SMT Soldering Handbook surface mount technology 2nd phần 10 pdf

job:LAY09 page:5 colour:1 black–text Figure 9.3 The yes/no nature of soldering success Bridges and solderballs A circuit board cannot function if it contains a short circuit, i.e. a solder bridge. Wavesoldering without bridging demands special techniques, such as optimizing the configuration of the wave (Section 4.4.4) and the board layout (Section 6.4.1). Boards with a pitch below 1 mm40/mil are difficult to wavesolder without faults, unless soldered in a nitrogen atmosphere. With reflowsoldering, especially of fine-pitch boards, the type of paste and its quality and the precision of the printing of it are key factors in achieving soldering success (Section 5.2.3). Solderballs need not necessarily be classed as soldering faults. If a solderball sits between two neighbouring footprints on a fine-pitch board, it can constitute a shortcircuit, and prevents the board from functioning. Elsewhere, solderballs repre- sent potential shortcircuits, and as such reduce the reliability to an extent which is difficult to quantify. How solderballs are to be regarded is very much a matter of individual company policy. Even a single soldering fault on a board prevents it from functioning, and there are only two options: correct it or scrap the board. The choice between them depends on several factors, which will be discussed in Section 10.1. What must be stressed here is the following. The nature of the soldering fault The existence of a soldering fault is an objective fact. A joint is either soldered or it is not soldered. A bridge is either there or else there is none. The soldering fault presents a ‘yes/no situation’ (Figure 9.3). To pronounce upon it is in the nature of a verdict upon an observed fact, and two or more inspectors must necessarily reach the same verdict. Because of its objective ‘yes/no’ nature, the success/fault verdict can be entrusted to an automatic quality assessment system, which may be based on opto-electronic inspection or functional electronic testing (Section 9.5.5). Quality control and inspection 329 job:LAY09 page:6 colour:1 black–text Figure 9.4 Some soldering imperfections 9.2.2 Soldering perfection and soldering imperfections Assessing soldering perfection presents an inspector with a fundamentally different situation: imperfect soldering does not prevent the affected circuit board from functioning, but it can be seen as endangering or reducing its reliability. It may also affect its saleability where the buyer has specified precise criteria. Criteria for perfection may include the following features (Figure 9.4): Wetting angle Joint profile and amount of solder on a joint Alignment or displacement of components If an imperfection disqualifies the product in the eyes of the customer, it becomes a soldering fault, because it makes the product unsaleable. A product which is unsaleable does not work as far as the vendor is concerned. Being saleable is the first function any manufactured product must fulfil. A product which is not saleable in the market for which it has been made does not function from the point of view of its maker (unless it is still saleable elsewhere for less profit or at a loss). The offending feature must be corrected, or else the product must be scrapped. In contrast to the unequivocal yes/no verdict upon the verifiable fact of soldering success or fault, a pronouncement upon the soldering perfection of a joint represents a judgement, which is necessarily subjective. The judgements arrived at by different inspectors represent points along a scale, which separate the ‘perfect’ or ‘acceptable’ from the ‘imperfect’ or ‘non-acceptable’ (Figure 9.5). On either side of the accept/reject divide are areas of doubtful acceptability and false alarm. It has been found that only 44% of the quality judgements on the same set of soldered boards, made on two different days by the same inspector, agree with one another. The quality judgements of the same boards made by two different inspec- tors overlap by only 25%, while those made by three inspectors overlap by 14% (A. T. & T. Bell, Burlington, N. Carolina). To sum up: deciding between soldering success and a soldering fault amounts to a verdict. Deciding whether soldering perfection has been approached sufficiently is a matter of judgement, and the making of this decision can be automated only with great difficulty. The blowhole problem Blowholes in wavesoldered throughplated joints, caused by ‘gassing’ of the walls of the hole, are a special form of imperfection (Figure 9.6). 330 Quality control and inspection job:LAY09 page:7 colour:1 black–text Figure 9.5 The perfect/imperfect judgement scale Figure 9.6 Blowholes in a throughplated wavesoldered joint The causes of gassing of throughplated holes, and the measures which are needed in order to avoid it, are by now well understood. Gassing can be prevented by ensuring the smoothness of the drilled holes and the continuity and adequate thickness of the copper plating on their walls. It can be cured by a suitable heat treatment of boards which are liable to form blowholes before using them. Quality control and inspection 331 job:LAY09 page:8 colour:1 black–text Because blowholes do not interfere with the functioning of a circuit board, they are soldering imperfections rather than soldering faults, though their presence or absence is an unequivocal yes/no situation. Searching investigations have shown that they do not affect the life expectancy of joints or their reliability, in any way. Corrective soldering can only mask, but not fill, a porous hole, and it is bound to shorten the life expectancy of the joint. 