Printed Circuit Board Assembly & PCB Design SMT Electronics Assembly Manufacturing Forum

Printed Circuit Board Assembly & PCB Design Forum

SMT electronics assembly manufacturing forum.


Gary Simbulan

#15529

Cracking Capacitors and Solder Balls | 8 June, 1998

Earl, et al, Boy things get old and cold around here fast. I promised more detail on my capactior problem and I thought I could drop something completely different in the same message and tell a tale of solder balls. First the caps. We still have no solution but the details are starting to point to, if not faulty components, at least components which are being used outside parameters originally assumed by the manufacturer. These boards are used to construct voltage multiplyiers in which a ladder of capacitors and diodes bootstrap the voltage up. They typically are an inch wide and vary in length depending on the voltage desired and therefore the number of capacitors required. The caps are a 2020 package about .13 high. As noted before they are constructed of an NPO type of ceramic. The pads are .075 by .215 and are connected to the through holes for the diodes by very short, .010long, .030 wide traces. An interesting construction detail is that there is a .031 wide and .425 long slot routed beneath each capacitor. These slots are to limit voltage creep, a common problem in high voltage assemblies. The slots have the added advantage of allowing the potting material, which does have a high CTE, to expand below the board and thus limit any jacking that might occur. The slots extend all the way to the edge of the board so that the board has a series of flaps extending from the center of the board with capacitors connecting two flaps and straddling the slots. We use 12mil stencils with a 1 to 1 apeture to apply Sn62 paste. I know this seems like a lot of paste, but the engineering department insists that even this much paste is not as much as they would like. The contend, based on a DOE that they ran several years ago, that for these high voltage parts more is better. We apply a latex bead to hold the parts in place. This is because the proximity of the caps to each other causes them to pull together and the last guy in line ends up off his outside pad. We reflow, No Giggles Please, in our Vapor Phase reflow oven at 217 degrees C. My problem is that although we know there are many failures we cannot identify the exact process that a failed unit might have gone through. The parts are not marked in any way with serial or lot numbers or date codes as most marking techniques cause some problem in H.V. Therefore any rework performed by us or our customer (internal) is not traceable. Currently, since the use of the latex to secure parts we perform no rework at this level but we still cannot segregate our new boards from the previous builds. Suspects so far for the cracks are 1) improper rework techniques causing thermal shocking to the parts; 2) Defective parts; 3) mechanical stress caused by the temperature and voltage cycleing of the different materials and their different CTEs; 4) Flimsy board construction allowing excessive flex at the solder joint. In researching this, one term came up that I am not sure I understand completely and that is "neckdown". Could someone please enlighten me and let me know what effect this has on stress to these joints. Solder Balls If you waded through that last bit perhaps this story might interest you. I have recently had a severe problem with solder balls, especially, but not exclusively, on High Voltage non-soldermasked single or double sided boards. We use an RMA SN62 paste, Clean in Tri-Chlor 1-1-1 or Clean ATMS, and use the afore mentioned vapor phase reflow oven at about 215C to 217C. Except for the new cleaning solution, which is not used extensively, there has been no change in our basic processes for 8 years. Most of our boards are marked with a Serial Number and then baked to cure the ink. Then the boards are washed SMT components are placed. The solder balls I am seeing are differrent than anything I have seen before. The size of the indivdual balls is the same as the powder in the paste, ie. no coalescing at all. They are in a flood around the solder joint. The solder has obviously melted because there is a joint area of melted solder. We are able to view the relow process, albeit at a distance of about two feet, so that we can visually verify that the solder has reached liquidus. The effect is so extreame that I have seen mounds of unincorporated solder balls resting on top of the joint. On solder masked boards the problem is reduced but still enough to cause 100% rework. Now before we launch into suggestions concerning reducing the amount of paste, apeture adjustment, modernising our processes and materials and anything else you can come up with, let me tell you our current work around. By not washing the boards prior to relow the problem disappears. Our first assumption was contamination in the degreaser. Cleaned and recharged and still the same problem. I tried the new solvent, same story. I am convinced that there is something on the boards that the solvents loosen and allow to contaminate the joint area. I can reproduce this behavior at will with boards from the same manufactuing lot, clean one and not the other. What type of chemical or material would/could cause this and how would I test for it. We build a large number of double sided and mixed technology boards. Not being able to clean between reflow of the two sides with RMA flux is quite a hinderence. We are located in Washinton, north of Seattle. If anyone knows of a testing lab that could assist in the area I would appreciate that info also. Thanks for listening to my diatribes. Gary

reply »

