Design Rules For Miniwave Drag Selective Soldering
Miniwave Drag Selective Soldering Design Rules
Selective soldering is the process in which through hole components on a printed circuit board are processed by applying solder to individual sets of leads. This process typically follows reflow of SMT components and therefore requires a great deal of precision and repeatability to avoid contact and damage to existing SMT components. The selective solder process, by design, eliminates the need for “selective” pallets to protect SMT components as would be used in a wave solder process.
Selective soldering is the process in which components on a printed circuit board are processed by applying solder to individual sets of leads.
When a Printed Circuit Board (PCB) is designed with a selective solder process in mind, it provides a great advantage to the electronics assembly manufacturer as the number of defects and material cost is reduced significantly when compared to conventional wave or hand soldering operations.
To maximize the soldering results a certain number of process variables and conditions must be met. The reliability of the selective soldering process depends on many of the following factors:
- Environmental Conditions – Humidity, temperature, facility layout, and operator competency can all affect the final results of a selective process
- Pad Design for Through Hole Component – The pitch of the pads (distance between pad centerline) and the shape of the pad will affect results
- Pad to Pad Tolerance – The “keep-away” tolerance is the critical distance from a pad that is meant to be soldered and an area that is not. This distance could be applied to surface mount components and/or other through-hole barrels and via’s.
- Lead Protrusion – The lead protrusion defines the distance at which a component lead extends beyond the bottom surface (Source Side) of the PCB
The following design guidelines provide a template for proper design of a PCB assembly to be soldered using a miniwave drag soldering process.
The environmental conditions are considered to be anything outside the process of materials or selective soldering machines operation that may have an impact on the performance of a selective soldering process. Environmental conditions can be a fixed (temperature, humidity) or variable (facility layout, employee) and each condition have a specific approach to maximize a selective solder process.
- Fixed Conditions – These conditions are difficult, if not impossible, to change or control, however, they are typically the easiest to account for in the selective solder process. Most manufacturing facilities, for example, have standard HVAC controls that maintain building temperature and/or humidity. Simple process adjustments will allow for the selective solder process to be corrected for these fixed environmental conditions that do not frequently change.
- Variable Conditions – The variable conditions can be more difficult to control and require internal process and standardized practices to maintain a consistent soldering process. Examples of variable conditions are
- Employees – The human factor is the most difficult to control. Humans have the ability to make decisions and choices that can negatively impact the results of a selective solder process. It is critical to ensure that employees receive proper training in both maintenance and operation when performing and automated selective soldering process.
- Facilities Layout – When approaching a selective solder process it is important to consider the layout of the manufacturing facility. Overhead fans, HVAC returns, system exhausting, air, and nitrogen piping runs and lengths can all have an effect on the performance of a selective soldering process. Overhead fans, HVAC returns, and system exhaust can pull nitrogen (a critical item for selective soldering) away from the process area causing defects and wave instability. Nitrogen and airline plumbing should be routed as short as possible and be free of pin holes and leaks as they will negatively affect the selective solder process.
- Equipment Maintenance Schedules – The most important variable condition is selective soldering machine maintenance. A poorly maintained machine will perform poorly and be reflected in the results of the selective solder process. It is important that the manufacturers recommended maintenance schedule is followed and they
- Periodic maintenance checks are performed by management to avoid machine down time.
Pad Design For Through Hole Component
The through-hole PCB design is directly related to the lead pitch of the component chosen and may not provide the ability for independent adjustment, however, when available the following criteria should be used for plated through-hole design.
Centerline Pitch should be greater than 2.0 mm when possible
- Pad-to-pad edge clearance shall be no less than 0.5 mm
Pad To Pad Tolerance
Keep away tolerance is important to protect surface mount components from contact with the solder wave as this will likely reflow the component and can cause displacement. The keep away tolerance is not specific to surface mount pads and can also include components and other features that would be negatively affected if contacted by the molten solder wave. The recommended keep away tolerances are as follows:
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