9.3 Practical examples of soldering faults The nature of a soldering fault means that a circuit board is faulty and cannot function until every single fault on it has been corrected. Therefore, the most important task of any quality-control system is to find every one of them. In Tables 9.1–9.3, the various types of soldering faults are listed and illustrated. For complete- ness’ sake, faulty throughplated joints are included. 9.4 The ideal and the imperfect joint The criteria of perfection in a soldered joint go back to the days of handsoldering. They have to do with two parameters: first, the wetting angle between the solder and the substrate; and secondly the amount of solder in or on the joint. Together, they determine the so-called joint profile. The ideal handsoldered joint has a ‘lean’ profile: the solder meniscus has a concave shape, so that the sharp wetting angle can be seen clearly. Also, the contours of the ends of the joint members must be visible, so that an inspector can be sure that, in the case of the leadwires of inserted components, the wires do in fact project through the hole and that all leads have been properly tinned (Figure 9.7). The criteria of perfection of wavesoldered and reflowsoldered joints on circuit boards go back to these early days. They deal with surface contours, surface areas, the relationships between distances. It is possible to base judgements like good/bad, acceptable/unacceptable or beautiful/ugly on these criteria, provided every inspec- tor can refer to a set of pictures or samples of ‘perfect’ and ‘imperfect’ joints. It is difficult and certainly expensive to derive a clear yes/no verdict unless precise, time-consuming and therefore expensive measurements of individual joints are made. It is equally difficult, if not impossible, to base an automatic, opto-electronic inspection system on a ‘good/bad’ or ‘beautiful/ugly’ situation instead of a ‘yes/no’ one. Tables 9.4 and 9.5 illustrate practical examples of perfect and imperfect joints. There is one instance where an imperfection can become a fault: ‘fat’ joints with too much solder at the ends of a melf or chip-capacitor can cause the ceramic body of the component to crack under the mechanical stresses caused by temperature fluctuations during service. Fat joints hold the component as in a vice, lean joints can yield. 9.5 Inspection No circuit board should leave its soldering stage without having been inspected. To inspect means to view or examine closelyandcritically.Thisimpliesthatinspection is 332 Quality control and inspection job:LAY09 page:9 colour:1 black–text Table 9.1 Soldering faults I: Open joints Soldering Fault Possible cause method (C, if correction is possible) Wave No solder on both footprint and wire or lead (C) Solderwave did not reach the joint. (If the joint was not fluxed, it is covered by lumps of solder); or the solderwave did not overcome the shadow effect, because either layout or waveshape was unsuitable. Remedy: improve waveshape or layout, or change orientation of the board towards the direction of travel. Reflow No paste on footprint. Check screen or stencil for blockage Wave Solder on wire or lead, but not on land or footprint (C, but difficult and costly) Reflow Defect unlikely With wired components: land unsolderable through faulty solermask or misplaced marking With SMDs, as above or misplaced adhesive Reflow Wicking (C) Gap between leg and footprint, due to lack of coplanarity of leads. Remedy: quality control before placement. Unsuitable soldering parameters. Remedy: put right, or choose paste with higher-melting solder more than ‘just having a look’. To be meaningful and cost effective, every inspection procedure must have a set of well-defined targets or criteria, preferably in the form of written, and sometimes illustrated, lists, or explicit software when automatic image analysis is used. The distinction between ‘soldering success’ and ‘soldering perfection’ (Section 9.2) simplifies the task of inspection, in the same way that it is easier to umpire a horse race than a beauty contest. Without an umpire, both race and contest are pointless. Without inspection, a manufacturing process like the soldering of electronic circuit boards is incomplete. There is a basic difference between the inspection of engineering products like a crankshaft and the inspection of a soldered circuit board. The dimensions of the Quality control and inspection 333 job:LAY09 page:10 colour:1 black–text Table 9.2 Soldering faults II: Bridging and solderballs Soldering Fault Possible cause method Wave Bridging between neighbouring lands or footprints (C) Conveyor speed too high; unsuitable wave