Gary Simbulan

#15535

Re: Cracking Capacitors and Solder Balls | 8 June, 1998

| Earl, et al, | Boy things get old and cold around here fast. I promised more detail on my capactior problem and I thought I could drop something completely different in the same message and tell a tale of solder balls. | First the caps. We still have no solution but the details are starting to point to, if not faulty components, at least components which are being used outside parameters originally assumed by the manufacturer. | These boards are used to construct voltage multiplyiers in which a ladder of capacitors and diodes bootstrap the voltage up. They typically are an inch wide and vary in length depending on the voltage desired and therefore the number of capacitors required. The caps are a 2020 package about .13 high. As noted before they are constructed of an NPO type of ceramic. The pads are .075 by .215 and are connected to the through holes for the diodes by very short, .010long, .030 wide traces. An interesting construction detail is that there is a .031 wide and .425 long slot routed beneath each capacitor. These slots are to limit voltage creep, a common problem in high voltage assemblies. The slots have the added advantage of allowing the potting material, which does have a high CTE, to expand below the board and thus limit any jacking that might occur. The slots extend all the way to the edge of the board so that the board has a series of flaps extending from the center of the board with capacitors connecting two flaps and straddling the slots. We use 12mil stencils with a 1 to 1 apeture to apply Sn62 paste. I know this seems like a lot of paste, but the engineering department insists that even this much paste is not as much as they would like. The contend, based on a DOE that they ran several years ago, that for these high voltage parts more is better. We apply a latex bead to hold the parts in place. This is because the proximity of the caps to each other causes them to pull together and the last guy in line ends up off his outside pad. We reflow, No Giggles Please, in our Vapor Phase reflow oven at 217 degrees C. | My problem is that although we know there are many failures we cannot identify the exact process that a failed unit might have gone through. The parts are not marked in any way with serial or lot numbers or date codes as most marking techniques cause some problem in H.V. Therefore any rework performed by us or our customer (internal) is not traceable. Currently, since the use of the latex to secure parts we perform no rework at this level but we still cannot segregate our new boards from the previous builds. | Suspects so far for the cracks are 1) improper rework techniques causing thermal shocking to the parts; 2) Defective parts; 3) mechanical stress caused by the temperature and voltage cycleing of the different materials and their different CTEs; 4) Flimsy board construction allowing excessive flex at the solder joint. | In researching this, one term came up that I am not sure I understand completely and that is "neckdown". Could someone please enlighten me and let me know what effect this has on stress to these joints. | Solder Balls | If you waded through that last bit perhaps this story might interest you. I have recently had a severe problem with solder balls, especially, but not exclusively, on High Voltage non-soldermasked single or double sided boards. We use an RMA SN62 paste, Clean in Tri-Chlor 1-1-1 or Clean ATMS, and use the afore mentioned vapor phase reflow oven at about 215C to 217C. Except for the new cleaning solution, which is not used extensively, there has been no change in our basic processes for 8 years. Most of our boards are marked with a Serial Number and then baked to cure the ink. Then the boards are washed SMT components are placed. | The solder balls I am seeing are differrent than anything I have seen before. The size of the indivdual balls is the same as the powder in the paste, ie. no coalescing at all. They are in a flood around the solder joint. The solder has obviously melted because there is a joint area of melted solder. We are able to view the relow process, albeit at a distance of about two feet, so that we can visually verify that the solder has reached liquidus. The effect is so extreame that I have seen mounds of unincorporated solder balls resting on top of the joint. On solder masked boards the problem is reduced but still enough to cause 100% rework. | Now before we launch into suggestions concerning reducing the amount of paste, apeture adjustment, modernising our processes and materials and anything else you can come up with, let me tell you our current work around. By not washing the boards prior to relow the problem disappears. Our first assumption was contamination in the degreaser. Cleaned and recharged and still the same problem. I tried the new solvent, same story. I am convinced that there is something on the boards that the solvents loosen and allow to contaminate the joint area. I can reproduce this behavior at will with boards from the same manufactuing lot, clean one and not the other. What type of chemical or material would/could cause this and how would I test for it. We build a large number of double sided and mixed technology boards. Not being able to clean between reflow of the two sides with RMA flux is quite a hinderence. | We are located in Washinton, north of Seattle. If anyone knows of a testing lab that could assist in the area I would appreciate that info also. | Thanks for listening to my diatribes. | Gary Great to hear you are making so much reverse progress. Only kidding. Only have time and "talent" to address the neckdown thing. As I said earlier, if the chip device solder termination area (pad) is not allowed to float as an "island" on a sea of resin (the board's surface and bonding to conductive foil material), something has to give. That something often is the device attached to the termination pads. In high current, high frequency (R/F) designs, often neckdown features are ignored to ensure current carrying and impedance requirements are met. If you have proper neckdown features (per appropriate IPC design guidlines), for soldering, you usually won't experience this problem. If you don't, you very well might because, again, the copper expands at 17-19ppm compared with cermanic's TCE of only 5-7. Again, this is something I have encountered in the past as it applies to high current and very high frequency designs for reasons of getting electrons down the path as fast as possible, and without burning up the whole thing. Thanks for your informative response and more questions, Earl Moon

