IPC-A-600H-2010 中文版印制板的可接受性 (1).pdf - 第26页

Even when the potential failure mechanism mentioned above is analyzed, it is almost impossible to experience such (IR/ migration) failure. First, a measle(s) gapping conductive patterns is needed. Secondly, moisture in t…

Brief summary of the Second IPC Blue Ribbon Committee

on Measles

This second committee was formed in late 1978. This committee

reviewed the findings of the first committee, solicited the industry

for additional data, and reviewed the proprietary acceptance crite-

ria provided by IPC members. The Second Blue Ribbon Committee

came to the same conclusion. Measles are a cosmetic process indi-

cator and had almost no reported effects on a product's functional

performance in most applications. The major exception was high

voltage applications. There was still reluctance by some govern-

ment organizations and a few industrial companies to categorically

accept measles. As such, this committee established a set of

measling/crazing requirements that obtained consensus from all

IPC members. The result was a matrix of acceptance limitations for

the three major phases of the printed board electronic assembly

process: laminated material, printed board final inspection, and

after printed board assembly. These requirements included percent

reductions in conductor spacing (not exceeding minimum conduc-

tor spacing), and various amounts of measled area for each side of

the printed board (or assembly) based on the Class of product.

These requirements were added as an amendment to the first print-

ing of the IPC-A-600, Revision C, and were included in later print-

ings of the C revision and, in a different format, the IPC-A-600,

Revision D.

Comments: The primary concerns expressed by the reluctant indi-

viduals are summarized in the following list (with comments):

• Electrical Insulation Resistance, both volume and surface - sev-

eral reports and available test data indicates that insulation resis-

tance is not significantly affected by measling or crazing.

• Contamination - the concern was that ionic materials could dif-

fuse or be ‘‘pumped’’ (by alternating atmospheric pressure) into

measles or crazing and would result in lower insulation resistance

or conductive anodic filament (CAF) growths, shorts. Salt spray

tests indicated this was not a valid premise, and most ionic mate-

rials (such as salts) will not diffuse into the base material.

• Applied Voltages - high voltage applications are a concern (in

particular where there is the possibility of ‘‘corona’’ in the mea-

sling or crazing) the dielectric strength is reduced by 20-50% in

comparison to a similar non-measled/crazed area, in particular at

altitudes greater than 20 km [12.43 miles].

• Environmental - most measling/crazing did not appear to increase

in size or occurrence due to environmental testing.

IPC-A-600, Revision E, was the first revision to reflect the needs

for surface mounted component technology. As such, the accep-

tance requirements for measling and crazing were separated. For

measles, the acceptance requirements allowed bridging under sur-

face conductor spacing. This was done based on the definition of

measles, test data, and industry experience of measles having never

been documented to cause a functional failure. Crazing is much less

controlled separation in the base material forming ‘‘interconnec-

tions’’ between measles and possibly adjacent conductive patterns;

therefore, the acceptance requirements for crazing were set the

same as the similar conditions of delamination and blistering.

Over a period of time, governing specifications have become

excessively heavy regarding the presence of measles. In addition,

cosmetic appearance has become a major acceptance criterion. In

actual fact, no failure has ever been attributed to measling, based on

all military and industry testing to date. IPC, industry and various

military agencies have conducted extensive testing in severely

measled assemblies under extreme environmental conditions for

long periods of time with no evidence of growth, spreading or any

detriment to the function of the assembly. Measles should not be

the cause for rejection.

Measling is an internal condition occurring in the woven fiber rein-

forced laminated base material in which the bundles are separated

at the weave intersection. The term ‘‘crazing’’ is sometimes used to

describe an array of measles which appear from the surface to be

interconnected. When the measles look to be interconnected, this

condition called ‘‘crazing’’ is a form of delamination in that there

are separations along the length of the fiber/yarns and the resin. For

non-woven material, this condition resembles a measle but is ran-

domly located and has an irregular shape (see Figure 23a).

