How to choose a new solder paste

Phil Zarrow wrote a useful paper that could help in evaluating paste. Here read it ...

Evaluating Solder Paste � Not An Option
Contributed by Phil Zarrow of ITM an Independent SMT consulting firm
With soldering being the dominant source of assembly defects and solder paste being the
key component of the process, how come a majority of SMT assembly facilities have no
idea how they began using the solder paste that they do. While a few assemblers perform
comprehensive solder paste evaluations, the vast majority of users either don�t know how
Solder paste testing should be qualified for your applications in your facility. If you think
that solder paste is a commodity in that �they all perform the same�, you are in for a major
surprise.
Factors affecting solder performance include equipment and set-up parameters, stencil
fabrication methodology, component lead density, operator skills, component and board
solderability as well as ambient temperature and humidity The ideal solder paste must
perform as good as possible in the range of applications present and within the printing
environment variation range. You can�t take someone else�s word for it, either, you have
to test them yourself. Remember, In God We Trust, All Others Bring Data.
How does one go about testing solder pastes?
There are a number of procedures out there, including those published by the IPC and by
solder paste manufacturers, and there are a few consultants out there well versed in this
area that can help you. Some of these procedures use existing production equipment while
others may require elaborate laboratory equipment.
Here are some of the tests that can be used with the minimum of equipment and materials
and yet get you on your way towards specifying the best solder paste for your
application.
Quantitative Solder Ball Test
One of the most common soldering defect that occurs in SMT assembly is the formation
of solder balls. These can be especially deadly to circuit functionality (as they have a
propensity for causing electrical shorts) in a no-clean process. Here we don�t have the
benefit of cleaning to �wash our sins away�. There are numerous factors in the soldering
process that can cause fines of solder to separate from the main mass (fillet). These
include improper reflow profile, lack of solderability of components and/or lands,
misalignment of solder mask, misalignment of solder print, and many others but solder
paste must not be a contributor to this problem. Sadly, many formulations do indeed
exhibit a tendency towards solder balling. Regardless of whether or not we are cleaning
the assemblies or using a no-clean process, the solder paste itself should not be solder ball
prone. Therefore we utilize a test that eliminates all the other factors from the �stew�.
The most straightforward method is to print a circle of solder paste onto a ceramic
substrate. Here the solder has nothing to interact with but itself. Be sure that the ceramic
substrate has been profiled in the reflow oven to the time/temperature duration that each
solder paste manufacturer specifies. Using a stainless steel �stencil� as thick as you
normally use, manually print a circle of paste .250� in diameter. After reflowing the
substrate, take a look at the resulting fillet under 10X magnification. Evaluate the number
and size of solder balls that satellite the fillet (ideally there should be none).
Residue Examination
This particular test also allows for the opportunity to visually examine the residue of a noclean
formulation. Since the alumina substrate is white, one can assess the clarity (color)
and thickness of the resulting residue surrounding the fillet. Of course, one of the major
issues with very-low and ultra-low residue no-cleans is the ability to penetrate the residue
with test probes to accommodate reliable In-Circuit testing. The residue should not
impede contact with the test pads nor should it gum up the probe, both of which will lead
to false readings. One means of testing this is to hook up a 5 volt power supply and
digital voltmeter in conjunction with a force gauge. This would allow for comparative
measurement of the force required to penetrate the flux residue to accommodate good
contact.
Solder Wetting
The typical test for the ability of the paste to wet is to print the solder paste onto a
substrate of bare copper clad laminate. Of course, no one really solders to bare copper so
this test is one of �relativity�. It is best to equally prep the substrates by deoxidizing them
as much as possible either chemically or with abrasion. If you have the facilities for crosssectioning
the resulting fillets and measuring the angle that way, print the .250� diameter
by .006� thick circles. However, lacking this, obtain a stencil comprised of a series of
.025� x .050� apertures, arranged in a rows with spacing between the apertures varying as
follows: .050�, .040�, .025�, .015�, .010� then .010�, .015�, .020�, .025�, .040� and .050�
respectively. Print the paste onto the copper-clad laminate, reflow to manufacturers
requirements as profiled for the substrate. After reflowing, clean the substrates in
isopropyl alcohol and examine under 10 X magnification. Determine and record the
smallest unbridged gap between two adjacent pads within a row, on either side of the test
pattern. Note that the gap is considered �unbridged� if the reflowed solder is not in
contact whereas a simple contact between two tinning layers is considered �bridged�.
If you are using, or thinking of going to Organic Solder Protectant (OSP) coated
substrates, instead of HASL, this test becomes particularly pertinent and extremely
important. Use copper-clad laminates coated with the appropriate OSP and perform the
test as described. Evaluations performed by ITM with our clients has demonstrated that
many solder paste formulations, both OA and no clean, are not compatible with OSPs,
including several that specifically claim they are.
Slump
Solder paste performance during the printing operation can have a profound result on the
resulting solder joints. One of the factors that must be considered is the solder paste�s
ability to stay well defined once released from the stencil. The material can collapse on
itself or slump and spill over causing bridging. While a number of circumstances can
contribute to this, including printing parameters, stencil parameters and others, the paste�s
rheology must be such that it is not a contributor to this defect.
Use the stencil described in the wetting test or one similar such as those designed to IPCA-
21 or IPC-A-20.3 Carefully print onto ceramic substrates or glass microscope slides.
After carefully releasing the stencil, examine under 30 X magnification and record the
incidence of slump, particularly at which pitch it became prevalent.
Now comes the part that separates the �men from the milquetoast�. Most pastes perform
fairly well here yet, to the assembler�s surprise, there is still occasional bridging as a result
of slumping. What happens is that the rheology of the paste changes during the reflow
cycle? As the solvents are driven off, particularly during the Preheat Soak stage, the
chemistry of the paste is altered. This is typically where slump occurs. Thus it is
important that these conditions be simulated during paste evaluation. This can be done by
taking the previously printed and examined slump test printings that were evaluated at
ambient temperature and placing them in a box oven set at 150 deg. C for two minutes.
Remove the substrates from the oven and evaluate the occurrence of slump. ITM has
found that most mainstream solder pastes in the market perform well in ambient but
several leave a lot to be desired with regard to their performance during reflow.
Tack
The solder paste has to act as an adhesive to hold the components in place until the
soldering cycle is completed. Tack should be evaluated as pre-placement and postplacement.
For pre-placement evaluation, print several boards with each solder paste and
evaluate the tack at 1-hour intervals. You can use a force gauge to do this or you can
populate the boards at these intervals with discretes (i.e. 1206s, 0805s, etc.), invert the
boards and count the components that fall off per each respective solder paste. I
recommend you go at least 4 hours out but take into account whether you are printing inline
or in batch mode. We are also simulating placement machine breakdown � something,
of course, that rarely occurs.
For post-placement tack, you can use the same inverted board test but here, you populate
several boards (of each paste) and wait for 1-hour intervals before inverting and assessing
component fall-off. Again, consider going 4 to 6 hours out on this, depending upon you
manufacturing scheme and how long boards are likely going to sit after placement before
they are reflowed.
If you are using a recent vintage high-speed turret type chip-shooter, it is recommended
that you perform tack tests with the machine. Users have reported some pastes lacking
adequate tack resulting in components literally skidding off the pastes during placement.
Worklife
Solder paste worklife should also be assessed. How long can the paste remain on the
stencil and still print adequately. This is a matter of printing a board with the range of
component pitches you normally encounter at 1-hour intervals, leaving the paste on the
stencil in the interim. Evaluate the quality of paste at these intervals examining the shape
of the deposit (should be a �brick�) and watching for skips and bridges.
It is incredible how yields can be improved just by assuring that the proper solder paste for
the application has been qualified. This is one area you definitely do not want to take for
granted. We�re all in this together.
� i.e. a Chatillon DFG-2 Force gauge with motorized stand.
� These stencils are readily available from most stencil suppliers including Alpha Metal