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Gary Simbulan

#15534

Re: Cracking Capacitors and Solder Balls | 8 June, 1998

| Earl, et al, | Boy things get old and cold around here fast. I promised more detail on my capactior problem and I thought I could drop something completely different in the same message and tell a tale of solder balls. | First the caps. We still have no solution but the details are starting to point to, if not faulty components, at least components which are being used outside parameters originally assumed by the manufacturer. | These boards are used to construct voltage multiplyiers in which a ladder of capacitors and diodes bootstrap the voltage up. They typically are an inch wide and vary in length depending on the voltage desired and therefore the number of capacitors required. The caps are a 2020 package about .13 high. As noted before they are constructed of an NPO type of ceramic. The pads are .075 by .215 and are connected to the through holes for the diodes by very short, .010long, .030 wide traces. An interesting construction detail is that there is a .031 wide and .425 long slot routed beneath each capacitor. These slots are to limit voltage creep, a common problem in high voltage assemblies. The slots have the added advantage of allowing the potting material, which does have a high CTE, to expand below the board and thus limit any jacking that might occur. The slots extend all the way to the edge of the board so that the board has a series of flaps extending from the center of the board with capacitors connecting two flaps and straddling the slots. We use 12mil stencils with a 1 to 1 apeture to apply Sn62 paste. I know this seems like a lot of paste, but the engineering department insists that even this much paste is not as much as they would like. The contend, based on a DOE that they ran several years ago, that for these high voltage parts more is better. We apply a latex bead to hold the parts in place. This is because the proximity of the caps to each other causes them to pull together and the last guy in line ends up off his outside pad. We reflow, No Giggles Please, in our Vapor Phase reflow oven at 217 degrees C. | My problem is that although we know there are many failures we cannot identify the exact process that a failed unit might have gone through. The parts are not marked in any way with serial or lot numbers or date codes as most marking techniques cause some problem in H.V. Therefore any rework performed by us or our customer (internal) is not traceable. Currently, since the use of the latex to secure parts we perform no rework at this level but we still cannot segregate our new boards from the previous builds. | Suspects so far for the cracks are 1) improper rework techniques causing thermal shocking to the parts; 2) Defective parts; 3) mechanical stress caused by the temperature and voltage cycleing of the different materials and their different CTEs; 4) Flimsy board construction allowing excessive flex at the solder joint. | In researching this, one term came up that I am not sure I understand completely and that is "neckdown". Could someone please enlighten me and let me know what effect this has on stress to these joints. | Solder Balls | If you waded through that last bit perhaps this story might interest you. I have recently had a severe problem with solder balls, especially, but not exclusively, on High Voltage non-soldermasked single or double sided boards. We use an RMA SN62 paste, Clean in Tri-Chlor 1-1-1 or Clean ATMS, and use the afore mentioned vapor phase reflow oven at about 215C to 217C. Except for the new cleaning solution, which is not used extensively, there has been no change in our basic processes for 8 years. Most of our boards are marked with a Serial Number and then baked to cure the ink. Then the boards are washed SMT components are placed. | The solder balls I am seeing are differrent than anything I have seen before. The size of the indivdual balls is the same as the powder in the paste, ie. no coalescing at all. They are in a flood around the solder joint. The solder has obviously melted because there is a joint area of melted solder. We are able to view the relow process, albeit at a distance of about two feet, so that we can visually verify that the solder has reached liquidus. The effect is so extreame that I have seen mounds of unincorporated solder balls resting on top of the joint. On solder masked boards the problem is reduced but still enough to cause 100% rework. | Now before we launch into suggestions concerning reducing the amount of paste, apeture adjustment, modernising our processes and materials and anything else you can come up with, let me tell you our current work around. By not washing the boards prior to relow the problem disappears. Our first assumption was contamination in the degreaser. Cleaned and recharged and still the same problem. I tried the new solvent, same story. I am convinced that there is something on the boards that the solvents loosen and allow to contaminate the joint area. I can reproduce this behavior at will with boards from the same manufactuing lot, clean one and not the other. What type of chemical or material would/could cause this and how would I test for it. We build a large number of double sided and mixed technology boards. Not being able to clean between reflow of the two sides with RMA flux is quite a hinderence. | We are located in Washinton, north of Seattle. If anyone knows of a testing lab that could assist in the area I would appreciate that info also. | Thanks for listening to my diatribes. | Gary

