There can be several types of defects in solder joints post assembly. The major ones being—no solder at all, insufficient solder, dry solder, and excess solder. Assembly shops test printed circuit boards (PCBs) at various stages of assembly. Usually, they test at production level, board level, and system level.
At production level, the tests are more rigorous, with automatic optical inspection to check for component placement and soldering defects. At board-level, tests involve the PCB assemblies in checking their structural integrity using in-circuit testers (ICT), or boundary scan, or both. ICT also works as pre-check for the functional test at system level. This is a very important step, as defects skipped during an incomplete structural test can lead to damage as power is applied to the board assembly for a functional test.
Carrying out tests incrementally increases the cost of the product, leading to assemblers skipping board-level tests in favor of performing only system tests. Decisions to forego any testing should be made carefully, as it could result in massive incremental costs if defects are found at later stages such as at the shipment stage, where the repair costs increase sharply. For defective products in the market, the problem can be still more serious. For instance, a defective PCB assembly in a car may cause not only a simple malfunction, but may also result in an accident, injury, and even a fatality. Recall costs for defective assemblies in the market could reach more than a million times compared to the original cost of the board.
Product Category and Test Strategy
The severity of the tests conducted depends on the position the product occupies in the matrix defined by volume and quality of production. For instance, very little testing goes into low volumes of low-quality and low-cost consumer products. On the other hand, high volumes of high quality and higher cost products merit the most stringent tests.
Latent defects emerge only after an assembly reaches the customer. For instance, insufficient solder on a joint may cause a latent defect. As the assembly experiences vibration in use, the joint can open up completely. Such defects may not result in failure during initial or field testing, but cause a malfunction only after several hours of use.
Inspection Methods for PCB Assemblies
For ensuring both high quality and reliability of the product, PCB assemblers implement inspection of boards at different stages during the assembly procedure, thereby aiming to eliminate surface defects. SMT requires a tighter inspection as their solder joints undergo higher stresses. Insufficient solder on the device leads can compromise the structural integrity of solder joints, causing long term electrical reliability failure. Hence, there is a definite necessity for the several methods used for PCB assembly inspection.
Visual Inspection
This is the most common form of inspection that assemblers perform after a few specified steps during the assembly process, with the equipment for visual inspection depending on the position of inspection targets. For instance, inspection with naked eyes is enough to locate missing components and contaminated solder paste immediately after solder paste printing and component placement. Observing the light rays reflow solder joints reflect from different angles is another very effective visual inspection method.
Assembly devices can have over six types of solder joints, with each type possibly containing eight kinds of defect standards. Therefore, the validity of visual inspection is entirely dependent on the capability and consistency of, and the manner in which inspection staff apply inspection standards.
This also means effective structural process control requires something more than visual inspection for quantitative measurements. In addition, there are instances where visual inspection does not work—solder joints hidden under BGA devices, surface array flip chip, ultra micro fine square flat devices, or J-lead devices with high-density packaging.
Assemblers usually regard visual inspection as a technology with easy access and involving low cost, and mainly applicable for large defect inspection, as visual inspection is based on establishment of uniform and specific rules.
Automated Optical Inspection
Automated optical inspection (AOI) is a much faster process, taking up where volumes and severity of tests precludes the use of manual types of visual inspection. Using multiple programmable LED light sources and cameras, AOI depends on capturing light that solder joints reflect in preference to both the surface of the PCB and the SMD components, which reflect little light.
Analysis of light reflected from the surface of solder joints offers a lot of information about the quality and durability of the joint. This includes parameters such as completeness of the joint, whether solder quantity is sufficient, and if there is adequate wetting. AOI can also inspect the presence of solder bridging and the absence or displacements of components after reflow soldering.
Although AOI is a much faster process than manual visual inspection is, it suffers from the same limitations as its manual counterpart. While inspecting solder joints, AOI cannot distinguish weld height and the quantity of solder. It cannot inspect the solder joints hidden under such packages such as BGA, PGA, and under the J-shaped leads of some devices.
X-Ray Fluoroscopic Systems
Assemblers use the X-ray fluoroscopic system to observe solder joints that the optical inspection methods cannot. This method is similar to the X-ray process medical units use to inspect bones within the human body. The X-ray fluoroscopic system sends a beam of X-rays from a single-point light source vertically through the circuit board assembly.
The PCB material and the SMD components allow most of the X-rays to pass without hindrance, while the solder joint largely weakens the intensity of the rays. Digital X-ray graphics record this change in intensity, with the image generating a density map that indicates solder joint height, its distribution, and its internal integrity.
Therefore, X-ray fluoroscopic systems can accurately inspect solder joint defects and identify insufficient solder, cracks, voids, bridging, and misalignment. This method is very useful when inspecting hidden solder joints under passive chips, BGAs, gull-wing devices, and devices with J-shaped fins. It is also suitable for detecting missing and reversed components.
However, X-ray fluoroscopic systems can inspect only single sided assembly boards, as the overlapping X-ray images of solder joints from both sides of a double-sided board makes it difficult to accurately inspect and identify defects.
Automated Laser Test
Solder joints on both sides of double-layer boards can be effectively examined by the automated laser test (ALT). In principle, ALT systems use a laser beam to measure the height and reflectivity of solder joints. The reflected beam focuses one or more position sensitive detectors maintaining a specific angle with the laser beam. The reflected light position has the information of the surface height, while the power of the reflected light beam has information on the surface reflectivity.
The system uses a method of multiple measurements to overcome interference, shielding from neighboring components, and multiple reflections. ALT optimally measures the solder paste deposited by stencils for quantity and position alignment, before components are placed. It also provides information on cleanliness, fluidity, viscosity, squeezing speed, and stress on the solder paste, facilitating structural process control.
Determining the Most Optimum Method of PCB Assembly Inspection
Printed circuit board assemblers rely on multiple inspection methods rather than on any one of them alone. They base their choice of inspection method on three factors—type of defect, cost, and inspection speed. Automated methods are preferable when handling complex boards of high volumes. However, visual inspection at multiple stages of assembly still holds the highest favor.