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3-D Solder Paste Inspection
December 31, 1969 |Estimated reading time: 9 minutes
As the geometry of surface mountable advanced packages decreases while product packaging density increases, solder paste-related problems continue to top many manufacturers' lists as the leading source of defects and is compounded by the fact that the new devices are more difficult and expensive to rework. However, comprehensive, three-dimensional (3-D) solder paste inspection can help eliminate paste defects and cut rework costs. By Robert Kelley and David Clark
By Robert Kelley and David Clark
When people think of automated optical or surface mount inspection, their first thought is often of inspection at the end of the SMT line. Envisioned is a machine that can check for faults in solder joints, misplaced or missing components, components of the wrong value, and various other process problems that could end up in the final product. This type of inspection may prevent defective products from leaving the factory but does little to improve the quality of the product. A more effective strategy would be to prevent these defects from occurring in the first place.1
The Shape of ThingsSolder paste processing is still the backbone of SMT. If the solder joint fails mechanically, the product most likely will fail entirely in the near future. One of the most reliable predictors of a solder joint's strength is its shape. The right shape, or meniscus, of the joint generally assures an adequate cross-section for strength and stress-relief. And one of the best predictors of joint shape is solder paste volume. If there is too little paste on a pad, it most likely will result in a weak solder joint. On the other hand, too much paste can cause poorly formed solder joints and short circuits.
In-line process control for solder joint inspection is accomplished by putting inspection tools where the defects most often originate. The two most popular applications are solder paste inspection and component placement inspection. A well-suited inspection machine in those locations can provide various benefits.
In-line Process ControlFirst, by finding defects where they usually occur, in-line inspection can deal more easily and at less cost with rework problems before the reflow oven stage is completed. For example, poorly screened solder paste can be washed from the printed circuit board (PCB) and the board printed again. Other common defects an in-line solder paste inspection system can detect include bridging, insufficient or excessive paste, and paste misregistration (Figures 1 and 2). Also misplaced or missing components are corrected easily before reflow. After solder reflow, correcting such defects risks damaging the board or the components. Pinpointing the exact cause of a defect after reflow is difficult. The most commonly reported end of line assembly defect is solder bridging. Although bridging frequently is attributed to an excess volume of solder paste, it is difficult to make this judgement with certainty after reflow because other factors such as component leads or placement may be at fault. Using in-process solder paste inspection can eliminate solder defects as a source of problems and help clarify the remaining causes of problems in the process. By eliminating solder paste defects from the process, the remaining causes of problems can be uncovered and addressed.
Figure 1. 3-D Image of damaged solder paste deposit.
Second, a good in-line inspection tool does more than sort good from bad products. By providing accurate, repeatable measurements of important parameters, it is easy to get valuable process-control data from an automated system. Such data provides a valuable window into the SMT assembly process. The information includes not only that on defects from every board built, but also it "sees" normal process variations and identifies the causes of defects. X-bar R charts are frequently recommended as a good method of detecting abnormal variation in the paste printing process (Figure 3).
Figure 2. 3-D Image shows potential bridging defect.
Third, inspection tools that provide in-line process control can be used to accelerate process refinement and help reduce product introduction cycles. The leading edge of the SMT process changes continually, with new materials, components and assembly methods being introduced to reduce costs and improve product performance. Thus, when the assembly process changes, it should be requalified and characterized to ensure high yields. In many cases, process studies can be performed on existing production methods with a view to further improve yields. Such engineering work is undertaken more easily with reliable data from in-line inspection tools that are monitoring the critical process steps.
Focus on the VariablesSolder paste inspection usually focuses on the variables that are the best predictors of solder joint quality: solder paste volume, height, area, registration and whether the paste is smeared. To accurately measure solder paste volume and height, some type of 3-D sensor is required. Another requirement is that the tool not only conducts a pass/fail inspection, but also performs measurements. The measurements, in turn, should meet the requirements for accuracy and repeatability so that the inspection system can be qualified for the process to be measured. A system without the required precision will not resolve process variations that can affect product quality. Doing any type of inspection with a tool that is not sufficiently stable, repeatable and accurate will not be effective and most likely will cause delay and reduced throughput.
Smaller Packages and PitchesTwo developments with surface mount devices (SMD) also drive the demand for solder paste inspection.
Component size. Only within the last few years has the introduction of the 0402 and the 0201 passive components become popular in new designs. With their growing application, however, the opportunities for problems continue to increase. Each size reduction means that even smaller solder paste deposits must be applied correctly to hold the parts on the board during the rest of assembly and transport.
Figure 3. X-bar and R chart. An X-bar and R chart is a commonly used control chart for monitoring the solder paste print process.
