I. Overview

    Currently, thetypical process of the printed circuit board (PCB) processing uses a patternplating method. I.e. first on the board outer copper foil portion to beretained, which is the graphical portion of the circuit, on pre-plating layerof tin-lead resist layer, and then chemically etched away the rest of thecopper foil, called etching. It should be noted that, when a plate with twolayers of copper. Only in the outer layer etching process with a layer ofcopper must be etched away, and the rest will be formed to the final desiredcircuit. This type of pattern plating, which is characterized the copperplating layer exists only in that below the a pewter resist layer. Further Aprocess are copper plated on the entire board, the portion other than thephotosensitive film is only a tin or tin-lead resist layer (see Figure 3). Thisprocess is known as "full plate copper plating process. Compared with thepattern plating, the biggest drawback of the full board copper plating isthroughout the board should be twice plated copper etching must also beregarded etched away. Therefore, when the wire is a very fine line width willproduce a series of questions. Meanwhile, the side etching (see Figure 4) wouldadversely affect the uniformity of the line.

    In the the boardouter circuit process, there is another way is to use a photosensitive filminstead of the metal coating to do the resist layer. This method is verysimilar to the inner layer etching process can refer to the the innerproduction process in etching. Currently, tin or lead-tin resist layer is mostcommonly used, is used in the etching process of the ammoniacal etchant.Ammoniacal etchant is the commonly used chemical drug solution, without anychemical reaction with the tin or lead-tin. Ammoniacal etchant mainly refers tothe ammonia / ammonium chloride etching solution. In addition, you can also buyon the market ammonia / ammonium sulfate etching liquid medicine.

    The sulfate-basedetching liquid, after use, wherein the copper can be separated using theelectrolytic method, and therefore can be reused. Because of its low corrosionrate, generally rare, but in the actual production is expected to be used inthe chlorine-free etching. Some tests with sulfuric acid - hydrogen peroxide doetchant to corrosion of the outer layer of graphics. Due to many reasons, includingeconomic and effluent treatment aspects, this process has not yet on thecommercial significance are used. Further, sulfuric acid - hydrogen peroxide,can not for pewter resist layer etching, and this process not PCB mainly in theouter layer of production, so the decision most people rarely cares.

Etching quality and pre-existing problems

    Etching quality ofthe basic requirements is to be able to the outside of the resist layer belowcopper layer removed completely clean, ended and nothing more. Strict sense, ifyou want to define precisely the etching quality must include wire thelinewidth consistency and degree of lateral erosion. Not only downwards butalso to the left and right direction, due to the inherent characteristics ofthe etching solution currently produced etching action, so the side etching isalmost inevitable.

    The problem of sideetching of the etch parameters often been proposed to discuss the one, it isdefined as the ratio of the side etching width and the etch depth, known as theetching factor. In the printed circuit industry, it's a very broad range, from1:1 to 1:5. Obviously, a small side etching or low etching factor is the mostsatisfactory.

    The structure andcomposition of the etching equipment etching solution will etch factor orlateral erosion of impact with the optimistic words, it can be controlled. Theuse of certain additives can reduce the degree of lateral erosion. The chemicalcomposition of these additives generally is a commercial secret, each Owner isnot revealed to the outside world. As for the structural problems of theetching equipment, the later chapters will be devoted to.

    In many ways, theetching quality is good or bad, as early as before the printed circuit boardinto the etching machine already exists. Very close internal contact betweenthe printed circuit processing of various processes or process, not anunwelcome one without affecting the other processes affect the other processprocess. Been identified etching quality problems, actually in the film to goeven more in the previous process has existed. Etching process, the externallayer graphics, because it embodies "down the river." Prominentphenomenon than the vast majority of printed circuit board technology, so manyproblems eventually reflected in it. The same time, this is due to etching isthe last part of the self-foil, the beginning of a long series of photosensitiveprocess, after the outer layer graphics that transfer was successful. The morelinks, the greater the likelihood of problems. This can be seen as a particularaspect of the production process of the printed circuit.