geometry. Flux too thin or too weak. Remedy: check (and change) flux, or soldering parameters; if layout unsuitable for wavesoldering, change orientation of board in carriage (does not always work) Ditto, with fine-pitch multileads As above, or add solderthieves to layout. Change direction of travel by 90° Reflow As above (C) Paste tends to form solderballs. Remedy: carry out solderballing test, and if necessary use fresh paste. Paste-printdown too thick: check and if necessary, correct Wave Solderballs near footprints (C) Can happen with controlled atmosphere machines. Remedy: change to different or slightly thicker flux. Slow down conveyor Reflow Paste forms solderballs or spits. Remedy: if solderballing test confirms, use fresh paste Solderballs under melfs or chips (C: de-solder and resolder affected SMDs) Printdown too large or too thick. Remedy: check and correct screen or stencil Wave Scattered solderballs, or ‘spider’s webs’. (C: pick up with tip of soldering iron) Flux too thin, or insufficiently pre-dried. Remedy: check and, if necessary, adjust Reflow Scattered solderballs. (C:as above) Paste spits because it has picked up moisture, or temperature profile too steep. Remedy: check and, if necessary, use fresh paste or adjust temperature profile of reflow oven Note: As pointed out above, solderballs do not necessarily constitute a soldering fault, unless they are loose, or are liable to become loose, and can roll about on the board 334 Quality control and inspection job:LAY09 page:11 colour:1 black–text Table 9.3 Soldering faults III: Displaced components Soldering Fault Possible cause method Reflow One or more leads or faces fail to connect with their footprints (C: desolder and resolder by hand) Serious misplacement or floating. Remedies: check pick-and-place equipment; with chips or melfs, lack of solderability at one end, or lack of symmetry between footprints Floating or tombstoning (C): desolder and resolder One end less solderable than the other. One end solders later than the other because of asymmetry of size or thermal behaviour of footprints. Remedy: check quality of components or correct layout fault Figure 9.7 The ideal handsoldered joint former can be expressed numerically, and readily and automatically compared against prescribed standards, within a given set of tolerances. It is very difficult, and certainly expensive, to ascribe numerical values to a soldered joint. Hence the need for visual, optical or functional inspection of soldered circuit boards. 9.5.1 When to inspect Inspecting every soldered board when it has reached the end of the production line is certainly necessary, but it is not enough. Unless intermediate inspections are carried out after various stages of production, errors or defects which are carried over into the next production stage can be very expensive to correct later. The printdown of the adhesive which fixes SMDs to the board before wavesol- dering must be checked for completeness and correct placement. Missing adhesive means a lost SMD; misplaced adhesive can make adjacent footprints unsolderable. Mistakes in adhesive printdown are very difficult and expensive to correct once the joints have been cured. Most adhesives are given conspicuous, sometimes luminescent, colouring to make visual or opto-electronic automatic inspection easier. Similarly, the correct printdown of solder paste must be checked before the Quality control and inspection 335 job:LAY09 page:12 colour:1 black–text Table 9.4 Too much or too little solder Soldering Defect Judgement and possible method remedy Wave or None The ideal joint Reflow H 1  H ! +H % Wave Too much solder H 1 9 H ! +H % Acceptability depends on the product category and the customer. Remedy: change machine parameters, lower the wave, speed up the conveyor Wave Too little solder Reflow H 1 : 30%(H ! +H % ) Acceptability as above. Remedy: raise wave, slow down conveyor, more paste in case of reflow Wave Too little solder Reflow W 1 : 75%W ! Acceptability as above. Remedy: check solderability of footprint and component Reflow Too little solder on PLCC J-leg W 1 : 50%W ! Acceptability as above. Remedy: see ‘wicking’ Table 9.5 Unsatisfactory wetting angle; displaced components Wave Angle 990° Reflow Acceptability depends on product category and customer D 9 50%W May be unacceptable with fine-pitch layout. In that case, count as soldering fault SMDs are placed on the board. Faults are easily corrected at this stage. If detected after soldering in the form of empty joints or bridges, the cost of correction rises by at least one order of magnitude. With hand-placed components, a final check before the soldering stage is advisable. Much mechanized pick-and-place equipment is equipped with integ- rated checks for correct identity, polarity and placement of the components (Section 7.4). If it is not, a final visual or optical check before soldering is worth while. 