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High-speed Stamp Soldering
December 31, 1969 |Estimated reading time: 7 minutes
By Todd O’Neil,Juki Automation Systems
This article examines stamp soldering, a solution that produces consistent high quality by repeatedly applying accurate amounts of molten solder onto a PCB using static volumetric solder stamps. This guarantees total flatness during thru-hole soldering.
Increased use of double-sided reflow — as for high-density SMT/mixed-technology assemblies — makes PCB assemblies susceptible to warpage that adversely affects planarity during subsequent thru-hole soldering. Multi-nozzle selective soldering can be problematic due to inconsistent or unstable wave height caused by molten solder flowing through nozzle openings of various sizes. Stamp soldering produces consistent high quality by repeatedly applying accurate amounts of molten solder onto a PCB using static volumetric solder stamps. This guarantees total flatness during thru-hole soldering.
Board Warpage
The defects most frequently associated with thru-hole soldering are insufficient thru-hole fill, excessive solder, solder bridging, and the formation of solder balls. All of these conditions are affected directly by the contact between circuit board and molten solder. Lack of parallelism between the board and the solder surface, as well as non-uniform flux deposition, improper preheating, or an uneven solder wave height can result in any of these defect conditions.
Insufficient thru-hole fill commonly is linked to wave soldering with aperture pallets, since boards are exposed to excessive conditions. These include board lifting, uneven or entrapped fluxing, inadequate preheat, or insufficient wave height. At the same time, an excessive solder condition can occur with either wave or selective soldering if the board is exposed to uneven fluxing, contaminated solder, or an uneven molten-solder height.
Typically, solder bridging’s root causes are either insufficient fluxing, excessive preheat that burns off flux activators, excessively high soldering speed, or contamination of the molten solder. Solder bridges often can be eliminated through adequate design practice, since its occurrence is affected by the pitch of thru-hole connectors, component lead length, and flux type.
In many high-reliability applications, zero tolerance is shown to the presence of solder balls. These must be 100% removed to avoid a catastrophic field failure in a mission-critical environment. A common misconception exists that solder balling is affected by glossy versus matte soldermask finish of the PCB. In reality, solder ball formation is a phenomenon resulting from uncured solder resist that leaves tacky plasticizers on the surface, inhibiting solder dewetting.
The propensity of solder balling also is affected by non-uniform flux deposition, which exerts a direct influence on solder dewetting. Some flux formulations minimize solder balls. However, these should not be used at the expense of activity. Prior to changing fluxes, it is best to have a good working knowledge of the actual flux deposition in terms of volume, uniformity, and board-to-board repeatability. In most cases, it is a better practice to reduce flux consumption to a uniform minimum volume. Here, users avoid a situation where the porous solder resist absorbs flux components.
Point-to-point and Multi-nozzle Systems
For high-density SMT and thru-hole mixed-technology assemblies, typically either point-to-point or multi-nozzle selective soldering is used with a re-circulating supply of molten solder to form thru-hole solder joints. However, these methods can be problematic due to inconsistent or unstable wave height caused by molten solder flowing through nozzle openings of various sizes in conjunction with excessive board warpage.
Point-to-point selective soldering systems are flexible — maximum control can be exerted over the soldering angle, contact time, and fluxing volume. Point-to-point processing can be effected by maintaining control over the solder wave height. Maintaining parallelism between the board surface and solder nozzle of a point-to-point system often requires continuous adjustment of the Z-axis coordinate, compensating for board warpage.
Multi-nozzle selective soldering systems typically offer faster throughput than point-to-point systems, but the process is determined by the most thermally challenged component. The wave height of multi-nozzle systems can prove to be less than consistent or unstable, since molten solder is flowing through various-size nozzle openings. Dross contamination also is an important consideration; it can further limit the stability of the wave height within multi-nozzle systems. When combined with board warpage as a result of two heat histories of the double-sided reflow process, this unstable wave height can result in inconsistent contact and a higher tendency toward solder defects, such as insufficient hole fill, excessive solder, or solder bridging.
