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68 SMT007 MAGAZINE I JANUARY 2021 hole fill, but only if substantiated by other means through a process capability analysis. For example, if there is circumferential wet- ting on both source and destination sides of the plated hole, it can be inferred that the hole is filled. The process for assembly can be opti- mized and hole fill measured by cross-section- ing the plated through-hole. The process can be confirmed to be in control by linking this hole fill cross-section amount to the visual inspec- tion of both source and destination leads and lands along with the appropriate confidence interval. Otherwise, X-ray inspection (Figure 2) can confirm hole fill percentage by using built-in algorithms. While AOI systems can detect the presence of both source and destination fillets they cannot determine positively that there is solder in the barrel. Once the defect is discovered and it has been decided that the component should be reworked as part of the boards' disposition, there are several rework processes which can be employed. Adding additional solder to the plated through-hole can take on several differ- ent forms. A solder fountain with the proper preheater along with the proper flux applica- tion can, in some cases, fill the remainder of the barrel. Care must be taken to ensure that the dwell time above the soldering source is kept to a minimum, especially as copper disso- lution may result in thinning of barrel walls or hole fill. These are just some of the implica- tions of the design and layout challenges on the hole fill. Not only can board layout and design chal- lenges cause inconsistent or poor hole fill but assembly processing problems may result in the same deficiency. A recent work [1] indicated that choosing the proper flux was one of the biggest influences on hole fill during wave soldering. A second large influence of improper hole fill is poor penetration of the flux into the through- hole. Another source of improper hole fill is improper board preheating and a lack of dwell time in the wave soldering source. Too long a dwell time can lead to copper dissolution of the plated holes (Figure 1). If pin-in-paste solder printing is used in lieu of wave or selective sol- dering, insufficient solder paste volume or the location of the print may be cause for improper hole fill. These and other process problems are some of the more common causes of process- related hole fill deficiencies. Discovering improper hole fill requires that the proper inspection protocols are in place in order to detect a defect. Per the IPC A-610 stan- dards, there are a couple of ways to inspect for proper hole fill. Visual inspection can infer Figure 1: Plated through-hole cross-section indicating copper dissolution of barrel walls. Figure 2: X-ray inspection and hole fill calculation inspection routine.

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