336 Quality control and inspection job:LAY09 page:13 colour:1 black–text 9.5.2 Visual inspection It is useful to distinguish between two basic types of visual inspection, which one could call the ‘general picture’ and the ‘detailed inspection’. The general picture In small-scale production, where boards are soldered individually, by hand or on benchtop equipment, the operator will naturally look at every single board before it leaves his workstation. If there is an obvious fault due to a malfunctioning of his equipment, or a defective board, he or she will put matters right before carrying on soldering. With in-line soldering, the operator, or supervisor in charge of the soldering line, whether wave or reflow, should have a brief look at the boards leaving the line at regular intervals, perhaps at one board in every ten, in order to ensure that the line is running normally. Obvious major faults might be unfluxed areas or uneven solder- ing because of an unsteady solderwave, or reflowed boards which are scorched, or did not get hot enough for the paste to melt on all joints. Unless such disasters are spotted before many boards reach the next inspection station, the line may have been producing a good deal of expensive scrap. The detailed inspection The manner of the detailed visual inspection and the equipment used for it depend very much on the type and volume of production and the size of the boards. The type and specification of optical inspection equipment ranges from simple or illuminated magnifiers with a power of about five times, to sophisticated apparatus with zoom optics, binocular operation, stereoscopic vision, and facilities to look at the J-legs of PLCCs at an angle. The advent of low-priced, small, readily manipu- lated video systems has added a new dimension to visual inspection. As a general rule, optical systems where the operator has to look into a single eyepiece, or a binocular, which forces him to keep his head in a fixed position, are more fatiguing to operate than systems which show the object of observation on a screen. A good and flexible system of illumination is essential with all optical inspection methods. An easily operated handling system of the boards under test is equally important. With a number of systems, boards are mounted on a movable xy table, which allows for overall scanning, or indexing into preset positions where certain recurring faults tend to occur. Recent studies pinpoint the problems of visual inspection: operators, often female, are under increasing stress, mental rather than physical, as boards get more complex and the pitch gets finer. They rate their stress factors in descending order as intense concentration, burden of responsibility and time pressure. Faulty ergonom- ics and noise can be additional problems. With fine-pitch layouts and components, the rate of inspection falls dramatically, and the stress is greater. The solution is seen in systems where automated opto-electronic inspection precedes, and is linked with, inspection by a closely integrated team of about three operators, who visually inspect and manually correct faulty joints at the same time. Quality control and inspection 337 job:LAY09 page:14 colour:1 black–text The value of linking visual inspection with corrective soldering is increasingly recognized and practised (Section 10.1). 9.5.3 Automated opto-electronic inspection Unless the distinction between ‘soldering success’ and ‘soldering perfection’ is made, automatic inspection must recognize both of them and be able to evaluate features like joint contours. This demands expensive systems of great complexity. If the judgement on soldering perfection is omitted, existing technology, which is constantly being refined, permits relatively straightforward practical solutions for automatically recognizing footprints without solder paste, empty joints or the presence of bridges or solderballs. These systems are based on video scanning of a board surface, combined with an automatic comparison between the actual image and the ideal image of a faultless board. Equipment which operates fast enough to keep up with in-line soldering machines and reflow installations is commercially available. Opto-electronic systems are able to recognize the following soldering failures: Missing, misplaced or defective printdown of solder paste Missing or misplaced adhesive Missing, misplaced or displaced components Bridges, ‘spider’s webs’ and solderballs A recently developed automatic opto-electronic inspection system does not re- quire a pre-programmed ‘ideal’ image of a faultless board, but creates its own ‘learning curve’ by evaluating parameters such as shape of solder-fillet, identity and dislocation of components, bridges etc. ‘Self-learning’ automated visual inspec- tion equipment is expected to become commercially available before the end of 1997. 