Homepage » Allgemein »

Meeting IPC qualification for hole fill at slightly higher rate Ursula Marquez de Tino, Linlin Yang and Denis Barbini, Vitronics Soltec, Inc., USA

Selective soldering: Nozzle type critical to achieve exceptional results
Meeting IPC qualification for hole fill at slightly higher rate Ursula Marquez de Tino, Linlin Yang and Denis Barbini, Vitronics Soltec, Inc., USA

The appropriate selection of nozzle type and design utilized in selective soldering applications is critical to achieve exceptional soldering results. There are several distinctions between wettable and non-wettable nozzles.

The physical properties of liquid lead-free alloys employed in selective soldering of electronics board assemblies such as viscosity, melting temperature, surface tension combined with the nozzle material and process parameters affect the behavior of lead-free alloys which ultimately determines the quality of the formed electric interconnect, the solder joint. This investigation evaluated the use of wettable and non-wettable nozzles with varying nozzle diameters for the purpose of selectively soldering different components to printed circuit board assemblies (PCBA). The Design of Experiment (DoE) included a variety of machine and nozzle parameters.

The results of visual inspection and quantitative analysis of the electrical shorts formed during the designed experiment showed that wettable nozzles produced less bridging defects than non-wettable nozzles. Furthermore, bridging defects were minimized with the use of nozzle diameters equal to or greater than 6 mm along with the faster drag speed (2 mm/s). Soldering temperature did not exert an impact on bridging. For non-wettable nozzles a solder temperature of 300 °C or more is required.
X-ray analysis of the through-hole penetration showed that wettable nozzles are slightly better than the non-wettable versions. Better through-hole penetration was observed when a soldering temperature of more than 300 °C and nozzle diameters of 8 mm for the wettable and 4 mm for the non-wettable versions were used. Ideal through-hole design depends on component type.
Results from X-ray analysis of through-hole penetration showed that vias soldered with the wettable nozzle were statistically filled and met the IPC 610D Class 3 qualification for hole fill at a slightly higher rate than those vias soldered with non-wettable nozzles. Better through-hole penetration was observed when soldering temperature of more than 300 °C and nozzle diameters of 8 mm for the wettable and 4 mm for the non-wettable types were used.
Increased complexity of board assemblies and component density make the selective soldering method a popular technology in today’s state-of-the-art mixed (surface-mount and through-hole components) assembly lines. For soldering a small number of through-hole joints or special component, selective soldering shows advantages in both lead-time and assembly quality compared with traditional hand soldering or wave soldering with pallets [1] [2].
Meanwhile, the electronics industry has put more expectations on the performance of selective soldering. The demands include but are not limited to capable and stable soldering processes, low defect level and low impact of thermal effects on heat-sensitive components. Tight control over selective soldering process parameters is vital to the success of the assembly process. These parameters are similar to those in wave soldering, but the rules governing these parameters in selective soldering are altogether different. Preheating temperature and time, flux amount and solder temperature are some of the critical parameters. Apart from these factors that are in common with traditional wave soldering process, selective soldering has factors that are unique to the process and machine design. For robotic point-to-point soldering system, the choice of nozzle type (wettable or non-wettable), nozzle size, move pattern and drag speed are very important to the assembly quality level and lead-time [3] [4].
The main focus of this investigation is the characterization of soldering quality as a consequence of nozzle type, wettable or non-wettable. A wettable nozzle is one that allows for solder to completely wet the nozzle in 360°, whereas a non-wettable nozzle allows the solder to form a well-defined meniscus at the rim of the nozzle. In addition, the solder does not overflow or wet to the base material of the nozzle. Nozzle design and pump speed in both nozzle types impact the volume of solder displaced. As a result of nozzle type, there are many advantages to both technologies in terms of process window, machine design and soldering quality.
When wettable nozzles are utilized, it is possible to solder the PCBA from all directions and remain horizontal. In this case, the joints that need to be soldered are all exposed to the same uniform soldering process regardless of directional approach to the nozzle. Maintenance of the nozzles must be determined in order to keep the stability and performance of the solder height, oxide formation and repeatability. Determination of the maintenance schedule is heavily dependent on nozzle material, treatment of the nozzle, flux and solder used, as well as operational temperature.
When non-wettable nozzles are used, the solder alloy is forced to flow to the backside of the component being soldered. The same principle is being used in wave soldering. And similar to wave soldering, the use of a defined soldering angle is typically employed to minimize the formation of bridging. Clearance with adjacent components also becomes tighter.
The Design of Experiment (DoE) included machine and nozzle parameters. The machine parameters were drag speed of the robot mechanism and solder temperature, while for the nozzle, the parameters were nozzle diameter and type. These parameters have been identified as significant and will affect the first-pass yield. Diameter of the nozzle is critical since it affects through-hole penetration, especially in thicker PCBAs which require more heating energy. Wider nozzles are able to provide more energy but their size is limited by surrounding components. Use of larger nozzles adds the benefit of soldering more vias in one pass permitting shorter cycle times.
After finishing the soldering procedure, the assemblies were characterized using visual inspection and X-ray techniques to quantify soldering quality as related to bridging and through-hole penetration defects.
Characteristics of printed circuit board
A 16 layered, 93 mil (0,235 mm) thick FR4-board and three different through-hole connectors was used in the experiment (figure 1). The board material was TU 752 with a Tg (glass transition temperature) of 170 °C and a Td of 350 °C. The solder mask utilized on these boards was made of Probimer 65. The board dimensions were 5.5 by 7 inches (140 x 188 mm). The board was finished with Cu OSP (copper with organic surface preservative). The selection of this surface finish was due to its popularity as a replacement for HASL (hot air solder leveled or hot air surface leveling) and to its soldering difficulty to achieve good through-hole penetration. The board layout included through-hole designs attached to different layers and varying pad-to-hole ratios.
The board design contained footprints for both surface-mount and through-hole components. In this study only through-hole components were assembled. Table 1 shows the design dimensions for these through-hole components.
