Selective Printing for BGA Components

We use common BGA packages on many of our low-end products. To save cost, could we print paste only on the pads that have functional requirements?

For example, could we print all the perimeter pads on a 120 pin BGA, but skip printing on 50 or more of the interior pads since they serve no function?

A.J.

Expert Panel Responses

At what cost to the integrity , structural, of the component. Could you do it... sure. Should you do it ... no.

Lets think of this as a bldg. I've got 12 vertical columns that support the structure. In actuality only 6 support the weight of the bldg. So I will leave 6 inches below those remaining six columns to save the cost of rebar & concrete. Bldg is supported but not to its maximum capability.

I would say that not soldering all the contacts, functional or not could present several problems.

You are now relying onthe perimeter pads to support the entire BGA. The lack of physical contact to the board by the unsoldered 50 pads would, I think, create the enhanced possibility of cracking on the soldered joints. Flex of the component in all probability would be greatly increased by the lack of the stability of the solder connection.

When considering the minute cost of using solder paste to secure the component properly by soldering all the contacts you are stepping over the dollars to save the pennies ...

Weighing the cost differential of the solder versus product failure & customer bad will... I go with soldering...

Oh, major point would be.... are you a CM? If so is your customer going to allow this.

One mans opinion!

Jerry Karp
President
JSK Associates

Based in. Northern California since 1971. Founded JSK Associates in 1979. Actively involved in soldering, cleaning, chemistries. 30 years experience in EOS/ESD control.

In my opinion it is fine if some of the non functional points are not soldered. It should be understood that solder joints apart from electrical continuity provides mechanical strength. The usual thumb rule for PTH is 7 gms per solder joint is minimum required and similar rule can be extended to BGA also.

Secondly in case of BGAs it should be ensured that all four sides have sufficient soldered joints to support the device.

KN Murli
Head-Quality
Astra Microwave Products, Hyderabad, AP India

Holds Degree in Engineering, started off as Scientist/Engineer in ISRO (Indian Space Research Organization) in Quality Assurance of Space hardware Electronics Production. Worked in the area of Parts, Material and Process; DPA, FA and Process Qualification for space and ground hardware. Later moved into Private sector and worked in the area of Quality Management Systems & ISO 9001 certification. Currently hold a position as Head-Quality in RF/Microwave Product manufacturing for Defense and Aerospace segment.

We should stick to rule to maintain equal or better quality, reliability and keep working to reduce cost. The uneven space between non-print BGA balls and paste deposited balls will create reliability risk and is not recommended for partial print at all.

Even unused pads should be present in PCB rather eliminating or masking and should get soldered. Standardization,alternate BGA can be worked out for cost perspective.

Subrat Prajapati
Supplier Quality Leader
Ge Healthcare

Subrat has 10 year of extensive experience in PCB assembly process optimizing for quality, process includes screen printing, wave, reflow. He has a copyright in stencil design published in Apex Expo2010 at Las Vegas US.

If the BGA is depopulated but PWB lands are present, sure, you can simply not print the lands where no balls are present. If, however, a BGA ball is present at a given location, the land must be printed. The reasons for this are three-fold:

First, even if there is no electrical function, the component may well rely on the thermal path, and if it is not soldered to the land, there will be very little thermal conductivity.
Second, there will be a big impact to assembly success and reliability. If there is no flux on a given ball, it cannot wet the land and collapse.The presence of balls like this will hold the component up higher, possibly unevenly, which can create deformed joints or even opens in other areas.
Third, if you do manage to solder successfully, the reliability will be affected.

All this begs the question, just how much would we expect to save? Based on not printing 50 lands per BGA, the savings would be in the range of 0.1 cents per BGA. It's doubtful that the savings from this would be significant enough to offset the cost of a new stencil and the associated costs of customizing the BGA CAD decals.

