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

Printed Circuit Board Assembly & PCB Design Forum

SMT electronics assembly manufacturing forum.

solder balling

Ramon I Garcia C


solder balling | 5 August, 2000

Hello Friends: I'm new in SMT process, I have a problem with little balls of solder beside of the component, I'm tried reducing the apertures in the stencil and making home base, some times the balls desappear, but some time not, and some times and sometimes appear insuficientsof solder in some components, would somebody recommend me the way to calcula the correct ammount of solder in each component size, somebady know a good book that explain me to way to calculate this amount and when is recomemmended the kind of paste shape for each lead.

Thanks all, I'd appreciate all your information.

Ramon Garcia.

PS. excuse my english but I'm learnig.

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Re: solder balling | 7 August, 2000

Ramon, I have had experience in the problems you are witnessing. The problem can relate to a number of factors.

Obviously the printing process is the first place to start your investigations, try looking at the stencil apertures for bleed under the stencil. This would suggest poor gasketing between the stencil and the land. Maybe the lands have a domed appearance. (not flat, appears more on a HASL finish). If this is the case then try using your screen printers under screen wipe more frequently. Or clean by hand. This will avoid depositing the solder beads onto the PCB substrate every time you print and hopefully keep the solder paste on the lands.

We found that by re-designing the apertures on the stencil to the specified component type the problem was solved. In our example the solder balls were appearing on the chip resistors and capacitors and therefore we �opened� the distance between the apertures to a value slightly greater than the distance between the two inner sides of the lands. Effectively we removed the possibility of depositing solder paste from the under the component body. Keep the solder to a minimum under the component terminations. Things like home base etc are fine, but can make the task of stencil design very time consuming and tedious.

The solder resist on the board plays a big part in �satellite� solder balls, and whether it has been curtain coated or screen printed !. This is possibly the hardest process to change, so try your process improvements first.

The profile of you oven could also play a large factor in the beading. Keep it simple and try the other process changes first they should help.

Good Luck.

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Brian W.


Re: solder balling | 7 August, 2000

If you are satisfied with the apertures, then you need to check placement pressure, plant humidity, and the reflow profile. If the placement pressure is too great, it will separate minute portions of the solder from the pad. When the solder reflows, it has no place to go but out the sides. More than likely it is the reflow profile. If you ramp the profile too quickly, all the volatiles dry out too fast. The flux then begins to dry out, resulting in solder balls. Make sure that the max rising slope is around 2 degrees c per second. I like to ramp on more of straight line. When I bring it up fast and soak for a long time, that's usually when I see solder balls. You should also check the hunidity in the plant, and make sure it is between 30 and 65 %RH. Above or below that can affect the solder paste if it remains on the screen too long. I mention this because you did not specify what screen printer you are using. Many of the newer screen printer have temperature and humidity controls. Using a Nitrogen atmosphere in the reflow oven can also help.

Good luck, Brian W.

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Ramon I Garcia C


Re: solder balling | 7 August, 2000


Thanks frieds for your advising, I have a MPM UP2000, I'm going to chek the humidity control, too I'll chek the profiles again, this plant it's located close to the beach about 1/2 mile from. However if you know some table to calculate the paste ammuont for each lead I'l to appreciate.

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Jarno Py�ri�


Re: solder balling | 9 August, 2000


I think, good stencil apertures is 80% from pad. And stencil thickness is 0.15mm - 0.20mm. I use 0.15mm thick stencil almost every pcb. Sorry my very bad english but i too learning. :)

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Jarno Py�ri�


Re: solder balling | 9 August, 2000

And little more...

If you use chip components, then that 80% from pads is good stencils apertures size. But if you use fine pitch components, example QFP, then you can use apertures, 90% from pads. I hoping that you understand something about my "letter", because my english.

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Re: solder balling | 10 August, 2000


Solder ball' are a bitch aint they?, identifying the right location to fix em is a bigger one. As you and the rest of teh guy's have said yes you'll need to look at your stenil..but maybe your stencil is good so you'll need to look else where... Hint... Start at the easy stuff first. Check your set up of your MPM. Is you board position right, is it contacting teh screen or is there a snap off in there. What pressure are you using and what is your upport like under the PCB?...could it be flexing under poor support or excess pressure and loosing you the seal on you component lands. If you pads are domed them you will get a bad seal and allow the paste out from under the stencil. Is your envoironment dry and at teh right temps? As for your reflow profile, check what your paste supplier is saying, they're normally pretty good and like our colegue said watch your ramp rates and dwell times (if yor paste has one!)


