|Ask the Experts|
January 10, 2019
I am facing some tombstone issue on a medical project. The board is double-sided and the tombstone occurs on the bottom side when it is reflowing top side. I have already fine tuned my soaking profile by increasing the soaking time to prevent, but still futile. Could this be due to vibration from my Pick and Place machine from the neighboring line? We added a soft pad to prevent vibration to the floor, but it fails to solve the problem. Any solutions?
|Expert Panel Responses|
The primary reason for tombstone defect is the difference in wetting action / surface tension of the molten solder on the pads of the small passive components (especially 0201 devices). There are many factors that influence the tombstone defect occurrence. Component Related Uneven plating of terminations leading to varying oxidation levels Oxidized terminations leading to poor solderability PCB Related Uneven pad sizes Uneven heat sinking on pads Pad finish irregularities Varying oxidation levels on the pads Pad coplanarity Paste Print Related Paste volume mismatch between pads Difference in particle distribution amounts between pads Component Placement Component Placement Offset Reflow Soldering Excessive heating rate Improper thermal distribution Insufficient pasty region Insufficient flux activity
inspīre solutions LLC
This is a tough one. Frankly, we have never seen this before so I am not "expert" in this area. But here are some questions, guesses, thoughts and things to explore: 1) Is this a "standard" double sided process where one side is reflowed and then turned over and the other side is then reflowed? Or is this a process where both sides are being reflowed simultaneously? I have seen situations where simultaneous reflow has been attempted and it typically ends up causing more trouble than it solves. If simultaneous, suggest stopping and going back to the "standard" process. 2) Assuming that this is a "standard" process and the part was already reflowed once before, the surface tension of the solder should be far above any forces that could be imposed on the part. So I would think vibration would not be part of the equation. 3) So if the part was soldered once before, it should not come "undone". Which leads us to: was it actually soldered the first time around? Could it be that one of the terminations seemed to be in contact with the solder on the pad but is in fact just slightly above? And when the part goes through the 2nd pass, the tombstone effect is exacerbated. The part then pops up in the air-but in fact it was technically a tombstone all along? Or that it was a cold solder joint and was held mechanically but the 2nd time around it rose up? (Less likely but possible) 4) We have seen situations where the paste is not printed properly and is offset to the pad(s). In these cases, the paste doesn't always "self center" during reflow (especially true of lead free paste) and the terminations will reflow but the solder is off one or both of the pads. If one blob of solder is on a pad and the other is not, it could be that during the 2nd pass the blob on the pad has more force than the one off the pad and could pull it up at that time. Side note-Juki has seen that adjusting the placement machine to put the component centered on the paste and not on the pads in these situations can help to encourage the self centering effect-especially on very small parts. So the Pick and Place can be used to overcome bad printing or bad board mfg. and could be part of the solution. 5) One other thought, check the paste volume. Make sure that the amount is the same on both sides of the part and that the volume of paste is minimized. Less paste = less force and this could be a simple way to fix. If there are any questions or if additional information is needed, please feel free to ask. Best Regards, Marc
Heller Industries Inc.
Tombstones are common in double sided reflow processes. Tombstoning can be fixed with in a number of ways. As you described below it looks like your issues are happening in the second reflow process which can be effected if there is sufficient vibration seen at the reflow oven during reflow process. Items to check include Pad Design, Heat Sinks, Vibration during reflow and heat sinks. Quick Fixes: Chipbonder or Cornerbond can be applied to help hold down components during the 2nd reflow process. This is a common fix when the Pad Ratio's are not enough to hold the component in place during reflow. Cornerbond materials allow for Self Centering of component during the first reflow process. Chipboders are effective at holding the component in place but cure during the reflow process so you may end up with more skewed components. Look to put heat sink shield tooling on the bottom side specifically around areas where Tombstoning is prevalent. Check Reflow oven for vibration. Longer Term Fixes: Review Chip to Pad Ratio's refer to the IPC standards. Wave solder pad ratios are different to the reflow requirements for pad design. Some components may need to be moved from bottom side to topside.
A Tombstone is created as a result of imbalance in the wetting forces across the part. This can be a result of pad design, paste deposition or wetting characteristics on the individual Pads. The adjacent machines seem an unlikely culprit, but the possibility for dewetting on one of the pads, or solder wicking along a track during second side reflow could create sufficient instability. Are there any features that may be robbing solder mass during the second side reflow?
There are many ROHS components that have been causing soldering issues. Many are manufacturer specific. Tombstoning can be controlled by using an LT type of powder blend in your solder paste. This will eliminate the tomb stoning issue. Contact your solder paste supplier and request the LT powder, profiling normally will not eliminate the problem.
Deck Street Consultants
One solution is to paste and glue the chip components. This is done in a two print process where paste is printed first then a glue stencil much thicker is used to print glue between the pasted pads of the chip component. The glue stencil has relief pockets etched on bottom side for paste relief. Works OK for chip components larger than 0603's.
Vice President Technology
You can try gluing the parts on the bottom side of the board. Or you can reduce the subzone temperature of the reflow cycle.
Regional Sales Manager
OK International Inc.
We have achieved second side reflow using SAC305 Paste but this is fairly hit and miss and subject to pad and component limitations. We would then usually recommend a Tin/Bismuth/Silver alloy and reflow the second side at 180C which inturn ensures the SAC305 at 219C doesnt reflow and this fixes the issue every time. Don't be tempted to use a non silver Alloy as they are ver brittle. Hope it helps.
Technical Sales Manager
BLT Circuit Services Ltd
Most likely the culprit is an imbalance of wetting between the two pads on each end of the component, caused by any of the reasons my fellow Viking Warrior Bjorn Dahle has so aptly described. A very quick fix to try is to simply change the pick and place program so it purposely places the components onto the pads at a slight angle (25 to 30 degrees off the long axis).If you have ever lifted large, heavy rectangular boxes, you understand it is quite a bit easier to tilt them up onto their edges than onto their corners.
The same is true of the wetting action of molten solder paste on a chip; it will pull the component into alignment first before it is able to lift either end. In the few microseconds it takes to pull the component into alignment, both ends are pretty well wetted to equilibrium and thus no tombstoning. The other advantage is that if either end does NOT pull into alignment due to any of the reasons on Bjorn's list, it stands out like a sore thumb and is easily detected as opposed to a perfectly placed chip that did not wet on one end, for whatever reason, but also did not tombstone enough to be noticeable.
A couple of other comments I would like to add: 1. If you are using Type 3 paste, switching to a Type 4 with its smaller grain size will reduce tombstones, but you need a very well-controlled print process to avoid excessive solder fines wedged between the soldermask and the edges of pads.
2. Using different solder alloys as suggested by some of the experts or even using two alloys, one for side one and another for side two, or LT powder additions, etc. should ALWAYS be qualified first, and then with the customer's buy-in and approval. Nearly all assembly drawings call out the type of alloy and may even specify a certain manufacturer's part number, so any changes may require an ECN first. Don't just arbitrarily jump in and make changes without some form of qualification and approval, as this can lead to disastrous consequences.
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