最近发现每次回来翻看Jedec标准总是会有新收获，索性就写下来，供大家参考。水平有限还请见谅！另外，欢迎讨论！这次就讲讲湿敏等级(MSL)吧.

标准全文请见 J-STD-020E.

(1) Purpose:

identify the classification level of nonhermetic surface mount devices (SMDs) that are sensitive to moisture-induced stress so that they can be properly packaged, stored, and handled to avoid damage during assembly solder reflow attachment and/or repair operations.

开篇讲述了这份标准的目的，这非常重要，经常有人说MSL，但MSL是如何定义，这样定义的作用又是如何？目的：确定IC的湿敏等级，使之能够正确的被包装，储存或处理，从而避免在装配的回流焊时或者返修时候受到损坏。

(2) ‘popcorn' phenomenon:

The vapor pressure of moisture inside a nonhermetic package increases greatly when the package is exposed to the high temperature of solder reflow. Under certain conditions, this pressure can cause internal delamination of the packaging materials from the die and/or lead-frame/substrate, internal cracks that do not extend to the outside of the package, bond damage, wire necking, bond lifting, die lifting, thin film cracking, or cratering beneath the bonds. In the most severe case, the stress can result in external package cracks. This is commonly referred to as the‘popcorn’phenomenon because the internal stress causes the package to bulge and then crack with an audible ‘‘pop.’’

在大多数人的认知中，“popcorn”现象就是受热炸开了，但要用文字来详细描述这个过程就不是那么容易，这份标准中的描述很好。重要的是，上面这种描述体现了由内而外，扩大的过程。

(3) Procedure:

Sample preparation---Initial electrical test---Initial inspection (OM+SAT) ---Bake---Moisture Soak---Reflow---Final inspection (OM+SAT) ---Final Test

下面挑其中几个步骤详细说明：

1.Sample preparation:

Reclassification (Qualified Package without Additional Reliability Testing): minimum sample of 22 units from two nonconsecutive assembly lots.

Classification/Reclassification and Rework: minimum sample of 11 units from two nonconsecutive assembly lots.

Sample units shall have completed all manufacturing processing required prior to shipment.

规定了实验样品的数目等要求。

通常precondition的样品数目为77ea，这来源于抽样检验的原理：

2. Initial inspection(OM+SAT)：

Perform an external visual (at 40X) and acoustic microscope examination on all components to establish a baseline for the cracking/delamination criteria。

此40X并不是定义这个倍率下能看到的crack 或者delam才算是fail,当看到疑似异常时，往往有必要用高倍显微镜或者SEM去检查确认。同理 Final inspection.

Note: This standard does not consider or establish any accept/reject criteria for delamination at initial/time zero inspection.

此处需要注意的是，最初的检查包括拍照只是作为一个baseline,并不会在0小时时作出判定。

3. Bake: Bake the sample for 24 hours minimum at 125 +5/-0 °C.

Note 2: If a bake test is interrupted for greater than 15 minutes; then the total time of the interruption should be excluded from the bake time. The interruption time should be accounted and no greater than 1 hour, then re-incorporated to ensure minimum of 24 hours. For instance, if the interruption was 45 minute, then the total bake test time would be 24 hours and 45 minutes. If greater than 1 hour the bake should be restarted for a full 24 hours.

这个步骤的目的是保证样品处于干燥的状态。特别注意的是：烘烤过程中如果遇到中断(比如机器故障或者其他样品的进出)，中断时间若大于15分钟，则需要累计，并且后续需要补满实验时间24hrs.与此同时如果24hrs内中断时间超过1hrs,则需要重新开始烘烤。比如：中断时间为45min,则整个实验时间就需要相应延长至2hrs45min。

4. Moisture Soak:

Place devices in a clean, dry, shallow container so that the package bodies do not touch or overlap each other. Submit each sample to the appropriate soak requirements shown in Table 5-1.

其中几个项目的定义如下：

Floor Life: The allowable time period after removal from a moisture barrier bag, dry storage, or dry bake and before the solder reflow process.

如上面的表格所示，我的理解是所谓的floor life指的是特定条件下的“安全用料期限”，此处用这样形容有别于“保质期”。仔细对比 不同等级下floor life 和 test time，会发现MSL2a~MSL5a 下，floor life=standard tmie-24hrs,这不难理解：因为这些level下温湿度条件都是30 °C/60% RH，而24hrs正是对应MET(manufacturer’s exposure time)。拿MSL3 来说，产品实际的“floor life”就是192hrs,去掉MET24hrs,则留给用户的只剩下168hrs。此外，Level 1 的floor time 为Unlimited，但并不是说这样的产品永远都是好品， floor life并不能保证不出现湿气相关以外的失效，比如长期储存引起的失效。

Accelerated Equivalent Soak： A soak at a higher temperature for a shorter time (compared to the standard soak), to provide roughly the same amount of moisture absorption.

