Over the years, best practices have evolved and will continue to evolve with the changing environments, company needs and challenges and what may work for one company may not necessarily be best for another. A common area sometimes overlooked versus surface mount is wave solder, which can commonly introduce significant touch up and rework if not appropriately managed.
The following recommendations are based on experiences and best practices and is not intended to be considered hard and fast rules, but rather guidelines—your situation will dictate which practices work best for your company.
This article will be focusing on common wave defects and best practices to both address, predict and proactively prevent these issues from reoccurring. Contributing elements such as component selection/ considerations, design, tooling and process will be discussed.
By far the most common wave defect is bridging, which is the unwanted formation of solder between conductors. Defect contributors include component, design, tooling and process.
Lead Length – Specification of component lead length in the design vs. the PCB thickness provides the respective protrusion of the lead into the solder during this process. Ensuring the lead length is neither too short (i.e. solder cannot reach the pin to achieve capacitary action) or too long (i.e. provides a pathway for webbing from one pin to the adjacent) can prorogate bridging for the assembly.
Best practice when specifying the component lead length is to ensure the lead length is long enough to provide the necessary heat transfer for proper wicking to provide sufficient barrel fill while neither exceeding the maximum protrusion specified as per IPC-A-610. A good rule of thumb is the length should not be longer than the distance between the two adjacent annular rings. By ensuring this is met, the probability of webbing is significantly reduced as surface tension will draw the solder to the nearest copper area.
In cases where the lead length is too long, component prep and lead trimming are recommended to provide the desired length.
Figure 1: Bottom side surface mount component heights can drive thicker pallet requirements and further impact the ability of the solder to flow in and out of the pocket.
Other considerations related to the components themselves can be PCB contamination, component contamination, oxidation or solder mask issues.
Component Orientation – Particularly relevant to larger pin count connectors with at least 2 or more rows where orientation of the connector parallel to the wave can be result in significant bridging occurrences.
Best Practice to address is to ensure larger pin count connectors are orientated perpendicular to the wave to minimize the number of exposed trailing end pins of the connector where bridging is likely to occur. This is especially true for fine pitch. In situations where orientation cannot be accommodated, other methods such as solder thieves (effectively non-functional pads or copper features which are placed on the trailing edge to pull the solder away from the last lead to prevent bridging) can be designed either into the board or onto the selective wave pallet to minimize bridging.
Some best practices in selective solder pallet design include proper PCB orientation in the selective wave pallet. It is recommended to angle the board between 15-30 degrees can help mitigate the bridging to a few pins by ensure only a handful of pins end up as trailing pins. This is especially helpful where larger pin connectors are designed parallel to the wave direction.
Sufficiently large wave openings and solder flow channels on the bottom of the wave pallet provides sufficient solder flow and flux application preventing pooling or areas where solder is trapped resulting in bridging. Generally, constraints such as minimum clearance from the outside edge of the annular ring to a surface mount pad drive the opening size. Recommendation is 0.100” for this distance for proper design.
Bottom side surface mount component heights can drive thicker pallet requirements and further impact the ability of the solder to flow in and out of the pocket. The aspect ratio relates to the solder opening length/ width versus the vertical travel required for the solder to reach the bottom of the PCB. The minimum ratio is 1:1 for leaded solder but increases to 1:3 for lead free solder. i.e. if the length/ width is 0.150” then the maximum vertical dimension is 0.150”, for leaded solder. Violating this aspect ratio will obstruct proper flow and increase the chance of wave related defects.
Additionally, orientating the board on selective wave pallet at 15 degrees can help mitigate the bridging to a few pins, typically a hybrid solution of the above techniques provides the optimal solution.
Selecting the right flux for the application as well as the appropriate thermal profile can have a significant impact on the formation of solder bridging and selecting an appropriate flux for the thermal mass and heating profile required can have a significant impact on overall yield.
Generally, a higher solid content is more robust at higher temperatures and water-based fluxes do not perform as well at higher temperatures and better suited for lower thermal boards. Ensure the pre-heat temperature and dwell time for your board is appropriate for your flux can mean the difference between a good and bad result. Burning off the flux prior to wave can result in bridging.
Another common defect is Components Lifted after wave which is more predominant on smaller components such as axial or radial components (but just as common on connectors and other components) which are lifted during contact with the wave and are soldered in placement. The most common practice to address is through component lead pre-forming and/or pallet hold downs.
Ensuring components such as axial and radial components are properly prepped can avoid most lifting situations. Lead forming or clinching of the leads which mechanical hold the components in place are by far the most common. Common with bridging, leads which are too long can also exaggerate lifting which acts as a lever to push the component out of position.
Other components such as connectors which cannot easily be retained in place require additional hold downs which can be in the form of glue or over-clamps as part of the selective solder.
When considering over-arms for clamping, the additional thermal mass introduced by these features and must be considered into the profile and may potentially require a different flux for better performance.
Wave height and the use of lambda vs. laminar flow can also contribute to increased occurrences of component lifting. Ensuring wave heights are set to no more than 50% of the PCB thickness relatively to the pallet and the use of turbulent flows should be minimized.