    Theory, the printedcircuit into the etching phase, the status of its graphical section should beas shown in Figure 2. In the pattern plating method for processing printedcircuit technology, and ideally should be: after plating of copper and tin orcopper and pewter total thickness should not exceed the thickness of theplating photosensitive film resistance, completely coating both sides of theplating pattern " wall "blocking and embedded inside. However, in thepractical production, the printed circuit board around the world after plating,coating graphics must be considerably thicker than the photosensitive graphics.Since the plating height exceeding the photosensitive film, in a process ofelectroless copper plating and pewter produced laterally accumulation trend,the problem would be resulting. Above in the lines covered with a tin ortin-lead resist layer extends to the sides, forming the "edge", thephotosensitive film of the small portion of the cover in the "edge"below (see Figure 5).

    Tin or pewter,formed along so can not go to film photosensitive film thoroughly removed,leaving a small portion of the adhesive residue along below (see Figure 6)."Adhesive residue" or "residual film remained in the theresist" edge "below, it will cause incomplete etching. Formed on bothsides of the lines in the post-etch "copper root, copper root so thatnarrow line spacing, resulting in printed circuit board does not meet the Partyrequirements, and may even be rejected. Rejection will greatly increase thecost of the PCB production. In addition, in many cases, since the reaction isformed dissolved in the printed circuit industry, plastic film, and copper mayalso be formed in the etching solution stacked and stuck in the nozzle of theetching machine and the acid pump had to stop treatment and cleaning , whichwill affect the work efficiency.

Equipment adjustment and the interactionbetween corrosion solution

    Processing printedcircuit, ammoniacal etching is a more sophisticated and complex chemicalreaction process. Conversely it is also an easy-to-work. Once the process wasraised to pass, it may be performed continuously production. The key is oncethe boot is on the need to maintain a continuous state should not be working onand off. Etching process to a great extent dependent on the equipment in goodworking condition. For now, regardless of the etching solution, you must usehigh-pressure spray, and in order to obtain a neat line side and high-qualityetching effect, structure and spray nozzle must be strictly selected.

    In order to obtaina good side effect, there have been many different theories, the formation ofdifferent design and device structure. These theories are often very different.However, the theory of all relevant etching recognize such a basic principle,that is, as soon as possible so that the metal surface contact fresh etchingsolution. Chemical etching process mechanism analysis also confirmed this view.Ammoniacal etching, it is assumed that all the other parameters constant, thenthe etching rate is mainly determined by the etching solution of ammonia (NH3).With fresh solution and etched surface role, the main purpose two-fold: Firstwash away just copper ions; continue to provide the reaction of ammonia (NH3).

    Traditionalknowledge in the printed circuit industry, especially printed circuit materialssuppliers, recognized ammoniacal etching solution of a cupric ion content islower, the faster the reaction rate. Confirmed by experience . In fact, many ofthe ammonia etching liquid products containing the monovalent copper ionspecific ligand (some complex solvent), and its role is to reduce themonovalent copper ion (these is the ability of their product has a highreactivity technical tips ), visible the cuprous ion impact is not small. Themonovalent copper etch rate dropped from 5000ppm to 50ppm, more than doubling.

    Due to the etchingduring the reaction generates a lot of monovalent copper ions, and a monovalentcopper ion complexing group always with ammonia tightly together, so close tozero is very difficult to keep its content. Converting the monovalent copperinto divalent copper to monovalent copper can be removed by the action ofoxygen in the atmosphere. The way of the spray can achieve the above object.

    This is to pass airinto the etching tank, a functional reason. However, if the air is too much,will accelerate the loss of ammonia in the solution leaving the PH valuedecreased, the result is still the etching rate is reduced. Ammonia in solutionis also a need to control the amount of change. Some users using pure ammoniapassed into the etching reservoir practice. To do so must be a PH meter controlsystem. When the PH results automatically measured below a given value, thesolution will be added automatically.

