12 Flexible Printed Circuit Boards
13.17 Re-work and Repair of Printed Circuit Boards
When printed circuit boards are inspected and tested, whether bare or fully loaded (assembled), and defects are found, it is necessary to evaluate the cost-effectiveness of repairing the board, and at the
same time provide the user with the same reliability as the original product. In case of simple boards with a few defects, it is usually not economical to re-work on them. However, many boards are highly complex and a fully loaded board could be expensive. It may turn out to be more economical to re-work on the same so that it will pass all the tests.
Bare boards, in general, are often not repaired because of the reliability risks associated with their later utilization in assemblies and because of their comparatively low cost as compared to assembled boards. In view of these factors, repairs or re-work of bare boards are not allowed in high reliability and military applications. Bare board re-work is permissible for boards which are used for commercial applications, as part of in process connections. However, the repaired board must meet the original design requirements and the expected reliability and quality standards.
Further, repair and re-work is also required on the boards received for repairs from the field. In most such cases, there may be a requirement of removing and replacing a component with a new one. The exercise is normally undertaken manually. For plated through-hole boards, the repair work can be done by simple tools such as soldering iron and wicked braid. For surface mount components, on the other hand, special re-work stations are required which depend upon hot air re-flow soldering units. During repair, a number of chemicals are used specially for cleaning, moisture displacement, flux removal, wiper lubricants and freeze sprays to locate thermally sensitive components.
13.17.1 Approaching Components for Tests
Most designers provide test points at convenient locations on the circuit board. These points are defined by specific dc and ac voltages, along with the waveform pattern. Figure 13.46 shows a test point as it would appear on a circuit board. This is usually a vertically mounted pin to which a test prod can be attached.
TP1
Test pin TP3
Fig. 13.46 Typical test points indication in a printed circuit board
Soldering, Assembly and Re-Working Techniques 543
If specific test points are not provided, measurements can be made at various points on the circuit by approach- ing various components. In that case, proceed as follows:
(a) For transistors, make test prod connection to the legs under the case
(b) To read the signal on a circuit board trace, lo- cate a component that is connected to the trace as shown in Figure 13.47. Clip your test lead onto the leg of the component that is connected to the trace.
(c) Connections to the ICs can be made more con- veniently by using an IC test clip. This is illus-
trated in Figure 13.48. Be careful not to touch more than one conductor at a time, otherwise you can easily create a short-circuit. Since digital circuits are usually densely packed on a board, make use of only as narrow a prod as possible.
(Wire gripper holds IC pin when clipis attached) (a)
IC test clip
IC
(b)
Fig. 13.48 Use of test clip for taking measurement on IC pins (a) test clip on the IC directly (b) Test clip on the IC connector
(d) Flexible flat wire with connectors often offer good place to take readings. The connector pins them- selves are usually well protected, but you can take readings at the conductors behind the connectors as shown in Figure 13.49.
13.17.2 De-soldering Techniques
Desoldering means removal of solder from a previously soldered joint. The two techniques common in soldering are:
a Wicking, and a Sniffing.
You can attach your test clip here
To test the signal on this trace Fig. 13.47 Taking measurements from the circuit
trace by connecting a test prod on the component
Take readings at the exposed pins at the rear of the connector
Fig. 13.49 Taking test readings from a connector
13.17.2.1 Wicking
In the wicking process, a heated wick, well-saturated with rosin, is placed on top of the joint to be de-soldered. The solder will flow rapidly into the rosin area due to capillary action leaving the joint to which it was previously affixed.
A wicking solder remover may consist of a braided shield wire with the core removed or it may be a piece of multi-strand wire. Wicks are available commercially which are suitable for de-soldering work. The de-soldering technique using the wicking process is as follows (Figure 5.15):
a Place the wick on top of the solder joint to be de-soldered.
a Position the iron tip on top of the wick. The heat of the iron will melt the solder. The solder will readily flow into the wick.
a Cut off the wick containing the removed solder. Repeat the process until all the solder is removed from the joint.
Take extreme care to ensure that the solder is not allowed to cool with the braid adhering to the work, otherwise you run the risk of damaging PCB copper tracks when you attempt to pull the braid off the joint. This technique is more effective specially on difficult joints where a desoldering pump, described below, may prove unsatisfactory.
13.17.2.2 Sniffing
In sniffing, a rubber ball (Figure 13.50) is employed as a solder sucker (sniffer). The sniffer uses the forced air pressure to accomplish the sniffing (removal of solder) action.
