Solve capillary issues by increasing solids content.
It is very important to control conformal coating thickness on a printed board assembly. Problems with coating over different surfaces, particularly sharp corners that can lead to shorts from tin whiskers, have been demonstrated many times.
FIGURES 1 and 2 show capillary action on an SOIC and QFP, respectively, where the thickness of the coating is much higher around the leads and the body of the devices. With very high fluidity and spray coating, liquid capillaries under the package are starving the area of the board close to the edge of the pads.
Using x-ray in a nonstandard way is useful in process development.
Here are three different examples of paste after placement or prior to through-hole component insertion. The images shown here are not necessarily defects, and it may seem strange to use x-ray to inspect paste deposits after printing, but it can be very useful for in-process control and ideal for preproduction runs.
FIGURE 1 shows a QFN/LGA. In this case we are interested in the placement force and the degree of paste displacement on the center pad under the device. Normal practice is to look at the degree of paste variation between printing and placement. It’s even more important when there are multiple rows of outer terminations.
Warping during reflow can leave solder balls distorted.
In cases where pad size, solder paste volume and solder spheres on a BGA are consistent in size, the solder joint size variation shown in FIGURE 1 is typically soldering-related. Modern x-ray systems can measure the joint sizes automatically and output a spreadsheet with the data. It is not uncommon to take these measurements on pre-production prototype builds, when product is working. This provides a permanent record of the ball variation on a satisfactory product. If problems are experienced in volume production, the results can be easily compared.
If parts are turning up where they shouldn’t be, check the feeder and paste.
Chip components sometimes appear on the surface of a board completely out of position. On some occasions they may be under other components or, as in FIGURE 1, even shorting under parts. If it’s a random chip component, then there is little chance of finding the cause. If all the parts are the same resistor or capacitor, it may be a feeder problem where the parts are being dropped after pickup, or are jumping out of the tape due to static, which can occur on small parts (below 0201).
Less-than-optimal preheat can lead to moisture-related defects.
Typically, when copper, organic solderable protectant (OSP) coated boards are left exposed after soldering, the copper oxidizes and darkens in color. It will quickly become unsolderable but does not corrode. If exposed to activated flux residues that have not deactivated after preheat and soldering, it is possible to see green verdigis on the copper surface. FIGURES 1 and 2 show a printed circuit board that has been soldered, and the exposed copper on the topside of the board around the pad shows the green deposit referred to as green verdigris.