Tb363

Guidelines for Handling and Processing Moisture
Sensitive Surface Mount Devices (SMDs)
Technical Brief
October 2000
Introduction
internal moisture concentration reaches an equilibrium withthe ambient relative humidity. Thus the higher the relative Certain plastic encapsulated surface mount devices (SMDs) humidity, the greater the amount of absorbed moisture within if not handled properly can incur damage during the solder reflow attachment process to printed circuit boards (PCBs).
The damage occurs as a result of internal package cracking(commonly referred to as popcorn cracking) and / ordelamination between internal package interfaces (i.e., diesurface and mold compound). Such internal damage canlead to a number of possible failure modes including brokenbond wires and lifted ball bonds. If they reach the exterior of REFLOW AND
DELAMINATION
the package they may provide an entry pathway for externalcontaminants. Furthermore, separations in the die attachregion can lead to increased electrical and thermalresistances, which may affect device performance in certainpackage styles where such a conduction path is required.
The root cause of this type of failure mechanism is the rapid SWELLING AND
heating of the moisture absorbed within the plastic PACKAGE CRACKING
encapsulant. All plastic packages absorb moisture. Duringtypical solder reflow operations when SMDs are mountedonto a PCB, the entire PCB and device population are exposed to a rapid change in ambient temperature. Anyabsorbed moisture is quickly turned into superheated steam.
This sudden change in vapor pressure can cause the FIGURE 1. CROSS SECTIONAL VIEW OF A MOISTURE
package to swell. If the pressure exerted exceeds the SENSITIVE PLASTIC PACKAGE UNDERGOING
SOLDER REFLOW. ABSORBED MOISTURE
flexural strength of the plastic mold compound, then it is TURNS TO VAPOR, WHICH CAUSES THE
possible to crack the package (see Figure 1). Even if the PACKAGE TO SWELL.
package does not crack, interfacial delamination can occur.
A number of factors can influence the moisture sensitivity of As mentioned earlier, the profile of the solder reflow process, a package. These include both the internal dimensions and which includes preheat, ramp up and maximum temperature design of the lead frame, the external dimension of the exposure, also affect the moisture sensitivity of an SMD (see package and the physical properties of the die attach Tech Brief TB334, Guidelines for Soldering Surface Mount material and mold compound. Also, the die dimensions and Components to PC Boards). In general, the slower the ramp even the type of passivation can affect moisture sensitivity.
rate and the lower the maximum temperature, the less the The last two factors which can influence the moisture probability of potential damage due to moisture sensitivity.
sensitivity of a SMD are the amount of absorbed moistureand the solder reflow temperature profile. For any given Moisture Sensitivity Classification
moisture sensitive SMD, once assembled, it is only the latter In order to establish common criteria for the classification of two factors that a PCB manufacturer has control over.
moisture sensitive SMD packages several industry The amount of moisture absorbed within a plastic package is specification were drafted. The more widely accepted includes JEDEC STD22B, Test Method A112-A andIPC-SM-786A. These have recently been combined into • Physical properties of the mold compound IPC/JEDEC J-STD-020A (Moisture/Reflow Sensitivity Classification for Plastic Integrated Circuit Surface Mount • Relative humidity of the ambient atmosphere and, Devices). These specifications outline the test methods toclassify the moisture sensitivity of a given SMD to one of eight different levels (see Table 1).
The diffusion rate of moisture into the mold compound istemperature dependent. The higher the temperature, thefaster the surrounding moisture will penetrate the moldcompound. The absorption process will continue until the 1-888-INTERSIL or 321-724-7143 | Intersil and Design is a trademark of Intersil Corporation. | Copyright Intersil Corporation 2000 Technical Brief 363
TABLE 1. MOISTURE SENSITIVITY LEVELS
may be used. The solder reflow must be accomplishedwithin the specified floor life shown on the warning label.
FLOOR LIFE
Failure to do so may result in damage to the product.
CONDITIONS
TIME (NOTE 1)
Unused product can be stored in a cabinet with a controlled ambient ≤ 20% RH when not in use. When product is returned to production any previous floor exposure shall bededucted from the floor life indicated on the warning label.
When a variety of moisture sensitive SMDs are being used, it is extremely important to maintain the sensitivity level and total floor life exposure of each device. The floor life shownon the label is for a maximum factory ambient of 30oC / 60% RH. Derating calculations have been published for use of moisture sensitive SMDs in other temperature / humidity As mentioned before, the ramp rates and maximum 1. Time after removing from dry pack in a ≤ 30oC / 60% RH ambient.
temperatures have a direct effect on moisture sensitivity.
2. Dry pack not required. Maximum conditions 30oC / 85% RH.
Moisture sensitivity classification is performed at a maximumtemperature of either 219oC/225oC or 235oC/240oC The classification test procedure involves a specified soak depending on the package dimensions. Higher reflow duration at the stated floor life conditions for levels 3 through temperatures may increase the moisture sensitivity of a 6. Accelerated conditions are used for level 1 and 2.
particular device type because of the associated increase in Following the humidity soak, the packages are subjected to the vapor pressure of the steam. Other precautions should 3 reflow cycles with either vapor phase or IR reflow. The be observed in instances where reflow is being performed specified maximum reflow temperatures are 219oC/225oC with radiant heating, such as with IR reflow. Heating is not or 235oC/240oC depending on package dimensions (refer to uniform across the PCB. Outer edges of the board tend to J-STD-020A). The product is then subjected to electrical get hotter. Also smaller packages with less thermal mass test, visual inspection, cross-sectioning and/or inspection can achieve higher temperatures than larger packages in a with acoustical microscopy. The package is assigned to the mixed PCB design. Profiling of IR reflow systems should be lowest level of moisture sensitivity for which it passes.
performed to account for these differences.