Non-molten processing is a selective soldering method that can minimize the frequency of solder bridges when soldering boards with a high degree of warpage. Non-molten systems use either solder wire, solder paste, or a solder preform that becomes molten as the thru-hole solder joint is formed. A significant drawback, however, is that they are relatively slow and require more expensive solder materials. This makes them less cost-effective for most production applications beyond prototyping or low-volume assembly.
Volumetric Control
Stamp soldering is a selective soldering process that combines the high throughput and minimal bridging of multi-nozzle and non-molten systems with a high tolerance for board warpage. It achieves a low level of solder bridging because it encompasses a stable solder height along with volumetric control of the molten solder. This degree of control over both the solder volume and solder height results in consistently high quality levels with low defect rates.
The stamp soldering process comprises several basic process steps. The PCB assembly (PCBA) arrives at a fluxing station, where it is held securely between top and bottom contour plates, negating any warpage. Flux is applied at predetermined locations on the board by means of flux stamp application brushes. These use surface tension to apply a uniform and consistent volume of flux irrespective of solids content. Preheating occurs to dry the flux solvent and stimulate flux activators.
PCBAs then transfer to the soldering location over a stationary solder pot. They are secured with top and bottom contour plates. The top surface of the molten solder is skimmed with an automatic solder skimmer — removing oxides — and a door opens above the solder pot, which can be contained in a nitrogen atmosphere. Solder stamps rise above the surface of the solder, transferring the volumetrically controlled molten solder to specific locations on the board. The entire soldering process is carried out in a flat and uniform plane.
After soldering, the solder stamps lower to their original position beneath the surface of the solder and the nitrogen sealing plate closes to maintain the nitrogen atmosphere. The PCBA is released and exits. All of these actions are carried out with tooling that is specially coded to ensure the correct tools are loaded with a given program. All flux stamp, contour plate, and solder stamp tooling should suit quick changeovers (Figure 1).
Figure 1. A nitrogen atmosphere ensures consistent solder joint formation.
Solder stamps have fully wetted surfaces to ensure uniform solder flow and excellent thermal transfer properties. Since the stamps always are maintained below the surface of the molten solder, they are tinned continuously. These stamps use a still pool of molten solder to transfer solder to the board and form thru-hole joints (Figure 2).
An advantage of stamp soldering is that volumetric control of the molten solder minimizes solder bridging and significantly reduces formation of solder ball defects. Processing a board at as slow a soldering speed as possible reduces overall contact length between the boards and solder, resulting in improved peel-back conditions. Controlled, uniform flux deposition optimizes solder dewetting, minimizing ball formation.
Narrowing the Gap
The stamp soldering technique offers several quality and operational benefits. Stamp soldering produces boards of a consistently high quality level with high yields and low defect rates. It leans naturally toward high-reliability, mission-critical applications. It also guarantees board flatness during the thru-hole soldering process, irrespective of warpage from previous reflow processing.
Figure 2. Volumetric control of molten solder minimizes solder bridging and solder ball formation.
Stamp soldering has a fast cycle time and high throughput. Rates are demonstrated to hit 25 sec. for 1 to 10,000 solder joints per board assembly. However, a major advantage of stamp soldering is its high tolerance of any amount of board warpage and ability to solder bare boards or boards in housings by means of top and bottom contour plates (Figure 3).
Figure 3. Quick-change contour plates ensure board flatness during thru-hole soldering.
When comparing stamp soldering and multi-nozzle selective soldering, several key factors can be noted. Stamp soldering has a stable solder height, guarantees board flatness, reduces dross formation, eliminates solder balling, and has greater process repeatability (Table 1).
Stamp soldering systems typically are available with either a turntable or in an in-line configuration to support batch or continuous operation, as well as special configurations to meet more specific application needs. Machines can be configured with convection preheat, infrared (IR) preheat, or various combinations of preheating for water-based or alcohol-based fluxes.
Conclusion
To obtain high throughput rates while maintaining consistently high quality levels, several high-volume manufacturers of safety-related management electronics systems have turned to stamp soldering. It is a well-accepted solution for repeatedly forming accurate solder joints and is able to guarantee total board flatness during thru-hole soldering. SMT
Todd O’Neil, product manager, selective soldering, Juki Automation Systems, may be contacted at toneil@jas-smt.com.