Three types of through-hole components were used in this experiment. All through-hole components were rated for lead-free processing and leads were Pb-free finished. The four 64-pin connectors were high-temperature terminal strips with gold finish. They had a pin diameter of 25 mils (0.64 mm) and pin pitch of 100 mils (2.54 mm). The three plastic dual-in-line package (PDIP), had 16 pins featuring a 100 % Sn matte finish. They posses a pin diameter of 15 mils (0.4 mm) and a pin pitch of 100 mils (2.54 mm). The 25 axial resistors had the usual two leads with 100% tin finish with a pin diameter of 22 mils (0.55 mm).
For all through-hole components with exception of DIPs, the leads were mechanically trimmed to achieve lead protrusions of 1 mm while the DIPs had a minimal, visible lead protrusion. This is in accordance with the worldwide accepted IPC 610-D standard.
General difference between nozzle-types
Two types of nozzles were used for soldering: non-wettable (NW) and wettable (W) types. The main difference between them is how the liquid solder flows. For the NW-nozzles the solder flows in a preferred side. Therefore, a robot unit is required to tilt and rotate the boards to properly solder the pins of the components. On the other hand, for the wettable nozzle (W) the liquid solder wets the nozzle all the way around. Therefore, the rotation as well as the tilt of the board is not necessary.
The nozzle diameters for both types varied. The inner diameters were 4, 6 and 8 mm. The Diameter of the nozzles is very critical because it affects through-hole penetration, especially in thicker boards, which requires increased thermal energy. Wider nozzles are able to provide more energy, but their size is limited by surrounding components on the board.
Test-assembly of 72 boards
Low-solid (1.8 %), no-clean flux (Interflux type IF2005) was used. This flux is a water-based (aqueous) L0 type of flux. The lead-free solder alloy is made of Sn 3%Ag 0.5%Cu (the SAC 305 type).
A full factorial design was performed as shown in table 2, and two replications per combination were done.
A total of 72 boards were assembled in this experiment. The through-hole components were manually placed. A 6748 Vitronics Soltec MySelective soldering machine with a dropjet fluxer, two heating zones with IR lamps, and a select wave was used for the soldering process. The selective temperature profiles were developed and optimized based on flux supplier specifications and observations made on initial runs.
The settings for the open time and frequency for the dropjet fluxer were 2 ms and 150 Hz respectively. The preheaters were set at 55 % for 65 seconds. The average topside board temperature at the end of the preheating cycle was 116 °C.
Depending on the type and diameter of the nozzle, the pin connectors were assembled using several passes. The pin connectors were placed in a group of two. Each pin connector had two rows of pins. Therefore a total of 4 rows needed to be soldered.
For the wettable (W) nozzles, when using 4 and 6 mm nozzles, the soldering of two rows of pins per pass was possible. While using an 8 mm nozzle, 4 rows per pass were possible. The movement pattern was forward and backward.
For the non-wettable (NW) nozzles, the boards were tilted in a 10° angle for appropriate soldering. When using a 4 mm nozzle, it was only possible to solder one row of pins per pass, while for 6 and 8 mm nozzles, two and three rows of pins per pass respectively could be processed. The movement pattern was forward.
Results of visual inspection
The PCBAs were visually inspected and soldering defects such as bridging, solder excess and pad exposure were recorded. Figure 3 shows the statistical average of defects per board we have found.
Figure 3. Statistics on soldering defects
Bridging was the dominant defect. It was measured by counting the number of pins that were involved in a bridge. Joints soldered with W-nozzles showed less bridging than NW-nozzles. The other defects were not significantly different at alpha level 0.05.
Main effects plots for NW-nozzles show that bridging is affected by nozzle diameter, soldering temperature and component type. The data favors a soldering temperature of 300 °C or more. More bridging was observed for the pin connectors. This is due to the larger amount of opportunities. For the nozzle diameter, the 6 mm nozzle shows an increased amount of bridging. The explanation for this behavior is still unknown.
The main effects plot for W-nozzles shows that bridging is mainly affected by nozzle diameter, component type and drag speed. The data favors a nozzle diameter of 6 mm or more and drag speed of 2 mm/s. More bridging was observed at the resistors.
In general, higher temperatures (above 300 °C) and faster drag speed (2 mm/s) resulted in less soldering defects. In this discussion of bridging and the mechanics of the formation of electrical shorts, it is important to note that elimination of bridges prior to the pins leaving the dome of solder is possible with the use of solder drainage technology. This technology is available on the non-wettable nozzle only and was not employed in this experiment. Use of this technology would have resulted in superior results for the non-wettable nozzle as related to bridging. However, this investigation was designed to derive and compare the optimum process for both nozzle types.
Through-hole penetration
As mentioned, X-ray analysis was used to inspect through-hole penetration. Selected vias were inspected as shown in figure 1 (rectangles). For the pin connector only vias connected to 8 layers were inspected. The output numbers of the inspection was 0 and 1. A value of 0 was assigned to those soldered joints with a through-hole penetration less than 75%. A value of 1 was assigned to those soldered joints with a through-hole penetration that is equal to or greater than 75%. The analysis of the data shows that W-nozzles performed slightly better than NW-nozzles. The mean percentages of good joints were 67 % and 62 % respectively.
The main effect plot for the W-nozzles shows that diameter, temperature and component type are significant at alpha level 0.05. The data favors 8 mm nozzle diameter and 300 °C solder temperature.
The main effect plot for NW-nozzles shows that diameter, temperatures and component types are significantly different at alpha level 0.05. The process window for NW-nozzles is 4 mm nozzle diameter and more than 300 °C solder temperature.
Through-hole vias had different designs as specified in table 1. Plots for the mean percentage of good through-hole joints based on connection and component type for W and NW-nozzles respectively show remarkable results:
For the W-nozzle, the DIP connector had more bad joints when the vias were connected to 6 layers, while for the resistors more bad joints were observed when connected to 4 layers. The pin connector shows minor effect on the connection type.
For the NW-nozzles, similar trends were observed for the DIP and the resistors. For the pin connector, more bad joints are observed when using hole diameters larger than 43 mils (0.11 mm) and pad diameters of 70 mils (0.23 mm).
An in-depth analysis of the interactions and dependencies between the various related parameters of process steps and material shows: The pad and hole columns only relate to the header connector. For both non-wettable and wettable interaction plots, the results for hole fill at layer 6 shows poor quality. This is dominated by the DIP component.
 