In a case study done, the prime cause of the observed measles was

a combination of moisture, which diffuses readily into epoxy-glass,

and component soldering temperatures. The application of local

high temperatures for component mounting caused entrapped

moisture to vaporize and break the epoxy-glass bond at the

‘‘knuckle’’ (intersection of the warp and fill of the e-glass cloth).

From previous experience, it is known that epoxy-glass absorbs

atmospheric moisture, and when moisture content exceeds 0.3

wt%, it can give rise to measling during solder dip/level and/or

assembly soldering operations.

There are other factors that can contribute to measles/crazing such

as: resin composition, method of making laminates, coupling

agents, T

, etc. In the past, reports were compiled which revealed

that measles and crazing with over 50% spacing violation were not

adverse to the reliability of the hardware. Why, if all test reports

showed no problems with measles and no reported field failures,

are we so concerned about measles and crazing? Because it appears

feasible, in theory, that if measles with 100% conductor spacing

violation combines with moisture or some other contaminant, cop-

per migration (IR failures) should be experienced between conduc-

tors.

2.3 BASE MATERIAL SUBSURFACE（基材表⾯下）

Introduction (cont.)（引⾔（续））

17IPC-A-600H-2010 2010年4月

Even when the potential failure mechanism mentioned above is

analyzed, it is almost impossible to experience such (IR/ migration)

failure. First, a measle(s) gapping conductive patterns is needed.

Secondly, moisture in the printed board/ assembly, along with a

conductive or ionic contaminant such as chlorides, is necessary.

In this instance, a typical industry example, the measle is at the

center between two plated through holes (see Figure 23b). The

measle is 0.4 mm [0.0157 in] wide. In order to get possible copper

migration, the measle had to gap the two plated through holes. This

of course would be most unlikely. The second example (see Figure

23c) illustrates what is required for a potential failure mechanism

between two surface conductors. A (+) conductor directly over a

knuckle is required and a (-) conductor is also required directly over

a knuckle. For an electrical short to occur between these conductors

through the base material, there would need to be a conductive path

from one conductive pattern, through the remaining dielectric

materials (resin and yarn) to the separation (measle), along the

separation in the direction of the other conductive pattern, once

again through the remaining dielectric materials (resin and yarn),

and to the second conductive pattern. In order to induce a failure all

of the above mentioned ingredients are required along with a volt-

age potential between two adjacent conductors. This occurrence is

highly unlikely and is most likely why the industry has not experi-

enced any adverse reliability problems due to measles.

When making acceptance calls on electronic hardware, consider all

the possible concerns mentioned above. Measles should not be con-

sidered a nonconforming condition. It should instead be considered

a process indicator, telling you that the process is on the verge of

going out of control. Correct the problem, but do not scrap the

product, taking into account all of the above mentioned variables.