9.5.4 X-ray inspection X-ray inspection represents an optical system, which operates at two levels. The board with its joints is scanned by penetrating X-ray radiation, which is absorbed most strongly by the lead-containing solder in either joints or paste printdown, less so by metallic conductors, ICs and other semiconductor devices, and least of all by organic substances like FR4 and ceramic or plastic component housings. The resulting X-ray image is converted to a monochrome image in the visible range, which can be evaluated visually by an operator, or processed photo-electronically as described above, and compared with the image of a faultless board. One of the problems which might perhaps be encountered in this context with lead-free solders (Section 3.2.3) is the reduced contrast between such a solder and its surroundings in an X-ray image, unless the solder contains bismuth. X-ray images are shadowgraphs. In the last decade, X-ray sources have been developed with emitter-spots small enough to provide shadows of sufficient sharp- ness to allow even micrographic evaluation. By controlling the voltage applied to 338 Quality control and inspection [...]... as soldering imperfection, 330 causes, 331 inadvisability of correction, 332, 343, 344 Board conveyors: reflow ovens, 208 wavesoldering machines, 126–128 Board-handling robots, 129 Board templates for wavesoldering, 129 Boundary-scan electronic testing, 340 Bridging: as a soldering fault, 00 measures for avoiding it in reflow soldering, 245 in wave soldering, 108 –112 removing bridges, 348 roles of surface. .. become available The wick acts towards molten solder like blotting paper against ink: for desoldering, it is pressed into the corner between the endface of the component and the footprint (Figure 10. 1) with the end of a chisel-shaped soldering Figure 10. 1 Desoldering a melf or chip with a solder-wick job:LAY10 page :10 colour:1 black–text 350 Rework tip held at a temperature of about 250 °C/480 °F–300 °C/570... handsoldering with paste instead of wire leaves more flux Figure 10. 5 Filling an open joint with a soldering iron Place the end of the solderwire on the footprint in spot (1) Then place the flat side of the soldering iron against the footprint or component leg as shown job:LAY10 page:16 colour:1 black–text 356 Rework residue on the soldering iron and on the joint, and needs more cleaning up after soldering. .. chisel-shaped soldering tip is more convenient for removing them Excess solder is removed from the tip of a soldering iron by wiping it with a piece of linen or cotton, but never with fabric made from synthetic or mixed fibre Many handsoldering stations are equipped with a heat resistant sponge for cleaning the soldering tip 10. 4.2 Desoldering SMDs Any of the following circumstances makes desoldering of... joint (Figure 10. 3) Figure 10. 2 Twisting a glued joint apart job:LAY10 page:11 colour:1 black–text Rework 351 Figure 10. 3 Desoldering with heated tweezers The same principle, using tweezers with interchangeable heated jaws which enclose the sides of a PLCC, is a useful option The jaws press against the sides of the J-legs, and the component is lifted or twisted off as soon as the solder melts Desoldering... other (Figure 10. 4) After desoldering operations such as these, the component is mostly a write-off because of its bent legs Also, unless skilfully executed, these methods put the footprints in danger of lifting off the board because of overheating For all these reasons, it is best to invest in a thermode desoldering tool job:LAY10 page:12 colour:1 black–text 352 Rework Figure 10. 4 Desoldering a SOIC... Z4, 10 January 1992, Swiss Fed Inst Technology, Zurich 2 Lea, C et al (1987) The Scientific Framework leading to the recommendations for the elimination of Blowholing in PTH Solder Fillets Circuit World, 13, No 3, pp 11–20 3 Lea, C (1990) The harmfulness of reworking cosmetically defective joints Soldering and SMT, No 5, pp 4–9 4 Strauss, R (1992) The Difference between Soldering Success’ and Soldering. .. resoldering BGAs and flip-chips are on the market which allow for accurate alignment (see Section 10. 4.2) When soldering a replacement BGA into position, the board, at least in the vicinity of the replacement, must be preheated from underneath, preferably by hot air, or with single-sided boards on a hotplate, to between 100 °C/ 210 °F–120 °C/ 250 °F, for reasons which have been discussed in Section 10. 2.3... Section 10. 3.2 Naturally, the ergonomic aspects of the hand-held soldering iron must not be neglected Precise control of the tip temperature, and a fast response to changes in the amount of heat demanded from it, are very important Recent years have seen rapid advances in the technology of the soldering iron, which had remained static through several decades Above all, methods of heating the soldering. .. technologies to take care of that are commercially available (Section 10. 5.2) 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 job:LAY10 page:3 colour:1 black–text Rework 343 10. 1.2 Desoldering and resoldering Rework itself often involves two closely linked operations: . 341 job:LAY10 page:2 colour:1 black–text 10 Rework 10. 1 The unavoidability of rework 10. 1.1 Rework in the production process In our imperfect world, zero-fault soldering does not exist. Soldering. joints. Soldering and SMT, No. 5, pp. 4–9. 4. Strauss, R. (1992) The Difference between Soldering Success’ and Soldering Quality’; Its Significance for Quality Control and Corrective Soldering. . 9.4 Some soldering imperfections 9.2.2 Soldering perfection and soldering imperfections Assessing soldering perfection presents an inspector with a fundamentally different situation: imperfect soldering