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Bob Willis


Re: solder balling | 10 August, 2000

Here is some thing that I wrote a while back on solder beading.

First what is a Solder Bead?

The term solder bead is used to differentiate it from solder balls. A solder bead is a solder ball but its location is normally constant unlike solder balls. Solder beads are located on the side of chip components between the terminations and away from the lands as illustrated in the photographs.

Solder beads are seen more commonly with no clean processes due to the fact that there is no cleaning process to wash them away. The solder beads are normally 0.010" in size but this can vary depending on the amount of solder paste available to form them. On some occasions there can be more than one ball at any one location.

Solder beads can also be found at the side of SOT89, SOT23 and on leadless ceramic chip carriers. Many of your production and quality engineering colleagues look to a specification for guidance on the acceptable size of a ball as they are very difficult and time consuming to remove. The real solution is find the cause and that will help you to eliminate the problem.

Figure 1 Example of solder bead on a SOT23 042.jpg

The solder bead is formed when solder paste is present under a component and can not form a joint at the solderable land or component termination. Solder paste can become trapped under the body of a component and then when the paste particles reflow it starts to coaless together to form a larger ball. Normally speaking the solder paste at the end terminations has already reflowed and wet the joint so the paste under the device can not form due to the limited space available under the part. This causes the liquid solder to flow from under the part at this point the solder is free to form a bead and hence the defect. You can see the solder beading phenomena demonstrated on my video tape titled "Reflow Soldering and Temperature Profiling".

After explaining what it is and how it forms let us together consider the causes and corrective action. Stencil Printing can cause the problem if the incorrect process parameters are used. The stencil aperture should be slightly reduced from the pad size 10% reduction is quite a common rule in our industry.

The stencil should make contact with the surface of the pad before and during printing this prevents paste being forced under the stencil causing it to be transferred to the surface of the next board. This correct initial set-up prevents the need for high squeegee pressures it also reduced the rate of under stencil wiping. Excess pressure will cause you problems paste is forced under the stencil and possibly where parts are to be placed.

Placement can cause the paste to be forced under the component body. Setting excessive placement force will displace solder paste between the component and resist coating. During reflow the beads will form. Removing components before reflow in areas where beads have formed in the past you can generally detect if this is the cause of the problem. The same thing can occur if you have a variation in the paste height. With the same placement force if the paste is higher you get the same end result.

Reflow Soldering inspection is where the solder beading fault is generally first detected. In-process inspection after reflow will allow you to detect the rate of solder beading. In the case of reflow it is the rate of paste slump which causes the solder paste to be present under the parts. When evaluating solder paste a key factor is paste slump which can occur during the early stages of pre heat or just before the paste moves into the reflow stage.

Figure 2 The most common place to see solder beads are at the side of a chip component. 029.jpg

Simple reflow tests using a board which has been printed and components placed can help you understand the process. Placing the board on a hot plate using similar profile parameters as the oven will allow the whole sequence to be viewed. An alternative is to take a populated board and pass it through the reflow oven without the final reflow spike. This can be achieved by lowering the final zone temperature which will give the sample board the complete pre heat cycle without reflow. The sample board can then be examined by removing selected chips and examining the amount of paste under the parts.

Solder slump tests on paste is a standard paste test you should conduct using a ceramic tile and printing small dots of paste. A measure of the diameter of the paste is made before and after heat exposure. As the paste does not reflow you can measure the worst possible slump for a given paste and oven profile. This test is normally refereed to as hot slumping.

As I have previously stated the oven profile and the point when the flux activates can also increase or decrees the amount of solder beads. That is why so often changing a paste can reduce the problem. This demonstrates that the original profile used was incorrect and poorly specified by the engineering staff. Remember that a poorly set-up oven profile can make the best paste on the market look poor.

Solder paste particles rely on their initial surface cleanliness and the activity of the paste to allow them to reflow and coaless in to a single ball. If the activity is low then it is quite possible for any paste to leave multiple balls at the side of chip parts. Inevitably as the components get smaller the distance between terminations on the chips becomes smaller. This may increase the presence of solder beads.

The solder mask thickness between the chip and board will exaggerate the problem as a limited space is available to allow the paste to flow back to the original print area. From a design point of view solder mask under chip components is only required if a track is placed under a part. If there is no mask the paste will flow back to the termination points more readily. Solder mask apertures are a necessity around pads when a track or via hole is present under a component.

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