拿MSL3 的Soak条件来讲：30 °C/60% RH 192hrs 和 °60C/60% RH 40hrs 的条件下样品吸收的湿气量等同。比方说，fast precond情况：周一晚上进 bake 24hrs,那么加上soaking 40hrs,在加上reflow的时间等等，本周四就能有 fast precond的结果；而换成Normal条件需要等下周三。所以在很多工厂里面为了尽快拿到precond的结果，通常normal 和fast条件的实验同时进行，如果fast的结果是fail 了，就可以尽快让产线调整参数或者更换材料等。 那么问题来了，fast 和normal 条件下的结果会不一样么？比如fast precond pass, normal precond fail,在实际实验中真的有这样的可能出现，我个人的理解是：还是以normal precond的结果为准,因为fast precond的过程中由于时间短，误差造成的影响更大，比如说打开温湿度箱时腔体内部的条件需要一定的时间来达到设定值，这个过程容易带来误差。

关于表格的Note5,加速实验对于采用没有filler的树脂作为塑封料的产品不适用，这其中的原因还不清楚！与SiO2的吸水性有关？只是猜测。

关于eV: Active energy.我翻阅了JESD22-A120B。文中只是介绍了理论的计算公式并阐述道与材料的Tg(Glass Transition Temperature)有关：

实际应用中，基本上取eV=0.4，其中的缘由目前还不明白。

5. Refolw:

Not sooner than 15 minutes and not longer than 4 hoursafter removal from the temperature/humidity chamber, subject the sample to 3 cycles of the appropriate reflow conditions.The time between reflows shall be 5 minutes minimum and 60 minutes maximum.

通常建议是，每次进行reflow之前，先跑一遍profile,看是否符合要求，有的客户会有特殊要求，比如特殊的ramp up rate 等。

Reflow profile 的几个重要参数：

1. Tp: Peak temperature;

2. ts: preheat time;

3. tp: the time when Tc-5

4. Ramp up/down rate;

(Classification Temperature (Tc): The maximum body temperature at which the component manufacturer guarantees the component MSL as noted on the caution and/or bar code label)

Tc和Tp 的关系取决于面对的是用户还是制造厂：

下面展示一个实际的profile:(通常需要三个或更多的测温点，来确保乘放IC的盘子上所有位置都满足要求，并且测温点要放在IC package上。

(4) Failure Criteria after Reflow Simulation:

A device is considered a failure if it exhibits any of the following:

a. External crack visible using 40X optical microscope. It is highly desired to use 100X optical magnification or low vacuum scanning electron microscopy (SEM) to better observe any cracks that could be precursors to problems during the stress of operational life.

b. Electrical test failure.

c. Internal crack that intersects a bond wire, ball bond, or wedge bond.

d. Internal crack extending from any lead finger to any other internal feature (lead finger, chip, die attach paddle).

e. Internal crack extending more than 2/3 the distance from any internal feature to the outside of the package.

f. Changes in package body flatness caused by warpage, swelling, or bulging not visible to the naked eye per JESD22-B101.

(5) Criteria Requiring Further Evaluation---delam

Metal Lead-Frame Packages:

a. No delamination on the active side of the die.

b. No delamination on any wire bonding surface including the downbond area or the lead-frame of lead on chip (LOC)devices.

c. No delamination change >10% along any polymeric film bridging any metallic features that is designed to be isolated(verifiable by through transmission acoustic microscopy).

d. No delamination/cracking >50% of the die attach area:

1. In packages with exposed die pad used for thermal conductivity or

2. For devices that require electrical contact to the backside of the die

e. No surface-breaking feature delaminated over its entire length. A surface-breaking feature includes lead fingers, tie bars,heat spreader alignment features, heat slugs, etc.

Substrate Based Packages (e.g., BGA, LGA, etc.):

a. No delamination on the active side of the die.

b. No delamination on any wire bonding surface of the laminate.

c. No delamination change >10% along the polymer potting or molding compound/laminate interface for cavity and overmolded packages.

d. No delamination change >10% along the solder mask/laminate resin interface.

e. No delamination change >10% within the laminate.

f. No delamination/cracking change >10% through the die attach region.

g. No delamination/cracking between underfill resin and chip or underfill resin and substrate/solder mask.

h. No surface-breaking feature delaminated over its entire length. A surface-breaking feature includes lead fingers, laminate,laminate metallization, PTH, heat slugs, etc.

Note 1: On substrate based packages, the C-mode acoustic image is not easy to interpret. Through Transmission Acoustic Imaging is recommended to supplement and verify the C-mode images because it is easier to interpret and more reliable. Ifit is necessary to verify results or determine at what level in the package the cracking/delamination is occurring, crosssectional analysis should be used.