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Elusive BGA Intermittent Connections
December 31, 1969 |Estimated reading time: 5 minutes
By Zulki Khan, NexLogic Technologies, Inc.
BGA intermittent connections (BICs) can originate from various root causes and produce periodic faulty operation involving BGAs. The worst characteristic about this problem, however, is its elusive nature, which can consume inordinate OEM engineering time and money, delay product launches, and detrimentally affect time-to-market and time-to-revenue. At this point, an OEM's top management is forced to intervene to determine causes of delays in its product introduction. Engineering and executive management time and costs escalate even higher.
It's common for OEM engineering to spend one or two days finally pinpointing a BIC after exhausting all possible failure causes during PCB debugging. In most instances, a BIC is the farthest thing from their minds when engineers start the debug process and go after the "usual suspects." Consider that we're talking about only one BIC. Based on today's engineering costs, $2,000 to $3,000 can easily be expended to detect a BIC. That's one way to look at this problem. Another is to consider wasted engineering time and costs that could otherwise be used more productively in another project. Dealing with one BIC is certainly a problem. But a complex PCB with 2,000 to 4,000 components is quite another. It may take two weeks to debug and test that particular board. If it is populated with 810 BGAs on each side, the OEM has its work cut out to determine if BICs are causing the failures, and then try to find which one or more BGAs have intermittent connections. This task is particularly challenging with micro-BGAs.
A BIC's domino effect is the most damaging, cost and frustration wise, to an OEM's engineering ranks. A newly arrived PCB subsystem at an OEM's location undergoes several engineering levels. One set of engineers checks BGA-related connections; sometimes another engineering group checks the subsystem after programming the EPROMs and installing firmware.
Some complex products require sets of checks and balances, debug procedures, and testing instructions. These follow from one set of engineers to another. A BIC or group of BICs can slow down or completely stop a well-structured, disciplined, and time-sensitive engineering queue, thus crippling the complete operation. For example, debug and test can be halted for extended periods as engineers repeatedly search for an unknown problem, which eventually turns out to be a BIC.
The 23 weeks of chasing down BICs take a major toll on time-to-market. It means an OEM loses its competitive edge in many instances. In most cases, BGA intermittent connection problems aren't anticipated and not factored into a product's delivery schedule. When they do occur, BICs create major financially associated issues. At this point, OEM top management steps in, and BICs take on whole new meanings.
That's just the tip of the iceberg. Once the OEM locates one or more BICs on a PCB, the next move is to ship it to a contract manufacturer (CM) or EMS provider. Depending on how busy EMS provider schedules are, there will likely be a relatively long lead time to perform BGA inspections and repairs. That may involve a new device or if the old one is used again, it needs to be extracted from the PCB, reballed, cleaned, and finally placed back on the board. This eats up more time and money.
Root CausesA less-than-100%-perfect thermal profile used for reflow soldering is a main cause of BGA intermittent connections. When all the zones of a thermal profile are dialed in properly, the result is a perfect reflow (Figure 1). This can be observed in an X-ray of BGA connections.
A common industry practice is to develop and apply three basic thermal profiles, one for small PCBs, another for medium-sized PCBs, and a third for large PCBs. Unfortunately, each poses a host of assembly issues, BICs being the most prominent. In each case, EMS providers and CMs are not following the solder paste manufacturer's specifications when they create a thermal profile, a main cause of BICs.
Key factors determining thermal profile accuracy include PCB layers, PCB thickness and dimensions, number of planes inside a board, type of PCB material used, component types, and related aspects. A poorly defined thermal profile can produce undesirable results such as opens under BGAs or shorts, which then need to be reworked. Also, if the board either isn't reflowed to a long enough presoak or to required peak time cycles, BICs can occur easily.
Further, by not following manufacturer specifications precisely, operators create insufficient flux activity at the BGA level, preventing solder balls from collapsing properly. The challenge in BGA placement is to make sure that sufficient heat is applied to the BGA's middle section, allowing heat to penetrate and collapse the balls there, and creating an even collapse of all the BGA balls.
Untented vias and poorly defined stencil apertures, both associated with imperfect thermal profiles, are two other BIC causes. Inexperienced designers often fall into the habit of placing vias next to the BGA pads or around BGA peripheries. This hinders perfect reflow soldering because solder is sucked into vias during reflow rather than being consumed by the BGA balls. That occurs when vias are not properly masked or tented. The result is a BGA with voids. Figure 2 shows poor reflow, when either not enough paste is dispensed on BGA balls or temperature ranges in the reflow oven are incorrectly calibrated.
An experienced designer, on the other hand, will mask or tent BGA vias properly at the design and layout level. Even when this step is omitted, a design-for-manufacturability (DfM) check mapped out at the planning stages or CAM level can catch this issue. Otherwise, reflow becomes a major challenge and undetected vias with solder paste traces can cause evasive BGA intermittent connections. Likewise, if stencil apertures aren't correct, too much or too little solder paste is dispensed at certain BGA solder points, creating BICs.
There are a few other BIC root causes, but most of them stem from not creating a 100% thermal profile. Regardless of its root cause, a BGA intermittent connection affects an OEM at various engineering levels and can reach the company's top echelons. All along the product cycle, BICs leave a path of inordinate waste.
Conclusion It's important for OEMs to partner with reputable and reliable EMS providers who thoroughly understand BIC issues and how to best avoid them during PCB design, fabrication, and assembly. OEMs should select EMS providers that are equipped with advanced gear such as high-powered X-ray machines, inspection devices and cameras, rework stations, and reballing systems to assure these obscure, well-hidden BGA intermittent connections do not occur.
Moreover, the OEM has to be assured a 100%-perfect thermal profile is used for its specific PCBs. A reputable EMS provider should make available to its OEM customer the BGA image before board is shipped. It will show how ball collapse was captured at the time these BGAs were installed. These images will assure the OEM that the BGA installation is not the issue when debugging the board.
Zulki Khan, founder and president, NexLogic Technologies, Inc., may be contacted at 2075 Zanker Road, San Jose, CA 95131; (408) 436-8150 ext 102; zk@nexlogic.com.