    In the field of arelated chemical etching (also known as photochemical etching or PCH), theresearch work has already begun, and reached the stage of the structural designof the etching machine. In this method, the solution used for the divalentcopper, ammonia - copper etching. It may be used in the printed circuitindustry. PCH industry, etching the copper foil of the typical thickness of5-10 mils (mils), the thickness in some cases is quite large. The its etchingparameters require frequent than in the PCB industry harsher.

    One from the thePCM industrial systems research, not yet officially released, but the resultwill be refreshing. The strong project fund support, researchers have theability to change from the long-term significance of the the etching devicedesign thinking to study the effect of these changes at the same time. Forinstance, compared with the conical nozzle, the best nozzle design fan-shaped,and the spray manifold (i.e., the nozzle screwed into that pipe) also has amounting angle 30 degrees to the workpiece into the etching compartmentejection. If such a change is not performed, then the manner of installation ofthe nozzle in the manifold will cause the injection angle of each of theadjacent nozzles are not entirely consistent. The respective spray plane of thesecond group of nozzles with the first group corresponding to a slightlydifferent (see Figure eight, it shows the work of the spray). So that the shapeof the ejected solution a superimposed or cross state. Theoretically, if thesolution shape intersect with each other, then the part of the ejection forcewill be reduced, can not be effectively flush out old solution on the etchedsurface while maintaining the new solution in contact therewith. In the sprayedge of the surface, this situation is particularly prominent. Ejection forcethan in the vertical direction is much smaller.

    The study foundthat the latest design parameter is 65 pounds / square inch (4 + Bar). Each ofthe etching process, and each solution has an optimum injection pressureproblems, but on the current situation, the etching compartment ejectionpressure reaches the case of no more than 30 lbs / square inch (2Bar) minimal. A principle thatthe best injection pressure should be higher the density of an etching solution(ie the proportion or glass Mido). Of course, this is not a single parameter.Another important parameter is the relative mobility of the solution to controlthe reaction rate (or mobility).

On the upper and lower plate surface, theleading edge with the posterior side etching state problems

    Involving etchingquality problems are concentrated in the portion of upper plate surface isetched. Understanding this point is very important. Was the impact of theseproblems from the printed circuit board on the upper plate surface etchantcolloidal compaction. Gum compaction was deposited on the copper surface, onthe one hand affect the ejection force, on the other hand barrier supplementedby fresh etching solution, resulting in a lower etching rate. It is preciselybecause of the different etching degree of gum compaction was formed andstacked so that the board on the graph below. It also makes easy etchingthoroughly or likely to cause corrosion in the part of the the etcher boardfirst enter has not yet formed, because then piled faster etching. Conversely,into part of the board to enter accumulation has formed, and slow down thespeed of the etching.

Etching equipment maintenance

    Etching equipmentmaintenance, the most critical factor is to ensure that the cleaning of thenozzle, non-blocking objects leaving the jet patency. Obstruction or slaggingforum under the impact of the jet pressure. If the nozzle is dirty, then thecause etching uneven leaving the complete PCB scrap. Clearly, the maintenanceof the equipment is to replace the damaged parts and wear parts, including thereplacement of the nozzle, the nozzle is the same wear problems. In addition,the more critical issue is to keep the etching machine does not exist slagging,and in many cases there will be accumulation of slagging Slagging excessiveaccumulation, even interested in the etching solution of the chemical balanceimpact. Similarly, if the etching solution to excessive chemical imbalance,slagging will become more serious. Slagging how the accumulation of problemscan not be overemphasized. Once the etching solution suddenly appears a largeslagging, is usually a signal, i.e. the balance of the solution is a problem.This should be properly cleaned with strong hydrochloric acid solution oradditional.

    The plastic filmcan also be generated slagging was a very small amount of residual film wasdissolved in the etching solution, and then forming a copper salt precipitate.The plastic film formation slagging Description before the Road to film processis not complete. The poor to film often is the result of the edge of the filmwith too the electroplating common cause.