This first section is concerned with the consequences of having made defective parts, whilst the final sections focus on the practicalities of performing rework. Underlying this unit are the themes that rework is best avoided, and that the inspection/rework cycle is a vital part of the mechanism by which a company controls and improves its operation.
Every rework operation must involve three steps:
Diagnosis: Having located a fault which must be put right, try to find out first why it has occurred. Don’t start working on it until you have satisfied yourself that you have found the answer, and have made a record of it. Otherwise, you may destroy vital evidence, which could have helped prevent the fault recurring.
Remedy: Put the fault right.
Prevention: Make sure that whoever in the organisation could or should have prevented the fault from occurring knows what you have found and done about it, and if possible that this information is recorded. . . . The rework rate can be regarded as the fever thermometer of a manufacturing line. If nobody cares to read it, the patient may well be moribund before anybody has noticed that he is sick.
Rudolf Strauss in Surface Mount Technology
In this unit, the terms ‘defect’, ‘defective parts’ and ‘failures’ are considered in the context of manufacturing defects, although design non-conformances requiring product upgrade, or commercial non-conformances (resulting for example in returned unwanted goods), have equivalent cost and commercial impact.
After rework, there is always the unspoken question of the potentially adverse effect of the rework on product reliability, especially where uncontrolled hand processes are used. Engineering intuition is that a reworked joint will be less reliable than one which has not been reworked.
One confirmation of the damaging effects of corrective soldering was given by multi-company research carried out under the auspices of the National Physical Laboratory. The tests counted the number of through-hole joints on standardised sample boards which showed visible cracks after they had been exposed to temperature cycling between –20°C and 100°C. Boards which had been reworked with a soldering iron under controlled conditions were compared with boards which had not been reworked. Some of the NPL results are given in Table 1.
|% of cracked joints
(after 2,000 cycles –20°C to 100°C)
(Sample size: 2,000 joints)
The reasons for joint degradation are believed to be:
Three implications for rework methods are that:
‘Get at it early!’ cannot be emphasised too strongly. As far as possible the aim should be to identify problems on the machines as they occur and use appropriate in-process rework techniques to present good product to the stage following. However, when solder joints do not meet inspection acceptance criteria they can be reworked by one of two methods:
The full procedure is shown in Table 2 for both through-hole and SM components. The point is well made by Strauss that reworking is often a longer process than getting it right first time.
|Through Hole||Surface Mount|
|Insert component in hole||Apply solder paste|
|Wave solder||Target, align and place component|
|Clean PC assembly||Reflow PC assembly|
|Clean PC assembly|
|Through Hole||Surface Mount|
|De-solder old component||Unsolder old component|
|Remove old component||Remove old solder|
|Clean PC assembly||Clean PC assembly|
|Insert new component||Tin pads and leads or apply solder paste|
|Solder new component||Target, align, and place new component|
|Clean PC assembly||Reflow new component|
|Clean PC assembly|
In small-scale production in particular, where visual inspection and rework are carried out by the same operator or operators, it is important to keep the cost aspect in mind. It is tempting to say ‘I might as well touch up this joint while I am looking at it.’ Not only do joint quality and reliability suffer through this practice, but costs are liable to rise in an uncontrollable manner. ‘Rework for cosmetic reasons alone is an expensive and damaging luxury, and it should only be carried out if the customer or the market demands it and pays for it.’
Given that the FR-4 laminate is much thicker than the copper foil, the specific heat of the board is considerably higher that of the foil or solder joint. Typically four times as much heat energy is needed to bring the board up to soldering temperature as to heat the joint itself. Preheating the board, either locally or overall, before carrying out any rework, therefore considerably shortens the time necessary to complete a joint, and is particularly necessary with heavy multilayer boards. A stream of warm air directed against the underside of the board is the usual method of preheating, as this avoids localised hot spots which might distort the board.
For wave-soldered assemblies, where the SMDs are glued to the board surface, preheating to 60–100°C softens the adhesive joint and makes it easier to break during desoldering.