Just had time to read the second part of your misery. I know you guys and the good work you do from many past associations. As far as your lab goes, if you haven't already, try Surface Science Labs in Mt. View, CA in the 415 or new 650 area (don't have any of my numbers or spec's with me on this trip). They have every imaginable surface contamination detection device or process known or knows who does. Earl Moon

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Steve Gregory

#15532

Re: Cracking Capacitors and Solder Balls | 9 June, 1998

Hi there Gary, Your solder ball problem is very similar the problem I experienced, but I was using a water soluble paste at the time. It started by inspectors telling me that they were seeing cold solder on some SIMM's that we had built regularly. When I asked to see one of the assemblies, there was the exact same appearance that you speak of, a solid fillet covered with a layer of spheres nearly the same mesh as the raw paste. But the strange thing in my case was, one lead on a DRAM would show the appearance, while the lead right next to it was fine. When I spoke to the inspectors and told them that it wasn't cold solder, they started busting out laughing! "Well, Mr. Engineer, if it's not cold solder, what is it?" I replied; "I don't know yet, but it's not cold solder." I tried to show them the solid fillet beneath the layer of spheres, and also explained that convection heat doesn't work that way, reflowing one pad and not the one next to it, but they weren't buying any of it. It was really weird, we might run a 10,000 piece work order and not see the problem at all, but then run a much smaller lot and it would show-up on 50 or so SIMM's...and it would always be that random pad here, pad there type of pattern. Without getting into the long story of how I finally found out what was causing it, it turned out to be caused from an operator using a wet rag to clean the bottom of the stencil on the one printer we had that didn't have an automatic cleaner...a old SP-200 we used to do the generic 1Meg X 9 SIMM's with. His heart was in the right place, but sometimes his rag wouldn't be just damp, but pretty darn wet, sometimes so much so that water would remain in a few apertures and affect the flux chemistry so much that a good percentage of the solder printed from that water soaked aperture wouldn't coalesce into the main fillet. -Steve Gregory-

reply »

Aric Parr

#15533

Re: Cracking Capacitors and Solder Balls | 9 June, 1998

| Hi there Gary, | Your solder ball problem is very similar the problem I experienced, but I was using a water soluble paste at the time. It started by inspectors telling me that they were seeing cold solder on some SIMM's that we had built regularly. | When I asked to see one of the assemblies, there was the exact same appearance that you speak of, a solid fillet covered with a layer of spheres nearly the same mesh as the raw paste. But the strange thing in my case was, one lead on a DRAM would show the appearance, while the lead right next to it was fine. | When I spoke to the inspectors and told them that it wasn't cold solder, they started busting out laughing! "Well, Mr. Engineer, if it's not cold solder, what is it?" I replied; "I don't know yet, but it's not cold solder." I tried to show them the solid fillet beneath the layer of spheres, and also explained that convection heat doesn't work that way, reflowing one pad and not the one next to it, but they weren't buying any of it. | It was really weird, we might run a 10,000 piece work order and not see the problem at all, but then run a much smaller lot and it would show-up on 50 or so SIMM's...and it would always be that random pad here, pad there type of pattern. | Without getting into the long story of how I finally found out what was causing it, it turned out to be caused from an operator using a wet rag to clean the bottom of the stencil on the one printer we had that didn't have an automatic cleaner...a old SP-200 we used to do the generic 1Meg X 9 SIMM's with. | His heart was in the right place, but sometimes his rag wouldn't be just damp, but pretty darn wet, sometimes so much so that water would remain in a few apertures and affect the flux chemistry so much that a good percentage of the solder printed from that water soaked aperture wouldn't coalesce into the main fillet. | -Steve Gregory- You also could get that problem from wet/old paste or component outgassing. Improper profiling may also cause this problem, with the paste still wet prior to reflow. You could actually explode the paste out of the pad area. I once tried the IPC solderball test (screen and reflow on ceramic (use hot plate). Many "good pastes" had some of these balls.