Chip scale package (CSP) technology. CSPs are array-type devices, i.e., their solder joints are located under the package, making inspection much more difficult. Accordingly, it is easier to inspect the process steps of solder paste application and component placement to assure that the final product features the required quality. Additionally, CSPs are smaller than their high-lead count counterparts. Hence, pad size is smaller and the lead pitch tighter. For these reasons, CSPs have been the object of several studies related to their reliability in surface mount applications. They have reaffirmed that solder joint reliability is critical if products are to meet the dependability of that associated with quad flat packs (QFP) and ball grid array (BGA) packages.
This means that printing solder paste deposits for the advanced packages is more difficult by an order of magnitude. The smaller apertures in the stencil mean less volume of solder paste in each aperture with which to attach the component to the PCB. And as pitch and pad sizes shrink, aperture areas decrease much faster than that of their sidewalls. This increases the tendency of the paste to adhere to the stencil during printing rather than release cleanly onto the board. Monitoring the paste printing process also becomes more difficult because of decreased deposit size plus their increased number on the PCB.
Rework CostsThe usual way to justify any in-line inspection tool is to estimate the production costs with and without that capability. Apart from the inspection system, this includes the cost for programming, maintenance and result monitoring. It also requires some estimate of the costs involved in creating and repairing defective products. In the case of the newer and smaller package types, more compact working areas and higher precision will be required for the rework, which often must be performed on more fragile boards and components. As a result, in the case of some products, the break-even point will be reached in which it may be more cost effective to scrap a defective board and to assemble another rather than attempt rework. This is especially true when CSPs with underfill material are present.
With rework becoming more difficult and in some cases unfeasible, the only choices remaining may be to accept the increased costs or improve first-pass yields (FPY). If the manufacturer chooses the latter, it only makes sense to improve the process step responsible for the greatest number of defects, i.e., most commonly, the solder paste printing process.2 When the same problems occur before the next processing step, the cost to correct often can be reduced by 10 times or more.3
ReliabilityReliability is an important consideration for all products, but for medical, automotive, hand-held communications and portable computing, reliability takes on a whole new importance. Many such products must perform reliably even when subjected to temperature extremes or mechanical shock and vibration. Solder joint integrity thus is a doubly important consideration for these systems. And although CSPs and 0201 packages ultimately will find their way into various products, their most immediate use will be in hand-held communication and portable-computing products in which smaller dimensions are imperative.
Solder paste volume plays a critical role in the reliability of all solder joints. Recent studies have shown that for CSPs, solder paste volume (too much or too little) is critical for long-term joint integrity.4 An in-line solder paste inspection system capable of measuring paste volume can be used to track this parameter during production. Also, many inspection systems now incorporate statistical process control (SPC) tools that help the engineer detect processing trends and take corrective action before producing out-of-spec parts.
ConclusionWith the high production volumes typically seen in today's SMT manufacturing, in-line solder paste inspection can pay attractive dividends. In addition to a lower rework cost, the advantage of a system that can look at every board as the solder paste is printed becomes important. Because the paste printing process is inherently difficult to control, it is common to see not only random but also systematic defects in which many consecutively built defective boards could pass through. In such instances, the number of defective boards produced can be many times greater than that those identified as the problems occur.
In-line solder paste inspection provides numerous benefits over other inspection strategies. These include the ability to analyze the process and the performance of the stencil printing sequence in real time. Further, SPC can help operators see trends in processing before they result in defects. If defects are produced, they are caught before expensive rework or scrapping is required. Finally, automated solder paste inspection can help improve FPYs and reduce rework costs.
REFERENCES
- Ray Prasad, "Step-by-Step: Part 9 Test & Inspection," SMT, October 1999.
- David M. Mendez, "An Integrated Test And Inspection Strategy," APEX Proceedings, 2000.
- Donald Burr, "Solder Paste Printing Guidelines for BGA and CSP Assemblies," SMT, January 1999.
- Shelgon Yee, "Optimization of Design and Process Parameters for CSP Solder Joints," APEX Proceedings, 2000.
ROBERT KELLEY, product manager, may be contacted at GSI Lumonics, 22300 Hagerty Rd., Northville, MI 48167; (248) 449-8989, ext. 2648; Fax: (248) 735-2460; E-mail: kelleyb@gsilumonics.com. DAVID CLARK, general manager, may be contacted at GSI Lumonics, 105 Schneider Rd., Kanata, Ontario, Canada; (613) 592-1460, ext. 1418; Fax: (613) 592-5706; E-mail: clarkdi@gsilumonics.com.