In repair or rework, the main consideration is to avoid excessive heating. Tests suggest that, during component removal, the internal temperature of components should stay below 100°C and adjacent solder joints below 150°C.
For most users this means:
The rationale for this is given in the sections which follow.
Hand-held, conductive (heating by contact) tools are better at targeting heat on the solder joints than are comparable hot gas/air (‘convective’) tools. Much of the reason for this lies in the fact that molten solder is a good heat transfer medium, having high thermal conductivity and the ability of a liquid to make contact with board, joint and components whatever their profile.
This generally means that suitable soldering irons can be used for multi-leaded component removal without worrying about over-heating adjacent components. Contrast the small amount transferred to adjacent components using a heated tool in Figure 1 with the removal of the same component by hot gas in Figure 2. The process is accomplished so quickly that the internal die temperature remains quite low, making the process safe even for sensitive components.
The tips must be designed to target heat at the lead/land interfaces. A number of modified soldering iron bits actually provide heat at the shoulder of the leads; from where heat is eventually carried down to the pad area. A heated tweezer allows the tip to be slipped over a component then squeezed together to direct heat much more directly and quickly into the lead/land area. One performance comparison showed that a tweezer could remove a large PLCC package in less than 10 s, whereas an adapted soldering-iron tip required nearly a minute for the same task.
Removal tools have to be able both to heat the component and to lift the component after reflow. With soldering iron tips, and smaller components, the surface tension of the solder between the tip and component is usually adequate to lift the component away with the tool once reflow has occurred. With larger parts, tweezers are used or a vacuum nozzle may be built into the tool. Whatever approach is taken, care is needed to ensure that the equipment does not require operators with three hands!
With practice, most operators are also able to mount components using single point, conductive tools with small conical or chisel tips. With fine pitch quad flat packs, or the hidden leads of SM sockets, solder bridges or inadvertent damage to the plastic portion of the package can occur, and fine point convective tools often work better.
For component replacement, however, hot gas tools have the advantage in that they provide a non-contact process, although to avoid overheating adjacent components, the gas flow must be focused.
The gas velocity should be kept low enough to avoid disturbing the part. Many hot air systems can actually blow small components off the assembly and require the operator to secure them with a tweezer or similar. This action can disturb the component after reflow, creating a cold joint.
There is a debate as to the best way to place components accurately. A number of ‘joystick’ micro-manipulators with X, Y, Z and θ manoeuvrability have been developed, supposedly to assist this task. In practice, most operators prefer the convenience of a hand-held pipette, and normal hand–eye co-ordination makes it possible to place parts to within 125 µm.
One issue often overlooked is presenting replacement components to the rework operator. As with automated placement, key considerations are maintaining correct component identity, polarity and orientation.
Before components can be replaced, the pads have to be restored to something like their original condition. Where the amount of excess solder is limited, this can be done by lightly fluxing the area and passing the flat tip of the iron over the pads. Excess solder is transferred to the tip and can then be wiped off.
The task is more difficult where there is a build-up of solder, as for example with through-hole assemblies. One option is to use vacuum to suck surplus solder away. Commercial tools are broadly of two types: in the hand ‘solder sucker’, a spring-loaded pump produces a single high-rate suction impulse at a non-wettable nozzle held close to the soldering-iron tip; in many systems for continuous operation, a vacuum hose from a pump is connected to a hollow heated tip. In both cases, there is the practical problem that the solder thus removed solidifies, and systems tend to clog and become less efficient.
The second option is to use solder braid, or ‘solder wick’. In the early 1950s, engineers discovered that if they stripped a piece of screening braid from a coaxial cable, and stretched and flattened it, they could use it to desolder joints: adding a little liquid flux greatly improved performance. Capillary forces are the primary reason why solder flows up a plated-through hole: braid has thousands of tiny crevices, almost a continuous length of ‘hundreds of parallel plated-through holes’.