第⼆届IPC蓝带委员会关于⽩斑的概述

第二届委员会成立于1978年底。它复审了第一届委员会的发

现，向业界征寻了更多的数据资料，并审阅了IPC会员提供的

专有验收准则。第二届蓝带委员会得出了同样的结论。白斑

是一种外观性的制程警示，在大多数应用中，几乎没有任何

有关影响产品性能的报道。一个主要的例外是高压电应用。

此时仍有一些政府机构和业界公司反对无条件地接受白斑。

因此，该委员会制定了一套由所有IPC会员一致同意的白斑/

微裂纹要求。形成的验收要求适用于印制板电子组装过程中

三个主要阶段：层压板材料、印制板终检和印制板组装后。

这些要求包括导体间距减小的百分比（不超出最小的导体间

距）和基于产品级别的印制板（或组装件）每面的白斑区域

的大小。这些要求作为修订本补充到IPC-A-600C版的首印版

中，并纳入IPC-A-600C的再版中，且在IPC-A-600D版中以不

同形式出现。

说明：反对方阐明的主要顾虑归纳如下：

• 电气绝缘电阻，包括体积电阻和表面电阻 - 一些报告和现

有的测试数据都表明：绝缘电阻受白斑或微裂纹的影响不明

显。

• 污染 - 离子残留物可能会扩散或被“抽吸”（由于大气压发

生变化）到白斑或微裂纹中，并会导致绝缘电阻的降低或导

电阳极丝（CAF）的生长进而短路。盐雾测试表明，这个根

据不能成立，大多数离子物（例如盐）不会扩散到基材中。

• 所用电压 -高压应用关系是一个顾虑（尤其是白斑或微裂

纹中可能出现“电晕”的情况下），与类似的无白斑/微裂纹

区域相比，有白斑/微裂纹的区域的绝缘强度降低了20%～

50%，尤其在20km（12.43英里）以上的海拔高度下。

• 环境

- 大

多数白斑/微裂纹不会因环境测试而增加或尺寸扩

大。

IPC-A-600E版本是第一个反映表面贴装元器件技术需求的修

订版。因此，对白斑和微裂纹的验收要求便分开了。就白斑

而言，验收要求允许其跨接表面导体间距。这样规定基于白

斑的定义、测试数据，而且业界的经验中从来没有白斑导致

功能性失效的记载。微裂纹是产生于基材内的一种更不受控

的分离现象，形成白斑间的互连，其范围可能跨接相邻的导

电图形，因此，对白斑的验收要求与类似的分层和起泡状况

相同。

相当长一段时期，一些指导规范过于看重白斑现象。再者，外

观也成为一个主要的验收准则。事实上，迄今为止，根据所

有军方及工业界当前测试发现，白斑从未导致过任何失效。

IPC、业界及各军方机构在极端环境条件下，对出现严重白斑

的组件进行了广泛的测试，并没有发现白斑有增长、扩散或

有损于组件功能的现象。白斑不应该作为拒收的理由。

白斑是发生在编织纤维增强型层压基板内的一种内在现象，

基材内的纤维纱束在交叉处的粘合发生分离。“微裂纹”一

词有时用来描述表面上连成片的白斑阵列现象。这种称作微

裂纹的互连的白斑，其实是分层的一种形式，即纤维纱线沿

其长度方向与树脂发生了分离。对于非编织的材料，这种情

况类似于白斑但是随意排列且形状不规则（见图23a）。

在一项研究案例中，所观察到的白斑现象主要成因是能快速

扩散到环氧玻璃中的湿气和元器件焊接时的温度共同作用的

结果。元器件贴装时产生的局部高温造成裹挟的湿气蒸发，

并破坏环氧玻璃在“接合点”（环氧玻璃布经线与纬线交叉

处）的粘合。根据以往的经验，我们知道环氧玻璃会吸收大

气中的湿气，当湿气含量超过0.3%（重量比）时，在浸焊/热

风整平和/或组装焊接作业中会使白斑增加。

2.3

BASE

MATERIAL SUBSURFACE（基材表⾯下）

Introduction (cont.)（引⾔（续））

18 IPC-A-600H-20102010年4月

造成白斑/微裂纹状况的因素还有：树脂的成分、层压方法、

耦合剂，T

等。过去搜集到的报告显示，白斑和微裂纹范围超

过间距的50%以上时，也不会影响硬件的可靠性。既然所有

测试报告都显示白斑没有问题，而且没有使用失效的报告，

为什么我们还如此顾虑白斑和微裂纹现象呢？因为理论上看

来，如果导体之间布满白斑，同时伴有湿气或其他污染物，

那么在导体之间应该存在铜的迁移（即绝缘电阻失效）。

尽管上文分析了潜在的失效机理，但出现这样的失效（绝缘

电阻或迁移）几乎是不可能的。首先需要白斑填满导电图形

之间。其次，印制板/组件内需要有潮气，并伴有导电的或离

子的污染物，例如氯化物。

图23b图示了业界的一个典型实例，白斑位于两个镀通孔间的

中心处（见图23b），其宽度为0.4mm[0.0157in]。要想使铜迁

移成为可能，白斑必须占满两个镀覆孔之间的区域。这种情

况当然是不太可能发生的。另一个案例（见图23c）图示了两

个表面导体之间存在潜在失效机理的条件。它要求一根带正

（+）电的导体刚好落在纤维交织点上，同时还要求另一个带

负（-）电的导体也刚好落在纤维交织点上。要想这两个导体

通过基材产生电气短路，需存在以下这样一条导电通路：从

一个导电图形开始，经由尚存的介质材料（树脂和纤维）到分

离点（白斑），沿着分离处向另一个导电图形的方向，再经过

尚存的介质材料（树脂和纤维），最后到达另一个导电图形。

上述条件均符合的情况下，还需要这两个相邻导体之间有压

差。这是非常不可能的事情。这也是为什么业界至今尚无任

何由于白斑造成可靠性问题的原因。

确定电子组件验收准则时，要考虑上述所有可能发生的情

况。不应该把白斑看作不符合条件。应该将白斑看作为制程

警示，它告诉你制程正面临失控的可能。纠正问题，而不是

报废产品，分析上述种种可能，排查隐患。

Brief summary of the Third IPC Blue Ribbon Committee on

Measles

In 2004, the issue of printed board laminate degradation caused by

internal Conductive Anodic Filament (CAF) growth came to the

forefront of discussions among the IPC printed board assembly

standards groups (including the 7-31b IPC-A-610 and 5-22a

