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RELIABILITY OF PRINTED CIRCUIT ASSEMBLIES
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Similar errors in drilling setup can cause rough hole walls, loose fibers, or burrs. These defects are not serious in and of themselves, but can lead to rough plating or copper nodules, which introduce stress concentrations. Rough walls are typically associated with an incorrect feed rate or drill speed, or insufficiently cured material. Loose fibers may be caused by incorrect drilling parameters or improper cleaning. Burrs are usually associated with too fast a drill feed or a dull drill. Poor drill registration can also decrease reliability of innerlayer via connections or the soldered connection to through-hole components. Poor registration can cause breakout on innerlayers; i.e., the drill hole may fall outside the pad on the innerlayer it is intended to connect to. Breakout increases the probability of PTH barrel failures. Breakout on outerlayers means that the solder fillet for a through-hole component will be partially missing, resulting in decreased reliability for some critical components. Whether caused by excessive resin smear or not, poor etchback can result in a weak connection between the plating in the hole and the innerlayer copper. Etchback (see Fig. 57.16) removes laminate resin and woven glass in the hole so that the internal copper projects slightly into the hole, permitting the plating to make contact with the innerlayer foil on three sides. This strength is important to prevent cracking at the interface under thermal shock conditions. A review of innerlayer cracking in or around the electroless copper at the junction between the innerlayer foil and the electroplated Cu in the hole suggests that negative etchback, in which the electroplated Cu projects into the laminate may also give good results. Zero etchback, when innerlayer foil is flush with the hole wall, is the most dangerous case because the bond line between the foil and the plated copper is located at the point of maximum stress.28 Causes of insufficient etchback include improper lamination and curing, hardened epoxy smear, a depleted smear removal bath, or a host of process control issues, including improper bath temperature, agitation, or time exposure.
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FIGURE 57.16 Schematic illustrations of positive, flush, and negative etchback. (After M.W. Gray.28)
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57.4.1.4 Plating. Defects from the plating process can be responsible for a variety of PTH reliability problems; in addition, as previously described, problems in earlier process steps such as drilling and desmear often show up as plating defects. Uniform coverage of the hole with electroless Cu is critical to the strength of the throughhole and the adhesion of the metallization to the laminate. Oxidation of the innerlayer copper prior to electroless Cu plating is one source of poor plating adhesion. Poor control of bath composition can have the same effect. The adhesion of the electroplated Cu to the electroless Cu and the ductility of the electroplated Cu also strongly affect PTH reliability. If the adhesion between the layers is poor, this interface may be the weak point where failure initiates when the PTH is subjected to thermal stresses. Causes can include tarnished electroless copper that is insufficiently microetched, burning the electroless copper with too much current in electrolytic plating, and film contamination
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in the electrolytic copper.28 Susceptibility to innerlayer cracks may be identified by looking for cracks in microsections after a solder float test. The fatigue life of the copper is directly related to its ductility. Plating process parameters and plating additives can strongly affect the plating ductility. For example, Mayer and Barbieri29 found that good thermal shock resistance of electrodeposits of acid copper depended on proper concentrations of three additives: 1. A leveling agent, to smooth over surface imperfections. (Without the leveling agent imperfections are reproduced in the deposit.) 2. A ductility-promoting agent, which functions to produce the equiaxed grain structure. 3. A carrying agent, to guide the other two components to create the equiaxed structure. (Striations occurred with insufficient carrying agent.) Additive levels below a certain threshold make the bath more susceptible to impurity effects. For example, iron contamination of 100 mg/L without the recommended concentration of ductility-promoting agents was found to produce a columnar grain structure at the hole corners. Similarly, organic contaminants such as photoresist can produce laminar deposits. Insufficient plating thickness in the barrel also directly reduces PTH reliability because the stress and, consequently, the strain in the Cu are increased. Overall insufficient plating thickness can be caused by a depleted bath or insufficient plating time, among other things. Insufficient plating thickness in individual holes can also occur as a result of nonuniformities in plating current caused by nonuniform copper feature density. It is particularly difficult to obtain adequate plating thickness in the center of high-aspect-ratio PTHs; good process control is important for aspect ratios greater than 3:1. Good coverage is difficult to obtain for aspect ratios greater than 5:1 by electroplating. What constitutes sufficient plating thickness in PTHs is a subject of some controversy. Specifications range from 0.5 to 1 mil (12 to 25 mm) Cu thickness in the barrel. There are at least two reasons why there is no one right specification. First, different applications provide different levels of thermal stress and demand different levels of reliability. Second, design factors such as the aspect ratio of the plated holes determine the susceptibility of the PTHs to thermal fatigue. The IPC recommends an average minimum copper-plating thickness of 0.5 mil for consumer products (Class 1) and 1.0 mil for general industrial and high-reliability applications (Classes 2 and 3). Poor coverage at the PTH knee can significantly accelerate PTH failures because it means the plating is thin at a point of high stress. It can be caused by excessive concentration of the organic leveling agents added to an electroplating bath. 57.4.1.5 Solder Mask Application. If it is properly applied, solder mask plays an important role in reducing the possibility of insulation resistance failures on PCBs. The solder mask protects the substrate from moisture and contaminants, which would otherwise promote shorting under electrical bias. The ability of the solder mask to perform this function depends on good conformity and adhesion of the solder mask to a clean, dry substrate. If solder mask conformity or adhesion is poor, moisture and other contaminants may accumulate at crevices or delaminations between the solder mask and the substrate. Substrate cleanliness is particularly critical, because in addition to causing poor solder mask adhesion, it can also provide the ionic species needed for rapid electromigration.When the laminate material absorbs moisture readily (e.g., polyimide, aramids), baking before reflow may be required to prevent solder mask delamination (as well as reinforcement/resin delamination). Other causes of poor adhesion or conformity include moisture on the board when the solder mask is applied, improper solder mask lamination or coating parameters, and improper solder mask cure parameters. Incomplete solder mask cure can create local soft pockets, which are common sites for delamination or contaminant entrapment. Solder mask over solder should also be avoided because delamination caused by reflow of the solder may allow contaminants to be entrapped.
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