For today’s desoldering braids very fine copper wire is woven on machines modified to produce a braid with more spaces to enhance the capillary effect. As the braid is wound, it is tensioned and gently flattened to produce constant widths to suit the application, then cleaned and coated in a special low-residue flux.
Braid is used by placing a suitable sized piece on top of the solder to be removed and placing a solder iron with a dry tip onto it, being sure not to press too hard (Figure 3) Thermal energy is transferred from the tip into the braid; this liquefies the flux which improves thermal conductivity. The flux will also break down any oxide on the surface of the solder, which in turn melts. As the solder melts, the capillary forces in the braid take over and absorb it.
Fluxing a circuit is generally recommended to protect cleared areas against oxidation. However, whether an assembly should be fluxed from the outset is a matter of debate. Some operators, especially those with a preference for hot gas, always apply mildly activated flux before heating the component; others prefer to avoid flux when the task involves removing solder, particularly solder balls.
Flux, and often solder, will normally be needed to dealt with dry joints, boards where solder has wicked away from the intended position, or when a component is to be replaced. A good quality liquid flux is recommended – this should be mildly activated and have no corrosive residues if cleaning is to be avoided. The coating should be even and thin, and applied with a soft brush or cotton bud just before rework – unlike wave soldering, it is best to pre-heat before fluxing.
Solder can be applied using fine solder wire. Often this is supplied with cores of flux to reduce the need to apply liquid flux, but make sure that the built-in flux is compatible with your application!
Solder paste can be dispensed, or even locally stencil-printed by hand for applications such as BGAs. Remember that paste formulated for dispensing has a lower metal content and viscosity than that normally used for printing – you may need to buy a different (but compatible) material. In general, paste is not compatible with using soldering irons, because applying direct heat to wet paste will cause spattering. Even with reflow methods (hot gas or infrared), care is needed to allow the paste time to dry before it reflows.
Cleaning after rework is often neglected or forgotten, and a common sign that a joint has been reworked is a conspicuous flux residue on an otherwise clean board. Unless it is deliberate quality policy to show where rectification has taken place, visible flux residues should generally be removed, to improve appearance and customer satisfaction. However, provided that no-clean fluxes have been used there is no technical reason for their removal unless the finished board is to be conformally coated.
Spray dispensers of non-flammable solvents are often used, but these merely disperse the flux residue and make it less conspicuous, and it will not remove it. The recommended method is to dab with a wet swab held with a tweezer. Synthetic fibre fabrics should not be used, because these do not soak up liquids very well, and cotton wool leaves fluff behind: use linen or cotton swabs. These should be soaked in isopropanol rather than more volatile solvents (such as methylene chloride) which dry so quickly that the flux residue is only spread around.
Explain to your manager in simple terms:
the basic methods for reworking an assembly
what aspects you have to bear in mind when specifying a rework process
why it is better to try and avoid the need for rework
This section focuses on soldering irons and hot gas systems, which are generally the preferred methods. It should be remembered however that any method which applies the right amount of heat in the correct place can be used: for example, infrared equipment, solder pots and even miniature wave-soldering machines may be used, depending on the application.
One approach has been to use hand-held hot air hand nozzles as a universal tool for removing and replacing all SM components. The tool is valuable, but fails consistently to meet process control requirements. For example, hand-held systems often lack the vacuum pick-up capability of large machines and removal requires a two-handed procedure which is more difficult to accomplish. The most critical limitation concerns safe handling of the product: air temperature, time of reflow and direction of air-flow are often poorly-controlled, compromising safety criteria and over-heating adjacent components (Figure 4). Quite frequently these tools are applied indiscriminately and the substrate is overheated producing warping or measling.
Desoldering stations, such as those designed by PACE Systems and Zevac, were developed to give better control, focus on the joint, and protect sensitive components. For repairing QFPs, a square head should be fitted. This has, coaxially-arranged, a vacuum pipe, a heat baffle, and an outer shield which can either sit over the device or clamp it. The vacuum pipe is fitted with a vacuum cup approximately ¼" diameter made of a resilient resin material.
