8 Common Errors in Surface Mount Technology (SMT)
Surface mount technology is not a zero-defect soldering process. In this article, we will look into simple defects in surface mount technology (SMT) that cause your PCB to fail and try to work out how to avoid them.
Do you remember when you first heard of saturation? Yes, the one you read about in analog electronics. Saturation can be defined as a condition where a quantity cannot increase or decrease above or below a certain value. Electronics is kind of in that patch. It’s more about reboots, versions, and updates right now. Please do not be under the impression that I am demeaning our technological upgradations. It’s just that new technology may sound very intriguing, but it is costly and takes quite a lot of time as well. It is far cheaper and faster to modify or adapt to existing electronic circuit technology.
Therefore, the more we change, the more they remain the same. Miniaturization, the transition to lead-free solder, has dramatically altered the manufacturing over the past decade. But as remarkable as these changes have been, the basics have experienced very little change. The basics of attaching a part to a board by SMT aren’t yet significantly different from the early package leads that came out of their holes. Solder is providing both the electrical and mechanical link for surface mount attachment of components.
This signifies even though the changes are dramatic, the failures have the same basic characteristics as those of bygone eras. We still talk about defects as simple as solder-bridging. This might sound simple, but it has multiple root causes and there are different ways to address the problems. Hence, using a hi-tech aid might not come handy when you are not well equipped with your basics.
Surface Mount Technology Troubleshooting
SMT Error #1
Solder-bridging or electrical bridging:
Solder that ‘bridges’ across two conductors that should not be electrically connected, causing an electrical short is bridging.
- A variety of reasons can account for bridging; however, the most widely recognized reason is an issue in the solder paste printing process. The print arrangement, or stencil arrangement to the PCB pad configuration, can be somewhat off.
- Too much deposition of solder paste can likewise cause bridging. This may happen when the stencil aperture to pad proportion is excessively high. The ratio of 1:1 for instance, where the stencil aperture and the PCB pad have precisely the same measurement.
- Solder paste cold slump can likewise prompt bridging.
- An incorrect solder paste metal to flux weight proportion causes a slump. High temperatures and moistness can likewise prompt solder paste slump.
- The reflow profile may likewise add to bridging. We know the purpose of the reflow process is to melt the powder particles in the solder paste. Along with that, it wets the surfaces being joined together, and finally, solidify the solder to form a strong metallurgical bond. The profile can be broken down into four zones – the preheat, soak, reflow, and cool-down zones.
- If the pre-heat area has excessively slow of a ramp rate it can account for bridging. Part contact with solder paste may skew the deposit making the solder paste to bridge. The extended soak will input more heat to the paste and result in paste hot slump phenomenon.
- Placement inaccuracy can further narrow the gap between pads, hence, escalating the chance of bridging. Too much of component placement pressure can squeeze paste out of pads.
- An appropriate application requires an appropriate solder paste metal to flux weight ratio. In other words, no slumping of the solder paste. For example, usually dispensable solder paste has a metal content of 85-87% metal. This ratio would slump if we use this for a fine pitch surface mount printing. Typically, 90% metal is, or at least should be used for a stencil printing solder paste application.
- Appropriate reflow profile is also very important.
- Proper attention should be given to the alignment of stencil apertures to pads unless you are using an automatic printer alignment.
- Ensure on point pressure and accuracy for component placement.
- Reduce the stencil aperture dimensions by 10%. Otherwise, the thickness of the stencil can also be reduced, that will reduce the amount of solder paste being deposited.
SMT Error #2
Insufficient solder joints or electrical opens:
When two electrically connected points become separated, or an area on the PCB which interrupts the intended design on the circuit is termed as electrical opens.
- Solder paste printing stage of a surface mount process contribute utmost to this defect.
- Suppose solder paste is never released to the PCB pad. This will result in an insufficient solder joint as solder being placed prior to reflow is insufficient. This can happen if solder paste clogs in the apertures of the stencil.
- Even if the solder volume is adequate, an open can occur if it is not in contact with both the lead and the pad during reflow. This is called component lead coplanarity.
- Opens may also be a consequence of the PCB fabrication process itself.
- First and foremost, the solution is correcting the aspect ratio. The aspect ratio is defined as the ratio of aperture width to stencil thickness. The solder paste clogging the apertures may be due to the aspect ratio being too small.
- Extreme environmental conditions are a strict no-no in the manufacturing process, to avoid solder paste contamination.
- An investigation regarding coplanarity is also very important when it comes to solving electrical opens.
- Fabrication must be checked with the PCB supplier.
SMT Error #3
The development of very tiny spherical particles of solder isolating from the main body which forms the joint. This is an essential concern for no-clean process since an enormous number of solder balls can make a fake bridge between two adjoining leads making functional issues to the electrical circuit. Solder balling isn’t as large of a worry with water-soluble procedures since they regularly are removed during the cleaning procedure.
- Moisture contamination of solder paste is one of the major reasons for solder balls. The moisture saturates during reflow leaving solder spheres behind.
- Lack of a proper reflow profile can also lead to solder balls. Fast preheat rate will not provide enough time for the solvent to evaporate gradually.
- Excessive oxides on the solder powder in the solder paste can also form solder balls.
- Solder balling can be caused by poor solder paste printing alignment as well where the solder paste is printed on the solder mask instead of the pad.
- Smeared solder paste on the bottom side of the stencil during the printing process is also an attribute.
- Coarser powder size is recommended as fine powder size has more oxides and tends to slump more readily.
