by Jon Roth (AcousTech, Inc. President)
My previous post
on this topic dealt with scanning acoustic microscopy (SAM) rejects that occur during the GEIA-0006
solder dip process, particularly those that are observed during the baseline inspection prior to solder dipping. This post further examines the value of SAM inspection to the solder dip process
, particularly in regard to the sampling methods prescribed by GEIA-0006
and common variants being used by a number of companies.GEIA-0006
requires SAM inspection for component types susceptible to internal delamination or cracking when subjected to the heat generated by the solder dip process. Two separate test scenarios are described: process qualification and testing of production lots. Process qualification requires a 50 piece sample of the part number or package type to be qualified. SAM inspection is performed before and after solder dip in order to identify any internal changes resulting from the process.
For production lots, it is important to test at least a sample from every lot to be solder dipped. GEIA-0006
requires pre-dip and post-dip SAM inspection on a 10 piece sample of each production lot. Occasionally, however, companies will modify their process to eliminate this requirement and perform only the process qualification testing. There are two reasons this is ill advised. First, as noted previously, SAM functions as a monitor to verify the consistency of the solder dip process. Second, qualifying the process to a particular package type or even a specific part number neither guarantees each lot construction is consistent with past lots nor guarantees a consistent SAM result from one lot to the next.
The problem is that when dealing with plastic-encapsulated microcircuits (PEMs), the behavior of the mold compound often varies from lot to lot. One lot of a particular part number may survive the dipping process with little or no degradation, while another lot of the same part number may delaminate significantly. By sampling each production run as required by GEIA-0006
, a component lot in which the mold compound adhesion is inherently weak can be eliminated or at least identified for further reliability testing.
Some users have found benefit in performing SAM beyond the GEIA-0006
sampling requirements for production lots. Some common variants are as follows...
• 100% inspection. This is particularly useful for package types that are prone to exhibit delamination in the as-manufactured condition. These are often smaller devices such as SOT, SMB, or small SOIC packages. By scanning the entire lot prior to processing, any rejects to the J-STD-020
criteria (required by GEIA-0006) can be removed prior to solder dip.
• Perform SAM on the normal 10 piece sample. If defects are observed in some percentage of samples (either pre-dip or post-dip), then subject the balance of the lot to SAM, rather than rejecting the entire lot. This may be a useful approach if an acceptable yield can be obtained.
• Rather than dipping the 10 piece sample concurrently with the balance of the lot, complete the SAM/Dip/SAM process on the 10 pieces first. If this sample passes with no rejects, then solder dip the balance of the lot.
• Eliminate pre-dip SAM and perform post-dip SAM on 100% of the lot. Though this method does not meet the letter of the standard, it does ensure that only acceptable parts get moved to production following solder dip. This may be a reasonable compromise between inspecting the required 10 piece sample and doing 100% inspection both pre-dip and post-dip, particularly with packages that are susceptible to delamination during the solder dip process (e.g. DPAKs when required to solder dip the entire exposed heat sink).Scanning Acoustic Microscopy
is not only an integral part of the GEIA-0006
qualification process, it is also useful for solder dip of production lots, both as a process monitor and to identify those lots that may lack sufficient robustness to withstand the solder dip process. Variations of the SAM sampling method that go beyond the prescribed 10 piece production lot sample size can provide additional benefit, particularly when dealing with component types that are prone to exhibit delamination as manufactured.