Question about self-organzing development team

A technical issue they cannot resolve by themselves is per definition an impediment, so the first step would be to communicate it to the Scrum Master.
Now the Scrum Master as a coach would work with the team to identify possible solutions and facilitate a team decision. Possible solutions include those mentioned by you.
The goal should be to remove the impediment, which means the team has to be able to solve this kind of issue in the future. This can be reached by including the expert to the team for a limited time, but not to solve the issue but to coach the team in this subject area.

Thanks for the answer,
1) You mean in this case the best solution is SM should facilitate and get the development team trained by technical expert from outside the team. So that in the future when team facing with the same issue. They can resolve it by themselves ?
2) I think the the solution by including the technical expert to the team seems not a really good solution as it can make the development team size exceed 9 (recommend by Scrum guide). As my understand, Scrum guide just recommend the DT team size should be in range of 3-9 but it's not a must right ?
If in there are just options given :
1) get the expert help from outside the team
2) include the expert to the team
What's your choice ?
Scrum did not clearly mention about how to train the development team to be a cross-functionality.

1) Yes, this appears to be a reasonable solution to me.
2) If your development team has 9 members, this means for a limited time it will have 10 members, one of which being only a coach for a specific subject area and not contributing to the increment. If you get the expert help from outside the team, you have an external dependency, which means the team is not able to build a "done" increment without external help. Both is not optimal, but consider it is only for a limited time. My choice as Scrum Master would be to let the team decide which option they prefer. If I was in the dev team, I would generally vote for 2, unless there are good reasons against it.

> Should they ask for help from technical
> expert outside the team or should they
> include the expert to the team to ensure that
> the team is cross-function ? 

Scrum requires a Development Team to have all of the skills and resources needed in order to deliver a Done increment. If a necessary resource, including third party expertise, is available to the team as and when it is needed, then by definition there is no impediment.

In other words you've only got a problem if the outside expertise is a *volatile* dependency. If that is the case then the team will need to consider recruiting a sufficiently expert member, receiving the required training themselves, or agreeing not to induct similar work into future Sprint Backlogs which they are unable to wholly complete.

Hi Lan,
Thanks for the clarification. Also as mentioned in Scrum Guide "Only member of the development team create the Increment". So I understand that the correct solution is to add the Expert to the team as long as we need him to be sure the definition of a Cross-functionality team right ? Having Expert in the team also get other benefit of Transparent-Inspection-Adaptation because he can join the scrum events ?

> So I understand that the correct solution is to add the Expert to
> the team as long as we need him to be sure the definition of a
> Cross-functionality team right ?

If that is the best way to be sure of the expert's availability, then yes. On the other hand, if the team have unfettered access to the expert then he or she does not need to be on the team. Examples of the latter case can include domain experts referred to the team by the Product Owner, or technical experts who facilitate the application of a selected technology. As long as these experts provide a utilitarian on-demand service, and are not making decisions about how the increment is produced, then there is no need to enfranchise them as team members.

Thanks all for your replies. I officially become a PSM1 with 93.5% scores