Head on Pillow Defects on BGA Assemblies

The growing use of lead-free soldering in electronics manufacturing has introduced new types of defects

that require proper identification and troubleshooting. One specific type of defect that has become

more prevalent on lead-free ball grid arrays (BGAs) is called “head-on-pillow”. Head-on-pillow is a

defect where both the paste deposit and the solder bump reach a full state of melt but fail to coalesce.

It is important to differentiate head-on-pillow from a defect caused simply by insufficient reflow

temperature, which is characterized by distinct solder spheres from the paste that have not been

properly melted on the pad and BGA solder bump. With head-on-pillow the soldering temperature is

sufficient to fully melt the solder bump and paste deposit, but an impediment to the formation of a

proper solder joint exists. One or both ends of the failed interconnect will show evidence of

displacement while it was melted, forming a shape that resembles that of a pillow with an indentation

of one’s head.  A “dye and pry” test is used to identify the presence of head-on-pillow. The dye

applied during this test can penetrate the gap formed between the two ends of the poorly connected

solder joint and can be identified when the component is removed from the PCB. An example of the

results of a dye and pry test on an assembly with head-on-pillow defects is shown in Figure 3-1.

Another method to identify head-on-pillow defects is through visual analysis. Some  head-on-pillow 

defects  are  readily  apparent using  relatively  low-magnification optical inspection. If those defects

are located on the outer rows of the assembly, an endoscopic inspection system may be sufficient to

definitively identify head-on-pillow as the cause of a failure. If the defect is present at an interior

location on the interconnect array, a cross-section may be necessary to optically verify the presence

of head-on-pillow. Once the solder joint is exposed, low magnification may be sufficient to identify

head-on-pillow depending on the severity of the defect. Figure 3-2 shows an example of a

head-on-pillow defect that was verified optically without the aid of mounting and polishing.

Some examples of head-on-pillow may not be severe enough to allow identification with low

magnification optical inspection. These may still be verified optically, but require polishing and

high magnification optical inspection. Figure 3-3 shows an example of head-on-pillow that

required high magnification to be clearly identified.

Verification of the presence of head-on-pillow leads to an investigation into the cause.

The ultimate root cause of head-on-pillow is a barrier that prevents the coalescence of the

solder bump and the solder paste deposit. That barrier may be contamination on the surface

of the solder bump that the flux in the solder paste is not able to remove. Testing to ensure

that the components are not entering the facility with the suspected contamination present

is an important first step. The recommended tests for contamination of this type is an

extraction based (non-destructive) method suited for detection of non-ionic contaminants,

such as UV-Vis Spectroscopy or Gas-Chromatography/Mass Spectroscopy. If no contamination

is found on raw materials in as-received condition, a careful assessment of the factory’

s handling practices is required. It is not difficult for contamination from factory personnel,

whether in the form of body oils, hand lotions, or machine lubricant to be transferred to a

raw component when improperly handled. Tasks such as removing or replacing BGAs on a

matrix tray or loading and unloading of trays from placement equipment can be avenues

of contamination if personnel are not trained and disciplined in the proper handling of

electronic components.

More commonly with lead-free soldering, an occurrence of head-on-pillow is related directly

to the reflow process. During a reflow soldering process, the flux present from the solder

paste is required to clean the initial oxidation from the parts to be soldered, as well as

protect the materials against continued oxidation during the reflow process. This oxidation

prevents proper homogeneity between the solder ball and the solder paste deposit and

will result in head-on-pillow.   One possible cause of excess oxidation is insufficient solder

paste (and thus flux) volume present. In this case, the solder paste printing process may

need to be optimized to ensure that sufficient paste is present. Another potential cause

of flux failure during reflow is poor material control. Solder paste materials need to be

stored and handled properly to ensure that they function as expected. Careful observation

of shelf life control, storage conditions, factory environmental conditions, and stencil life

performance are important to ensure that the flux constituent in the solder paste is able

to perform all of its functions in the reflow process.

The reflow profile can be a cause of head-on-pillow defects by exposing the flux to excessive

temperatures for durations beyond what it is designed to withstand. This causes the flux to

exhaust its oxidation cleaning abilities well before the completion of the reflow cycle,

regardless of the amount of flux present, and even if the raw component and the solder

paste have been properly handled for their entire life. The obvious change that can be

made to the process to prevent this occurrence is to reduce the profile length and/or modify

the profile temperatures to prevent the premature exhaustion of the flux.

A case of head-on-pillow was recently encountered during a project undertaken at the EMPF.

The project involved the attachment of a large BGA component
(approximately 2 in2 with over 2000 solder balls on a nearly full array) to a large PCBA using

the Metcal/OKI APR-5000 hot air rework equipment. The BGA incorporated a heat spreader

as a die cover and utilized lead-free tin-silver-copper solder balls. A no-clean solder paste was

applied to the PCBA prior to installation of the BGA and was to be reflowed along with the

BGA during the attachment process.

Due to the sensitive nature of the PCBA, and the warpage that had been previously observed

when attempting an attachment with a reflow cycle of standard length, an extended profile

was designed to minimize thermal gradients across the PCBA. This profile extended for over

1000 seconds from ambient temperature to peak. Although excessive thermal gradients

across the PCB were no longer present on the longer profile, an unintended consequence

was a large number of head-on-pillow defects on the assembly due to the extended duration

of the profile and the exhaustion of the flux during the reflow cycle. This was verified by

optical inspection after performing a dye and pry operation, followed up with a cross-section;

the results of this analysis by the customer are shown in Figures 3-1 and 3-2.

Since reducing the length of the profile would increase the risk of PCBA warpage, another

strategy had to be employed to prevent this defect. The use of an inert atmosphere would

prevent the formation of an oxide on the solder bumps after the flux was exhausted during

the reflow cycle and allow the solder to coalesce. For verification, another attachment was

performed after outfitting the rework machine with a nitrogen source. No changes were

made to the temperature settings of the rework equipment. A dye and pry analysis of the

soldered part showed no examples of head-on-pillow and demonstrated that the use of

nitrogen successfully mitigated head-on-pillow defects on this extended length profile.