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Gary Simbulan

#15531

Re: Cracking Capacitors and Solder Balls | 16 June, 1998

There are some common reason for capacitor cracking on my web page if you want to have a look. The document can be downloaded for your reference. | Earl, et al, | Boy things get old and cold around here fast. I promised more detail on my capactior problem and I thought I could drop something completely different in the same message and tell a tale of solder balls. | First the caps. We still have no solution but the details are starting to point to, if not faulty components, at least components which are being used outside parameters originally assumed by the manufacturer. | These boards are used to construct voltage multiplyiers in which a ladder of capacitors and diodes bootstrap the voltage up. They typically are an inch wide and vary in length depending on the voltage desired and therefore the number of capacitors required. The caps are a 2020 package about .13 high. As noted before they are constructed of an NPO type of ceramic. The pads are .075 by .215 and are connected to the through holes for the diodes by very short, .010long, .030 wide traces. An interesting construction detail is that there is a .031 wide and .425 long slot routed beneath each capacitor. These slots are to limit voltage creep, a common problem in high voltage assemblies. The slots have the added advantage of allowing the potting material, which does have a high CTE, to expand below the board and thus limit any jacking that might occur. The slots extend all the way to the edge of the board so that the board has a series of flaps extending from the center of the board with capacitors connecting two flaps and straddling the slots. We use 12mil stencils with a 1 to 1 apeture to apply Sn62 paste. I know this seems like a lot of paste, but the engineering department insists that even this much paste is not as much as they would like. The contend, based on a DOE that they ran several years ago, that for these high voltage parts more is better. We apply a latex bead to hold the parts in place. This is because the proximity of the caps to each other causes them to pull together and the last guy in line ends up off his outside pad. We reflow, No Giggles Please, in our Vapor Phase reflow oven at 217 degrees C. | My problem is that although we know there are many failures we cannot identify the exact process that a failed unit might have gone through. The parts are not marked in any way with serial or lot numbers or date codes as most marking techniques cause some problem in H.V. Therefore any rework performed by us or our customer (internal) is not traceable. Currently, since the use of the latex to secure parts we perform no rework at this level but we still cannot segregate our new boards from the previous builds. | Suspects so far for the cracks are 1) improper rework techniques causing thermal shocking to the parts; 2) Defective parts; 3) mechanical stress caused by the temperature and voltage cycleing of the different materials and their different CTEs; 4) Flimsy board construction allowing excessive flex at the solder joint. | In researching this, one term came up that I am not sure I understand completely and that is "neckdown". Could someone please enlighten me and let me know what effect this has on stress to these joints. | Solder Balls | If you waded through that last bit perhaps this story might interest you. I have recently had a severe problem with solder balls, especially, but not exclusively, on High Voltage non-soldermasked single or double sided boards. We use an RMA SN62 paste, Clean in Tri-Chlor 1-1-1 or Clean ATMS, and use the afore mentioned vapor phase reflow oven at about 215C to 217C. Except for the new cleaning solution, which is not used extensively, there has been no change in our basic processes for 8 years. Most of our boards are marked with a Serial Number and then baked to cure the ink. Then the boards are washed SMT components are placed. | The solder balls I am seeing are differrent than anything I have seen before. The size of the indivdual balls is the same as the powder in the paste, ie. no coalescing at all. They are in a flood around the solder joint. The solder has obviously melted because there is a joint area of melted solder. We are able to view the relow process, albeit at a distance of about two feet, so that we can visually verify that the solder has reached liquidus. The effect is so extreame that I have seen mounds of unincorporated solder balls resting on top of the joint. On solder masked boards the problem is reduced but still enough to cause 100% rework. | Now before we launch into suggestions concerning reducing the amount of paste, apeture adjustment, modernising our processes and materials and anything else you can come up with, let me tell you our current work around. By not washing the boards prior to relow the problem disappears. Our first assumption was contamination in the degreaser. Cleaned and recharged and still the same problem. I tried the new solvent, same story. I am convinced that there is something on the boards that the solvents loosen and allow to contaminate the joint area. I can reproduce this behavior at will with boards from the same manufactuing lot, clean one and not the other. What type of chemical or material would/could cause this and how would I test for it. We build a large number of double sided and mixed technology boards. Not being able to clean between reflow of the two sides with RMA flux is quite a hinderence. | We are located in Washinton, north of Seattle. If anyone knows of a testing lab that could assist in the area I would appreciate that info also. | Thanks for listening to my diatribes. | Gary

reply »