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Soldered joint Wick Soldering
iron
Component side Wiring side
IC
P.C.
board Solder filler Miniature
soldering tip De-soldering bulb
(a) (b)
Fig. 13.50 (a) wicking process for de-soldering (b) sniffing technique in de-soldering
Another vacuum type sniffer uses a spring loaded plunger.
The following steps are adopted in sniffing:
a The air is first squeezed out of the rubber ball.
Soldering, Assembly and Re-Working Techniques 545
a With the ball depressed, the pointed end of the sniffer tube is placed next to the solder to be removed.
a The joint is heated with the soldering iron. The tip of the iron should be kept in the solder and not on the sniffer.
a The pressure on the sniffer ball is slowly released to allow air to enter the ball through the sniffer tube. As the air enters, it pulls the molten solder into the tube with it.
a After the solder has been completely pulled into the tube with it, the sniffer is removed from the joint. By depressing the ball again, the collected solder can be forced out.
A de-solder pump is another device for solder removal. It uses a spring-loaded mechanism. For using the device (Figure 13.51), the spring is cocked and the tip of the vacuum pump is held against the solder joint. When the solder melts, the trigger is operated which releases the spring, thereby creating a powerful vacuum action. Some of these devices can generate a static charge. Be sure to get a type that is specified as ‘anti-static.’
Fig. 13.51 (a) vacuum de-soldering pump and (b) use of a vacuum pump for use in de-soldering
For stubborn joints or those connecting to the power planes (surface or multi-layer boards), you may need to add some fresh solder and/or flux and then try again. Generally, if you only get part of the solder off the first time, repeated attempts will fail unless you add some fresh solder.
A very important consideration which must be kept in mind while de-soldering is that the heat required may damage the base materials and adjoining components. The de-soldering should be carried out by using appropriate tools so that a minimum amount of heat is used during the de- soldering process.
During any repair work, it is well worth taking time and care so as not to damage or lift copper back from the printed circuit board, as the printed circuit board is usually a very expensive item.
Do not use a sharp metal object, such as a twist drill for removing solder from component mounting holes. Sharp objects may damage plated through-conductor.
Removing multi-lead components such as integrated circuits presents a special problem. If the component to be removed is still functional, it must be de-soldered quickly lest it be damaged by heat. Alternatively, if the device is defective, it also needs to be removed fairly quickly to avoid lifting of printed circuit foil conductors by excessive heat.
Specialized devices are needed to solve this problem. One such device is a special DIP-shaped soldering iron tip (Figure 13.52) and a spring-loaded IC extractor tool. The tool is placed above the IC to be removed and locked into position. When the tip is hot, it is applied to all the dual-in line IC pins or the foil side of the board. The extractor tool lifts
the IC off the board as soon as the solder holding it melts.
Special desoldering tools are available for use with other IC and transistor cases.
Circuits coated with silicone conformal coatings may be repaired after removing the coating using solvent- swell or mechanical abrasion techniques, the defective device can be de-soldered and removed. Standard burn- through techniques can also be used.
After removing the old solder, the area should be thoroughly cleaned with a solvent-soaked swab to ensure a good replacement joint. After component installation, re-coating can be accomplished.
13.17.3 Replacement of Components
Printed circuit boards used in modern equipments are generally the plated-through type consisting of metallic conductors bonded to both sides of an insulating material. Before a component replacement is attempted, the following precautions should be observed:
a Avoid unnecessary component substitution. It can result in damage to the circuit board and/
or adjacent components.
a Do not use a high power soldering iron on etched circuit boards. Excessive heat can dislodge a conductor or damage the board.
a Use only a suction device or wooden toothpick to remove solder from component mounting holes. Never use sharp metal object for this purpose as it may damage the plated through- conductor.
a After soldering, remove excess flux from the soldered areas and apply a protective coating to prevent contamination and corrosion.
The following steps are to be followed for replacing a component:
a Read carefully the replacement procedure from the service manual of the instrument.
a Switch-off the power, if applicable.
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For dual-in- line type For TO-5 type
Heating element
Fig. 13.52 Special de-soldering tip for inte- grated circuits
Soldering, Assembly and Re-Working Techniques 547
a Remove any assemblies, plugs, wire that will facilitate repair work.
a Label the component to be removed.
a Observe carefully how the component is placed before removing it. Record information regarding polarity, placement angle, positioning, insulating requirements and adjacent components.
a Be careful to handle the printed circuit board by the edges only. Fingerprints, which though invisible, can cause an accumulation of dirt and dust on the boards, resulting in low impedance bridges in portions of the circuit board, which normally should have a very high impedance.