National Standard for Soldering task groups.) It was observed that

circuit density, operating speed, band pass and reduced operating

voltage had impacted the ability of circuits to operate under condi-

tions supporting dendrite or CAF development.

With the proliferation of finer line conductors and reduced spacing

in current designs, discussions within the assembly standards

groups questioned the role of measles within printed board lami-

nate materials as a potential catalyst for CAF growth.

For years, IPC standards for electronic production contained no

restrictions for the occurrence of measles in printed boards and/or

assemblies. Theoretically, measles could be continuous between

conductors and could exist throughout the printed board. It was

noted that this allowance was based on studies conducted nearly 30

years earlier based on circuit designs from that time period. It was

recognized that a need existed to reconsider the measles require-

ments for today's production designs and product environment.

In the interim, as a means to call attention to this potential problem,

IPC assembly documents were changed to include pass/fail (defect)

criteria for Class 3 assemblies that exhibited the visual appearance

of measles.

The new measles criteria created in the assembly documents

defined requirements that were stricter than those given in the

printed board documents unless the additional IPC-6012, Class 3A

(Military and Aerospace) requirements were considered. IPC-

6012, Class 3A does not allow measles in bare boards for these

industry segments, however Class 3 printed boards produced in

accordance with 6012, which exhibited measles, were no longer

acceptable for use in the assembly documents for Class 3 in gen-

eral. This requirement conflict was brought to the attention of the

IPC Technical Activities Executive Committee (TAEC) and that

body directed both groups to work together to come to some reso-

lution, based on test data that would bring the documents into

agreement on the acceptance criteria.

The leadership of the printed board standards committees formed a

new Blue Ribbon Committee on Measles and designed and per-

formed testing to determine if measles contributed to CAF failures

in actual end product. This test protocol was completed in late 2006

and presented to the printed board standards committees at IPC

Printed Circuits Expo/APEX 2007. There was conclusive evidence

within the test that measles did not contribute to CAF growth; nor

did the presence of measles promote CAF failures in the end prod-

uct that exhibited CAF.

2.3 BASE MATERIAL SUBSURFACE（基材表⾯下）

Introduction (cont.)（引⾔（续））

19IPC-A-600H-2010 2010年4月