- A reflow process should be selected according to the solder paste.
- Solder paste’s interaction with the moisture and humidity should be absolutely avoided.
- Check minimum print pressure used.
- Print alignment should be verified on a consistent basis, before preceding to reflow.
- Ensuring proper and frequent cleaning of the bottom of the stencil.
SMT Error #4
A tombstone, sometimes called Manhattan effect, is a chip component that has partially or completely has pulled into a vertical position of the pad having only one end soldered. This results from force imbalances during the reflow soldering process.
- Uneven heating can cause a differential across the component terminals. To be more precise, if the heat distribution is not even, the solder will melt at different rates. Thus, one side reflows before the other resulting in the other lead to stand upright.
- Unequal heat sinks i.e. ground planes, if present inside PCB layers may draw heat away from the pad.
- Sometimes due to temperature and humidity exposure on the solder paste, there is an insufficient force of the solder paste to hold the component in place during reflow.
- Excess movement during and after the reflow operation can cause component misalignment which results in tombstoning.
- Unequal placement of components on pads prior to reflow results in unbalanced solder forces.
- The component body must cover at least 50% of both pads to avoid imbalance in solder forces.
- Ensure a high component placement accuracy.
- It is recommended to maintain a high preheat temperature so that the differential between the two ends is small while reflow.
- Minimizing the movement, during the assembly to the least possible during reflow.
- Least exposure to environmental extremes such as high temperature or humidity.
- Extended soak zone can help in balancing the wetting force on both pads before paste reaching to the molten state.
SMT Error #5
Non-wetting or de-wetting:
The condition in the solder joint in which the fluid solder has not adhered intimately with at least one the components. Characterized by a sudden limit between the solder and the component lead or conductor. A condition whereby a surface has contacted liquid solder, however, has had part or none of the solder holding fast to it. Once more, this is a phenomenon that can be related to all procedures.
- Poor PCB finish can be one of the major reasons. Suppose the base metal is visible, typically this is more difficult to solder to, hence, non-wetting occurs.
- It might also be due to soaking time being too long in the reflow process. Resulting in exhausting the flux prior to soldering.
- Possibly during the reflow process, the heat is insufficient, hence, the flux doesn’t get the proper activation temperature.
- Better quality metal surface finish such as higher Temperature Resistant OSP or ENIG needs to be adapted.
- Reduction in the total profiling time prior to the reflow stage.
- Proper flux for the given soldering task.
SMT Error #6
The formation of bigger solder balls situated close to discrete components having very low distances among each other. This deformity is like solder balling, yet it is discrete in the way that these solder beads hold fast to discrete components rather than multi-leaded gadgets.
- Typically, this kind of problem is due to an excessive amount of solder paste being deposited.
- Sometimes during the preheat stage, there is flux outgassing that can override the paste’s coalescence force.
- Too high component placement pressure can also be an issue. This can push deposited solder paste onto the solder mask. Hence, not being able to fuse back into the joint.
- Reducing the stencil thickness or reducing the aperture dimensions. On the side where the solder bead occurs a reduction of 10% should solve this problem.
- Reducing the component pick and place pressure.
SMT Error #7
Insufficient fills and insufficient solder:
Amount of solder paste deposited at printer station is much less than stencil opening design or, and after reflow, insufficient solder to form a fillet at the component leads.
- The stencil aperture can at times get blocked with dried up paste. This is one of the major causes of the issue.
- During the print cycle, it is of great importance that sufficient pressure is applied across the entire length of the squeegee blade. This will ensure a clean wipe of the stencil. Too much pressure can cause scooping of the paste especially from larger pads and this can result in the defect.
- Paste does not roll into aperture due to too high squeegee speed. The traveling speed of the squeegee determines the available time for the solder paste to roll into the apertures of the stencil and onto the pads of the PCB.
- When the solder paste viscosity and/or metal content too low.
- The large opening can be segmented into smaller apertures and checked for too much of a squeegee pressure.
- The stencil must be cleaned at regular intervals and the paste should be checked for expiry or dry. Also, sufficient board support should be ensured.
- Too much of squeegee speed is also undesirable, that should also be controlled.
SMT Error #8
Cold solder joint or grainy joint:
Some solder connections sometimes exhibit poor wetting and possess a greyish, porous appearance after soldering. It is recognized by dark, non-reflective, rough surfaces of an alloy that should supposedly be bright and shiny.
- One of the major causes listed is insufficient heat absorbed by the solder. This happens due to the heat present to reflow the solder is not adequate.
- Many times, during soldering, the flux seems incapable of accomplishing the task. This can be due to insufficient cleaning of the components and, or the PCB pads before they go for soldering. Too much of impurities in the solder solution can also lead to the defect.
- The maximum reflow temperature should be high enough so that the material is reflowed thoroughly.
- The assembly should not experience any sort of movement during or immediately after reflow.
- Alloy analysis must be done to check for contaminants.
As the sizes are getting smaller and smaller, the problems are happening more and more. The problems in the new age lead-free SMT are pretty basic. They are so basic that you cannot really differentiate them from the lead era problems. Though the issues are of primitive times, they are becoming more common. And sometimes more difficult to eliminate. Therefore, we need to go ahead by eliminating these problems, and when we say eliminate, we mean from the roots.
Even simple things can be disastrous as feature sizes go down. Accurate control of solder printing and precise component placement is absolutely mandatory. Therefore, by progress in SMT, we do not mean anything new. Real progress can be made through precise, pertinacious persevering and purposeful pushes.
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