Fritz Byle
Process Engineer
Astronautics

Fritz's career in electronics manufacturing has included diverse engineering roles including PWB fabrication, thick film print & fire, SMT and wave/selective solder process engineering, and electronics materials development and marketing. Fritz's educational background is in mechanical engineering with an emphasis on materials science. Design of Experiments (DoE) techniques have been an area of independent study. Fritz has published over a dozen papers at various industry conferences.

Interesting question. I don't see where this would be an issue. I would check whether or not an underfill would be necessary to enhance the strength of the attachment, but other than that, I see no reason why it would be an issue.

Leo Lambert
Vice President, Technical Director
EPTAC Corporation

At EPTAC Corporation, Mr. Lambert oversees content of course offerings, IPC Certification programs and provides customers with expert consultation in electronics manufacturing, including RoHS/WEEE and lead free issues. Leo is also the IPC General Chairman for the Assembly/Joining Process Committee.

At first sight this looks like a good approach - however I do not see the savings being significant. This is not an electrical connection question but a mechanical approach of the process.

BGA balls go through a reflow process so you will expect a consistent collapse at the end of this process. Missing lands and also paste deposits can cause that collapse not to happen in which case you are going to experience mechanical stress of the component.

You can try to experiment and perform some cross sections to have a better understanding of the effects taking inconsideration, of course the final application of your product is important too(excessive heat, vibration and other mechanical challenges can accelerate a potential failure in the field). Again, I really do not see savings to justify this approach.

Georgian Simion
Engineering and Operations Management
Independent Consultant

Georgian Simion is an independent consultant with 20+ years in electronics manufacturing engineering and operations.
Contact me at georgiansimion@yahoo.com.

Reader Comment

Not printing pads to save money? Has somebody tried to calculate how much savings would result? I would love to see that calculation! Unless you save your paste after printing (which you should never do) the miniscule volume not used is what? equal to a fraction of a gram?

The balls are there for a reason, mechanical, thermal or whatever. Your not going to save anything by not printing them. The package chosen is intentional. Nobody is going to waste packaging cost for their product on a center array that is not needed.

Alan Woodford, OnCore Manufacturing Services, USA

Reader Comment

What is one achieving by not dispensing paste in certain locations? Are we saving paste / cost - how much? One of the important feature of a BGA is it's ability to self align and depriving paste off certain locations are we compromising on this important feature?

Structural integrity of the device is based on the solder joint and it is equally important to ensure that the device will not fail structurally over a period of time. Surely, design & packaging engineers have thought about this while designing the structure of the pads.

More over, heat transfer of a device is another important aspect and in the cases of BGA's it will be transmitted through the solder joints (other than heat sink mounted). Why restrict the transfer through adequate points and induce other thermal related issues on the device?

Mahesh V Draksharapu, Aristos EMS Pvt. Ltd, India

Reader Comment

Is this a serious question? If you want to save money, use tacky flux on the BGA. No solder paste used. I say that with a smile! Seriously, if you only use some of the functionality of the BGA, look for a less powerful one. You could save more pennies with a cheaper component!!

Jerry Wiatrowski, General Dynamics

I absolutely think you should go ahead and do that, and save about 3 cents per BGA in paste. What most engineers do not understand about BGA balls is that they undergo huge lateral pressures during operation in the field, even inside a simple PC. The CTE mismatch between the standard FR-4 and most BGAs is significant, and for those with soldermask-defined pads, the extrusion forces that the soldermask puts directly onto the solder balls is even more so. The more solder balls you have, and the taller those solder balls are in height, the more modulus of elasticity and number of supports you have to collectively absorb these changes, the higher cycles to failure that component has. So yes, by all means, eliminate those "unnecessary" solder balls so you have even flatter loaves of cheese under there, and fewer of them, with a lot less modulus of elasticity, so they fail faster. Because you may be my competitor!

Richard D. Stadem
Advanced Engineer/Scientist
General Dynamics

Richard D. Stadem is an advanced engineer/scientist for General Dynamics and is also a consulting engineer for other companies. He has 38 years of engineering experience having worked for Honeywell, ADC, Pemstar (now Benchmark), Analog Technologies, and General Dynamics.

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