EPP411

Acknowledgment
The authors would like to thank Chris Curole for his support on the assembly of the boards.

References
[1] O’Neil, T.: Selective Soldering in Lead-free Assemblies. SMT Journal, November 2006
[2] Klenke, B.: Implementing Selective Soldering- Selective Soldering Significantly Improves the Conversion Cost and Quality of Through-Hole Interconnections in Complex PCB Assemblies. Circuit Assembly Magazine, May 2001
[3] Marquez, U., Barbini, D. and Szymanowski, R.: Selective Soldering with Sn3.9Ag0.6Cu Process Development. SMTA International Proceedings, October 2004
[4] Marquez, U.: Selective Soldering with Pb-Free Alloys. Circuit Assembly Magazine, January 2008
[5] Thompson, B.: Selective Soldering: The New Wave. SMT Journal, November 2006

Zusammenfassung
Mit unterschiedlichen Düsenkopf-Technologien können bei Selektivlötverfahren verschiedene Parameter gezielt beinflusst werden. Der Beitrag stellt die Vor- und Nachteile benetzbarer und nicht-benetzbarer Düsenköpfe bei unterschiedlichen Applikationsanforderungen gegenüber. Ergebnis: Den grundsätzlich besseren Düsentyp gibt es nicht. Je nach Applikation muss abgewogen werden.
Avec différentes technologies de têtes de gicleur, on peut influer de manière ciblée sur divers paramètres dans le cadre des procédés de brasage sélectif. L’article met en parallèle les avantages et les inconvénients des têtes de gicleur mouillables et non mouillables face aux exigences des différentes applications. Résultat : aucun type de gicleur n’est fondamentalement préférable à un autre. La décision doit être prise en fonction de l’application.
Current Issue
Titelbild EPP EUROPE Electronics Production and Test 11
Issue
11.2023
READ
Newsletter

Subscribe to our newsletter now

Webinars & Webcasts

First hand technical knowledge

Whitepapers

Find all current Whitepapers here

Videos

Find all current videos here


Industrie.de Infoservice
Vielen Dank für Ihre Bestellung!
Sie erhalten in Kürze eine Bestätigung per E-Mail.
Von Ihnen ausgesucht:
Weitere Informationen gewünscht?
Einfach neue Dokumente auswählen
und zuletzt Adresse eingeben.
Wie funktioniert der Industrie.de Infoservice?
Zur Hilfeseite »
Ihre Adresse:














Die Konradin Verlag Robert Kohlhammer GmbH erhebt, verarbeitet und nutzt die Daten, die der Nutzer bei der Registrierung zum Industrie.de Infoservice freiwillig zur Verfügung stellt, zum Zwecke der Erfüllung dieses Nutzungsverhältnisses. Der Nutzer erhält damit Zugang zu den Dokumenten des Industrie.de Infoservice.
AGB
datenschutz-online@konradin.de