Other considerations include conveyor vibration, angle, etc.
Another most common wave defect is insufficient solder and can be categorized as incomplete barrel fill or incomplete circumferential wetting.
Related but typically more related to contamination of the solder, board or component is de-wetting or non-wetting. For the purpose of this review we will assume the components are in good condition prior to processing. Best practices to prevent introduction of these types of defects include a well-established incoming inspection process combined with solder dip testing as per IPC-TM-650 for suspect contaminated or oxidized components.
Common design considerations are direct connection of plated through holes to large copper planes which act as a heat sink during wave soldering. To address, best practice is to provide thermal relief in these areas to allow proper flow during soldering. Thermal spokes provide isolation and can significant increase the probability of a good joint.
Figure 2: The direct connection of plated through-holes to large copper planes acts as a heat sink during wave soldering. The best practice is to provide thermal relief in these areas to allow proper flow during soldering.
Other considerations include component lead diameter to plated through hole diameter ratio mismatches. A plated through hole that is either too large or too small vs. the lead can equally result in insufficient. Recommended aspect ratio is typically 0.6 larger than the component lead will provide good results.
Generally, this comes down to heat transfer or insufficient flux as either can have an equally significant impact on solder fill. Lack of flux penetration or presence due to profiles which are too hot are the most common root causes.
Products such as Fluxometers which use acid paper and specially design PCBs with regular spaced plated through holes can be used to ensure the appropriate amount of flux and penetration (i.e. pressure) is applied for optimal use.
Regular or monthly reviews including lev-checks or wave riders can also provide an indication of the wave levelness, temperature profile and overall oven performance and is recommended to ensure process drift related to the equipment is not a contributor to defects.
Figure 3: A Fluxometer can be used to ensure the appropriate amount of flux and penetration is applied for optimal use.
Solder Voids or Out-gassing (Blow Holes and Pin Holes) occurs when a solder joint has a small hole that penetrates the surface of the solder connection. This is typically due to moisture entrapment that during the soldering process out gasses from the joint.
Like components, printed circuit boards are also moisture sensitive however commonly are not treated in the same manner as moisture sensitive components. As a general rule, all PCBs should be considered MSL 3 and be managed as any other Moisture Sensitive Device.
Best Practice is to ensure PCBs are sealed and only opened just prior to use. Extended periods between thermal cycle operations like surface mount reflow and wave should be considered when reviewing exposure time. If a board is not soldered within 72 hours after the previous thermal cycle operation, it should be baked to remove excessive moisture in accordance with J-STD-033 or kept in a dry cabinet with a relative humidity <5% to minimize the risk of such occurrences.
Solder Balls and Spatter defects are generally where a small sphere of solder adheres to the laminate, resist or conductor after wave soldering. There are typically 3 types, random, non-random and splash back which are all typically process related.
For Random solder balls, these are the easiest to address and are typically a result of an excessive flux prior to wave, uneven wave height. If you hear a “sizzle” while the board is going over the wave solder it is a good indication that the pre-heat is either too low or the flux application is too high or the wave temperature is set too high.
For non-random solder balls which the balls appear in the same location or trailing pin this is most commonly due to insufficient flux or pre-heats are too high.
For Splash back this is most commonly due to the wave height being too high or excessive turbulence in the wave. 95% of applications if designed appropriately can be soldered with laminar flow only and is recommended to help avoid occurrences.
Best practice is to utilize tools such as the Fluxometer and Waverider to check for parallelism and proper flux optimization to minimize such occurrences.
Areas of entrapment in the wave pallet can also contribute to solder balls, reviewing pallet designs for solder flow to ensure there is sufficient flow channels or vents to allow outgassing during soldering can help minimize the occurrences of solder balls and spatter.
Icicles, Flags and Excessive Solder
Icicles & Flags (Horns) and Excessive Solder occurs when a printed circuit board passing through a soldering process and either collects too much solder or develops an undesirable protrusion of solder from the joint. The most common contributor is process.
By far the most common reason is the wave solder pot temperature is too low or insufficient dwell on the solder pot. Best practice of 3-5 seconds of dwell is recommended for a proper joint formation. Tools such as Oven-riders can provide an indication of solder pot temperature drift is occurring, it is always recommended to measure the solder pot temperature regularly to ensure proper temperature. Wave Solder pot temperature readings from the machine do not always translate to actual and must be monitored.
Defect prevention is best performed through applying best practices through formalized design reviews and implementing process controls around key wave parameters such as solder pot temperature, pre-heat, dwell, parallelism and flux optimization.
Activities such as Design for Manufacturing (DFM) or Design for Assembly (DFA) can save significant time in applying design rules to ensure PCB design considerations, thermal requirements, manufacturing compatibility and related contributors are identified early in the design cycle where changes can be implemented at a fraction of the cost.
It is important to align with strategic manufacturing partners early on to provide relevant design feedback on all aspects on the design as the design decisions made early on can affect the long-term viability and cost of the product for the total lifecycle.
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