1 = quick change support
2 = vacuum cup
3 = nozzle nest (corresponds to the component housing)
4 = gas openings
Conventional soldering irons come in a variety of sizes, shapes and wattages, but all consist of three basic elements, a resistance heater, a heating block and a tip.
Although now no longer a mandatory requirement, American Defense Standard MIL-STD-2000 has had many important implications for the selection and use of rework and repair equipment. It laid down specific requirements for devices such as soldering irons including, for example, idle tip temperature stability (±6°C of pre-selected temperature), AC leakage (no greater than 2mV rms at tip), tip to ground impedance (no greater than 2Ω), and the requirements that zero power switching must be used, all handles must be static dissipative, and soldering iron holders should be non-heat sinking.
There was also helpful guidance: ‘the soldering iron . . . shall heat the connection area rapidly and maintain proper soldering temperature at the connection throughout the soldering operation’. A soldering or desoldering tool which cools off and cannot recover its temperature while being used is as potentially damaging as one which is too hot. The ability to maintain a proper temperature during the soldering or desoldering process is especially important in multilayer boards due to their very high heat absorption.
Single point tools are useful for accessing small spaces or fine leads, but they have limitations; they require relatively high operator skill levels for consistent repetitive work; multi-lead components must be attached one lead at a time, possibly creating undue stress in the assembled components; safe removal of multi-lead SM components is virtually impossible; and they may not meet requirements for safe use with delicate components.
These limitations hastened the development of multipoint irons but these are not ideal for rework. There are four limitations:
A lifted pad is one of the most serious types of damage facing the repairer. It is often caused by getting a joint too hot, heating it for too long during desoldering, or applying tensile stress whilst the board is above its glass transition temperature (for example, by pulling at a component whilst its leads have only partially reflowed).
Major board repairs, to replace lifted pads and tracks or even ‘rivet’ a defective plated through-hole, are possible, and reliable techniques have been developed, primarily to salvage expensive assemblies. These use preforms of foil, special epoxy adhesives and liquid solder resists to restore functionality and appearance. Such repair is, however, expensive and time-consuming, involving manipulation under a microscope, and some observers doubt whether a board is ever fully restored to its original quality and reliability.
Earlier sections have indicated that one needs methods appropriate to the specific job, and repair professionals often use a multitask work-station, or set of individual tools, where each head or tool is designed for a specific task.
Which combination of tools works best can be debated: the following combination has been used successfully in process-controlled applications, but some alternatives might be suggested:
Larger integrated circuits, and in particular Ball Grid Arrays, provide a challenge to the repairer:
The result is that specialist BGA repair stations have proliferated. Some work with thermodes or infrared heating, but most use hot gas for both removal and replacement, the design of the nozzles confining the heat to the joints, reducing damage to neighbouring components. Controls on the thermal cycle allow the affected part of the board to be subjected to almost a full reflow soldering experience. Alignment is mostly manual, with micro-manipulators to give better resolution in movement and rotation. Optical systems tend to be complex, commonly with split field microscopes to align opposite corners of device and pad, and/or with 45° mirrors to allow simultaneous view of board and BGA underside.
This kind of equipment is also often found in production rather than at a repair station, where it is useful for assembling small quantities of multi-lead components.
Some rework stations have additional features, such as:
So-called ‘paper-less rework stations’ can identify and record the bar coded serial number of the board that is being reworked, the identity of the reworked component and type of fault, the rework parameters and identity of the operator, and make the information available for statistical process control through the Factory Management System.
None of this complexity comes cheap! Equipment tends to be in the £5k–£15k range, and be restricted in terms of throughput. Fortunately BGA assembly yields tend to be high, so a single item of equipment will service a substantial factory.
There are many makers of equipment for rework and repair, and you will get an overview of what is available by visiting some of their sites. Be prepared to be irritated by the format of some sites!
