solder pads before soldering. The component is held in an adapted tool and is placed on the solder pads. Soldering is performed with hot air.

4.4.3 Resistance soldering:

This method can be used for medium or large quantities. Special tools are needed. The pre-tinned solder pads must be reflowed using hot air and a small amount of flux, to be sure that the solder on the pads is smooth. After placing the components and refluxing the pads, the components can be attached by resistance soldering or the hot block method. Care should be taken that all leads are soldered onto the board.

When components with many leads are used, it is necessary to carefully check the correct position of the leads on the solder lands, before soldering. Soldered joints must be checked with a magnifying glass (magnification of 5 to 10). Special attention has to be paid to icicles and loose solder balls. All ICs and many other semiconductors are susceptible to electrostatic discharges. Careless handling during repair can reduce life drastically. Operators must be connected to the same potential as the mass of the set via a wrist strap with a resistance of 100 Kohm. Components and tools must also be kept at this potential. Application of flux should be performed carefully, in order to avoid as far as possible unnecessary wetting of the adjacent components. Only noncorrosive should be used (mildly activated rosin flux).

5. Repair of damaged surface mount pads:

Damage to surface mount pads likely to occur during the removal or replacement of a component. It is relatively far difficult to repair a damaged pad area when compared with that of replacement of a component. However, by using proper materials combined with precise method it is possible to reliably replace a damaged surface mount pad to a large extent.

1. Selection of proper pad from circuit frame : Copper foil (1 or 2 oz.), chemically etched into a circuit frame of shapes exactly matching the shape of the pads needing replacement, is selected. The entire circuit frame, plated with 60/40 solder on the top surface, features an oxide treatment for adhesion on the bottom surface. A dry film application of thermoset epoxy is evenly applied to the treated side. The individual pads on the circuit frame are trimmed out and bonded to the board surface in a small bench-top bonding press.

2. The extent of damage is to be evaluated. If the SMD is still in place it must be removed and the area cleaned. A proper inspection of each pad can be made by using a stereo microscope at a minimum of 10X magnification. If any damage to the board surface is severe it must be repaired using epoxy before the pad is replaced.

3. Examine the board surface to decide if the area is suitable for a replacement. The new surface mount pads have a dry film epoxy applied to the back. The dry film epoxy is 0.0018 thick. Most minor scratches or defects do not require any resurfacing.

4. All defective pads are removed. The connection is trimmed back to the point where a good bond exists. Any soldermask is scraped from the remaining portion of the pad area where new pad is going to overlap.

5. A replacement pad is selected and the area on the PCB where old pad was resting, the area is cleaned of any epoxy residual.

6. The new pad is placed in position on the PCB and held in place with high-temperature tape. The tape helps in keeping the pad in place till the bonding is complete.

7. Bonding is completed using a press or iron. Pressure is 100 psi (2- 6 oz); temperature, 425 0 F (2180 C); bonding cycle ,30 sec.

8. The electrical connection is completed with a lap solder joint. The lap should be about 1/16.

Reliability of the method is assured through the use of proper material (replacement pads are not salvaged from scrap PCBs). Also, precise shapes are used, which are dry film epoxy baked, eliminating the use of frequently messy liquid epoxies.

In summary, there are various options available to the operator for removal of components, excess solder, and mounting of components. Appropriate method could be chosen on the availability of hardware, necessity to reuse the removed components, type of component being reworked etc. Also, a method of repairing damaged circuit pads is also discussed.

Repair of printed boards carrying SMCs Reflow soldering
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