Dry Pack
Several manufacturers employ wave solder for solderingSMDs to PCBs. Typically such soldering is performed at If a particular package style is determined to be moisture 260oC. This method and temperature range is not sensitive (levels 2 through 6), then the product must be recognized by the IPC/JEDEC J-STD-020A and as such shipped in dry pack. The dry pack bag is a tough, moisture SMD packages have not been characterized for moisture resistant bag. The moisture sensitive product is typically sensitivity under these conditions. If a user is employing
baked for 24 hours at 125oC. Following the bake the product this method, they should check with Intersil prior to use.
is placed inside a dry pack bag along with predeterminedamount of desiccant and a humidity sensitive indicator card.
PCB Rework
The bag is then sealed. A moisture sensitivity warning labelis then affixed to the bag. The label will indicate the floor life If a rework of a PCB with moisture sensitive SMDs is after the bag is opened as well as the date the dry bag was required, special precautions must be observed. Should the sealed. The label will also contain information on storage rework require complete exposure of the PCB to reflow conditions, then the manufacturer needs to take into accountthe shortest floor life of any moisture sensitive SMD on the PCB Assembly
board. If the floor life has been surpassed, then the entireboard should be re-baked.
Upon opening a dry pack bag with product, the user needsto check 2 items: Localized board repair with a soldering iron or hot bar shouldnot damage neighboring packages. Care should be exercised not to overheat individual leads of the replacement • the moisture indicator from within the bag.
device. If a hot air gun is used for making board repairs, care If the bag seal date is over 1 year and / or the humidity should be taken to shield any surrounding moisture sensitive indicator card shows >20% RH, the product needs to be packages. If a moisture sensitive SMD is to be replaced, the re-baked prior to reflow. If both the seal date and humidity new device should be within its floor life. Any special indicator card are within the requirements, then the product requirements, which might be on the product’s data sheet,should be followed.
Technical Brief 363
Re-Baking of Moisture Sensitive Product
Distributors
Moisture sensitive product which has been exposed to the Dry packed product should be turned on a First In/First Out factory ambient past its intended floor life or when the dry (FIFO) basis to insure dry pack does not go beyond its pack bag has been opened and the humidity indicator card one-year expiration date. Preferably factory dry pack should shows >20% RH needs to be baked dry again before not be opened. If, however, an order requires opening a dry reflowing. The baking process for dry packing is 24 hours at pack bag for part of its content, the bag should be resealed 125oC. Shipping trays can typically withstand this immediately and the removed contents transferred to a new temperature (check with product supplier to be sure).
bag. The new dry pack bag shall meet Class I barrier However, shipping tubes and tape and reel cannot. Two requirements per Federal Test Method Standard 101, Method 3030. Resealing of the new dry pack should occurwithin 30 minutes maximum and should follow the 1. For product in plastic tubes the product can be transferred requirements of EIA-583 (Packaging Material Standards for to metal tubes or placed on metal trays for the normal125oC bake out procedure. ESD precautions need to be Moisture Sensitive Items) and / or IPC/JEDEC J-STD-033 (Standard for Handling, Packing, Shipping and Use ofMoisture/Reflow Sensitive Surface Mount Devices). A 2. Tubes and tape and reel can also be baked at 40oC +5oC/-0oC at <5% RH for 192 hours or longer.
duplicate of the same moisture sensitive warning label as onthe original dry pack bag shall be affixed to the new bag. If Following the bake out procedure the product needs to be the product is exposed for greater than 30 minutes, processed through reflow within its assigned floor life or it can be returned to a storage cabinet with < 20% RH for useat a later time. In the case of extremely moisture sensitive References
components (level 6) it is advisable to process throughreflow immediately after the bake.
[1] “Diffusion Model to Derate Moisture Sensitive Surface Mount ICs for Factory Use Conditions”, R. L. Shook andT.R. Conrad, Proc. 45th Electronic Components andTechnology Conference, pp. 440-449, 1995 All Intersil semiconductor products are manufactured, assembled and tested under ISO9000 quality systems certification.
Intersil semiconductor products are sold by description only. Intersil Corporation reserves the right to make changes in circuit design and/or specifications at any time with-out notice. Accordingly, the reader is cautioned to verify that data sheets are current before placing orders. Information furnished by Intersil is believed to be accurate andreliable. However, no responsibility is assumed by Intersil or its subsidiaries for its use; nor for any infringements of patents or other rights of third parties which may resultfrom its use. No license is granted by implication or otherwise under any patent or patent rights of Intersil or its subsidiaries. For information regarding Intersil Corporation and its products, see web site www.intersil.com
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