Traditionally, almost all major assembly houses had equipment from one or both of the major players, Metcal (now subsumed into OK International) and PACE, but the market is actually quite fragmented. Manufacturers you will encounter are, in alphabetical order:
(Metcal) OK International (http://www.okinternational.com/)
Plato (soldering tips) http://www.platoproducts.com/
The Antex site still has some technical data, but the useful Metcal hand soldering tips and information about tip care, and all but one of the PACE series of pdf format Process Guides, have unfortunately disappeared.
Give a production colleague advice on the equipment that would be needed to rework a computer motherboard that contains a large BGA package, several QFPs, many chip components and a through-hole power module.
Except after wave-soldering, where the level of solder bridging can lead to in-line removal of solder bridges as a 100% task, rework is generally a discretionary process, which will only be used where made necessary by the presence of unacceptable defects. Typical defects requiring soldering and replacement are:
One major issue is identifying the components or board areas which need to be reworked. Traditionally inspectors and debug technicians have used self-adhesive markers indicating the area or component at fault, but this can take as long as the repair process itself. It is therefore common for inspection and rectification to be carried out by the same operator.
With the advent of automated vision systems, at least one manufacturer has integrated inspection and rework, using a projected spot of laser light to pin-point the fault, or a TV monitor display to advise the operator of the problem and its location. This is an area where considerable change is happening, as computing power becomes more effective and affordable.
Company procedures are generally generic rather than product-specific, and focus on the control of both rework processes and documentation.
The industry standards for ‘acceptability of electronic assemblies’, upon which many company standards are based, are defined in IPC-A-610. These visual standards, however, have certain drawbacks:
The acceptance standards for a reworked assembly are normally identical to those for assemblies submitted for preliminary examination. It is important to avoid inducing faults, such as dewetting during the rework process!
One factor which can be overlooked is the maintenance of product traceability during rework. Even where this is not part of the mandatory customer requirement, it is helpful to record what work has been done on a particular assembly. This information can be used to:
As you will have found during your study of the last Unit, static charges are produced when materials are rubbed together and separated, and operators can acquire a charge when working. Electrostatic discharge (ESD) can damage components, but the problem may not necessarily appear immediately: an early survey suggested that for every electronic device found at test to have failed through static damage, a further nine will have suffered latent undetected damage and are likely to cause early failure and intermittent equipment faults, with an adverse impact on the company’s reputation for quality and reliability. Operators can prevent ESD damage by:
Care should always be taken when handling populated boards. Ideally these should be stored in antistatic totes, with only one PCA in each slot. This allows easy removal and avoids damage to components. When being carried, boards should be protected with either antistatic totes or bags.
Operators should also practise basic workstation discipline and keep the area tidy - it is far too easy to let a repair bench become a ‘bone-heap’! There should never be too many boards on the work-bench at any time – boards should not be stacked, or lifted in a pile, as this leads to damage.
For components, the golden rule is never touch a component. However clean hands may appear, the contamination transferred is bound to reduce solderability. Parts should be handled with tweezers or (to reduce possible damage) with a vacuum pipette.
The soldering iron tip temperature is very important because too much or too little heat can be detrimental to joint soldering: the tip temperature should be checked at the start of each shift. Where the temperature is controlled by the tip, make sure that the correct type of tip is fitted. Before using any iron, the tip should be cleaned by touching lightly and quickly on a damp sponge which shocks off any remaining oxides. When not in use, the iron should be kept in a holder with its tip cleaned and coated with a small amount of solder.
Finally, remember that hand soldering releases fumes which are potentially harmful to health, and always make sure that there is adequate extraction for the task. Obviously the extent of the precautions taken should be matched to the level of rework being carried out, and a small amount of ‘unprotected soldering’ is normally acceptable.
The Circuit Technology Centre (http://www.circuittechctr.com/) provide a very useful resource in the form of a guidebook, which you will find in many linked pages starting at http://www.circuittechctr.com/guides/guides.shtml. This guide really is what it says on the tin – a comprehensive resource for circuit board rework and repair.
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