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Here's what I do:

- Test Surface Insulation Resistance (we're in the high reliability market and use a test significantly harder than IPC's). I also test SIR in combination with other assembly materials like conformal coatings and wave solder fluxes.
- Test wetting and solder balling per IPC-TM-650
- Test ambient and hot slump per IPC-TM-650. I want it to be equivalent or better than my current paste.
- Test print volume using SPI of "difficult" features at 25 and 125 mm/second print speed. Define "difficult" based on your products. I use 0201 chips and 0.5 mm pitch BGA sites
- Test abandon time at 30 and 60 minutes. I want the paste volume of the first board to look like the last one did before the line went down.
- Test coalesence of fine features such as 0201 or finepitch BGA after reflow. Don't place parts - just print paste and reflow. Check under a microscope to make sure the deposits are smooth and don't look like a pile of BB's.
- Place chips and reflow. Check for mid-chip solderballs and tombstones. These are typically more driven by design than solder paste, but a lousy paste can aggravate them.
- Assemble boards with a variety of parts and inspect to whatever level of IPC Workmanship Standards you work to.
- Make sure the paste doesn't stick to the squeegees. This is a particular pet peeve of mine.
- Ask your test engineers if the residues are going to give them grief with their ICT probes. I've never had a test engineer give me a good test method for this so I dump the responsibility on them.
- If you use AOI, make sure the paste residues don't mess things up.
- Make sure your conformal coatings will cure and adhere to the board properly. Some paste residues cause coatings not to cure or to peel off the board easily.
- Test pin-in-paste if you use that process. Make sure the solder wets back to the pin and barrel and doesn't leave solder balls. Try to see if it drips in the oven pre-heat section, although your sample size will probably be too small to catch it.
- Repeat all of the above tests (except SIR) with paste (still sealed in tubes or jars) that has sat in a 30C/70% RH chamber for a week to simulate being mishandled during shipment. I've seen temp/humidity preconditioning cause solder balls, lousy wetting, print problems, and sticking to the squeegees.

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Hi,

There are quite a few suppliers so you could have a lot of work on your hands trying them all.

Do a bit of research finding out what paste all the big guns use? they spend a fortune doing evaluations etc

But you should get at least 3 different suppliers and put them through your process and record performances.

I have my favourite but probably unfair to mention here but email me if you wish.

Rgd's

AJ...

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Thanks everybody for the answers.
I already chose 3 differents solder paste brands.
And, how many boards it's necessary to perform these evaluation tests to have a reliable result? 20, 50, 100 boards??
Thanks.

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