Gloves should preferably be worn to prevent fingerprint problem if the boards must be handled.
a Remove the dry film or the hermetic sealer from the solder joint that is to be worked on.
This is done by using a cotton tipped applicator dipped in the recommended chemical.
Large quantities of solvents should not be allowed to drip on the board because the impurities will then only be shifted from one place to another on the board. This cleaning is necessary because it can be difficult to burn through a layer of dry film with a soldering iron. In addition, if the dry film is not removed before heating, the appearance of the board will be badly changed.
a Heat the solder fillet on the solder side of the printed circuit board. Using the de-soldering tool (suction device), gently and carefully remove the component. Too much soldering iron heat should not be used otherwise the foil is lifted or plated through-holes get removed.
a In case of multi-lead component, the vacuum desoldering tool must be used to remove almost all the solder from the component leads before the component can be removed from the board. This procedure must be carefully followed because multi-lead components multiply the probability of printed circuit board damage during repairs.
a Some components are difficult to remove from the circuit boards due to a bend placed in each lead during machine insertion of the components. The purpose of the bent leads is to hold the component in position during a flow solder manufacturing process which solders all components at once. In order to make removal of machine inserted components easier, straighten the leads of the components on the back of the circuit board using a small screw driver or pliers while heating the soldered connections.
a After removing the component from the printed circuit board, the area around the removed component must be cleaned up by using the cotton tipped applicator in a solvent. Also, there may be solder in the plated through-holes or other areas of the board that must be removed in order to allow easy insertion of a new component.
a Clean leads of new component or element with a cleaning tool, such as a braided tool. Use abrasives if required. In case of a wire lead, the insulation must be removed. The secret to a good solder joint is to make sure everything is perfectly clean and shiny and not depend on the flux alone to accomplish this. In case of multiple strands, form the strands. Tin to about 3 mm from insulated part.
a Shape the leads of the replacement component to match the mounting hole spacing. Insert the component leads into the mounting holes and position the component as originally positioned. Do not force leads into mounting holes because sharp lead ends may damage the plated through conductor.
a Start with a strong mechanical joint. Don’t depend on the solder to hold the connection together. If possible, loop each wire or component lead through the hole in the terminal. If there is no hole, wrap them once around the terminal. Gently anchor them with a pair of needle nose pliers.
a Heat the parts to be soldered, not the solder (Raby, 1994). Touch the end of the solder to the parts, not the soldering iron or gun. Once the terminal, wires or component leads are hot, the solder will flow via capillary action, fill the voids and make a secure mechanical and electrical bond. Apply the soldering iron to the joint and feed solder into it. The solder should be applied to provide a complete seal covering all elements. Be careful with the amount of solder and the amount of heat. Check the component side of the board for good solder flow. Remember SN63 is the best type of solder for soldering electronic components.
SN60 is acceptable.
a Remove the soldering iron and allow the solder to cool and solidify. Do not disturb the board for a while, otherwise you will end up with a bad connection, what is called a ‘cold solder joint’.
a Clean the area of splattered rosin flux and residue using isopropy1 alcohol. Be careful not to leave cotton filaments on the printed circuit board. Allow the circuit board to air dry completely.
a Apply protective coating, if possible, on the repaired area and allow this to air dry.
a It is always advantageous to check the integrity of the joint soldered or repaired. This check can be performed with an ohmmeter (multimeter) by measuring the resistance between the solder and the component lead. Any reading except a short reveals a defective joint. Recognize defective solder joints, that are cracked, pitted and cold stressed, have excessive flux or the impure solder.
a When working with semiconductor devices and microelectronic IC circuit components, a heat sink may always be used, while soldering. Also, when working on equipment having components like these, the specifications of allowable soldering iron sizes, voltage ranges and other factors must be studied. This is essential to understand the damage one can do if a unit is repaired improperly.
a While replacing components, it may be noted that mechanical shocks can seriously damage the components. For example, semi-conductors can get damaged by the high impact shock if dropped in a concrete floor even from a table height. Cutting of leads can also cause shock waves which may damage delicate or brittle components. Therefore, cutting or scratching of surfaces of components by the careless use of tools or sharp test probes should always be avoided.
Soldering, Assembly and Re-Working Techniques 549
a It is always wise not to remove or replace any component while the power is on. This may well produce voltage or current surges that could damage the component itself and other sensitive components in the circuit.