Gary Simbulan

#15530

Re: Cracking Capacitors and Solder Balls | 16 June, 1998

There are some common reason for capacitor cracking on my web page if you want to have a look. The document can be downloaded for your reference. | Earl, et al, | Boy things get old and cold around here fast. I promised more detail on my capactior problem and I thought I could drop something completely different in the same message and tell a tale of solder balls. | First the caps. We still have no solution but the details are starting to point to, if not faulty components, at least components which are being used outside parameters originally assumed by the manufacturer. | These boards are used to construct voltage multiplyiers in which a ladder of capacitors and diodes bootstrap the voltage up. They typically are an inch wide and vary in length depending on the voltage desired and therefore the number of capacitors required. The caps are a 2020 package about .13 high. As noted before they are constructed of an NPO type of ceramic. The pads are .075 by .215 and are connected to the through holes for the diodes by very short, .010long, .030 wide traces. An interesting construction detail is that there is a .031 wide and .425 long slot routed beneath each capacitor. These slots are to limit voltage creep, a common problem in high voltage assemblies. The slots have the added advantage of allowing the potting material, which does have a high CTE, to expand below the board and thus limit any jacking that might occur. The slots extend all the way to the edge of the board so that the board has a series of flaps extending from the center of the board with capacitors connecting two flaps and straddling the slots. We use 12mil stencils with a 1 to 1 apeture to apply Sn62 paste. I know this seems like a lot of paste, but the engineering department insists that even this much paste is not as much as they would like. The contend, based on a DOE that they ran several years ago, that for these high voltage parts more is better. We apply a latex bead to hold the parts in place. This is because the proximity of the caps to each other causes them to pull together and the last guy in line ends up off his outside pad. We reflow, No Giggles Please, in our Vapor Phase reflow oven at 217 degrees C. | My problem is that although we know there are many failures we cannot identify the exact process that a failed unit might have gone through. The parts are not marked in any way with serial or lot numbers or date codes as most marking techniques cause some problem in H.V. Therefore any rework performed by us or our customer (internal) is not traceable. Currently, since the use of the latex to secure parts we perform no rework at this level but we still cannot segregate our new boards from the previous builds. | Suspects so far for the cracks are 1) improper rework techniques causing thermal shocking to the parts; 2) Defective parts; 3) mechanical stress caused by the temperature and voltage cycleing of the different materials and their different CTEs; 4) Flimsy board construction allowing excessive flex at the solder joint. | In researching this, one term came up that I am not sure I understand completely and that is "neckdown". Could someone please enlighten me and let me know what effect this has on stress to these joints. | Solder Balls | If you waded through that last bit perhaps this story might interest you. I have recently had a severe problem with solder balls, especially, but not exclusively, on High Voltage non-soldermasked single or double sided boards. We use an RMA SN62 paste, Clean in Tri-Chlor 1-1-1 or Clean ATMS, and use the afore mentioned vapor phase reflow oven at about 215C to 217C. Except for the new cleaning solution, which is not used extensively, there has been no change in our basic processes for 8 years. Most of our boards are marked with a Serial Number and then baked to cure the ink. Then the boards are washed SMT components are placed. | The solder balls I am seeing are differrent than anything I have seen before. The size of the indivdual balls is the same as the powder in the paste, ie. no coalescing at all. They are in a flood around the solder joint. The solder has obviously melted because there is a joint area of melted solder. We are able to view the relow process, albeit at a distance of about two feet, so that we can visually verify that the solder has reached liquidus. The effect is so extreame that I have seen mounds of unincorporated solder balls resting on top of the joint. On solder masked boards the problem is reduced but still enough to cause 100% rework. | Now before we launch into suggestions concerning reducing the amount of paste, apeture adjustment, modernising our processes and materials and anything else you can come up with, let me tell you our current work around. By not washing the boards prior to relow the problem disappears. Our first assumption was contamination in the degreaser. Cleaned and recharged and still the same problem. I tried the new solvent, same story. I am convinced that there is something on the boards that the solvents loosen and allow to contaminate the joint area. I can reproduce this behavior at will with boards from the same manufactuing lot, clean one and not the other. What type of chemical or material would/could cause this and how would I test for it. We build a large number of double sided and mixed technology boards. Not being able to clean between reflow of the two sides with RMA flux is quite a hinderence. | We are located in Washinton, north of Seattle. If anyone knows of a testing lab that could assist in the area I would appreciate that info also. | Thanks for listening to my diatribes. | Gary

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